urban clmatology and its relevance to urban design · trast, although not entirely neglectful of...

WORLD METEOROLOGICAL ORGANIZATION

TECHNICAL NOTE No. 149

URBAN CLMATOLOGY AND ITS RELEVANCE TO URBAN DESIGN

by

T. J . Chandler

Prepared with the support of the United Nations Environment Programme (UNEP)

WMO - No. 438

Secrétariat of the World Meteorological Organisation - Ceneva - Switzerland

THE WMO

The World Meteorological Organization (WMO) is a specialized agency of the United Nations of wlùch 145 States and Territories are Members.

I t was created:

— To facilitate international co-operation in the establishment of networks of stations for making meteorological and geophysical observations and centres to provide meteorological services and observations;

— To promote the establishment and maintenance of Systems for the rapid exchange of meteorological information;

— To promote standardization of meteorological observations and ensure the uniform publication of observations and statistics;

— To further the application of meteorology to aviation, shipping, water problems, agriculture, and other human activities;

— To encourage research and training in meteorology.

The machinery of the Organization consists of the following bodies:

The World Meteorological Congress, the suprême body of the Organization, brings together the delegates of ail Members once every fo\ir years to détermine gênerai policies for the fulhlmcnt of the purposes of the Organization, to adopt Technical Régulations relating to international meteorological practicc and to détermine the WMO programme.

The Executive Commitlee is composed of 24 directors of national Meteorological Services and meets at lcast once a year to conduct the activities of the Organization and to implement the décisions taken by its Members in Congress, to study and makc recommandations on matters affecting international meteorology and the opération of meteorological services.

The six Régional sissociations (Africa, Asia, South America, North and Central America, South-West Pacific and Europe), which are composed of Mcmbcr Governmcnts, co-ordinate meteorological aclivity within their respective Régions and examine from the régional point of view ail questions referred to them.

The eight Technical Commissions, composed of experts designated by Members, are responsible for studying the spécial technical branches related to meteorological observation, analysis, forecasting and research as well as to the applications of meteorology. Technical commissions bave bcen established for basic Systems, instruments and methods of observation, atmospheric sciences, aeronautical meteorology, agricultural meteorology, marine meteorology, hydrology, and spécial applications of meteorology and climatology.

The Secrétariat, located at Gcneva, Switzerland, is composed of an international scientific, technical and administrative staff undcr the direction of the Sccretary-General. I t undertakes technical studies, is responsible for the numerous technical assistance and other technical co-operation projocts in meteorology throughout the world aimed at contributing to économie developmcnt of the countries concerned. It also publishes specialized technical notes, guides, manuals and reports and in gênerai acts as the link between the Meteorological Services of the world. The Secrétariat works in close collaboration with the United Nations and other specialized agencies.

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WORLD METEOROLOGICAL ORGANIZATION

TECHNICAL NOTE No. 149

URBAN CLIMATOLOGY AIVD ITS RELEVAIVCE TO URBAN DESIGN

by

T. J.jchandler

Prepared with the support of the United Nations Environment Programme (UNEP)

U.D.C. 551.588.7 c i-m

WMO - No. 438

Secrétariat of the World Meteorological Organization - Geneva - Switzerland 1976

* Z / j

© 1976, World Meteorological Organization

ISBN 92 - 63 - 10438 - 7

N O T E

Thc désignations employcd and the présentation of material in this publication do not imply thc expression of any opinion whatsoever on thc part of the Secrétariat of the World Meteorological Organization concerning the légal status of any country, tcrritory, city or area, or of its authorities, or concerning the délimitation of its frontiers or houndaries.

CONTENTS

Foreword V

Summary (English, French, Russian, Spanish) VII-X

Préface XI

Introduction XIII

Population growth, urbanization and meteorology XIII The climatic input into building and urban design XIV The building climatology/urban climatology continuum XIV Scales of décision making XV Organizational aspects and information transfer problems XV Conclusion XVII

Chapter 1 — Climatological processes 1

1.1 Introduction 1

1.2 Scales of climate 1

1.3 The influence of urban areas upon the physics and chemistry of the boundary layer 3

1.3.1 Airflow in urban areas 3

1.3.2 The water balance 5

1.3.3 Heat balance 7

1.3.4 Air pollution: Emission and abstraction 11

Chapter 2 — Observational studies 15

2.1 Introduction 15

2.2 Airflow 15

2.3 Humidities 21

2.4 Précipitation 23

2.5 Hydrology of cities 24

2.6 Radiation and sunshine 25

IV CONTBNTS

2.7 Température 25

2.8 Air pollution 36

2.9 Visibility 38

Chapter 3 — Urban climates and urban planning 39

3.1 Introduction 39

3.2 The decision-making process. . . . . . . ' 40

3.3 Hydrology 42

3:4 Températures. : . . . . . . . . : . . . . . . 42

3.5 Pollution 44

3.6 Humidities 47

Références . . 49

FOREWORD

The expansion of urban areas is accelerating throughout the world. This expansion gives rise to many socio-economic, ecological and environmental problems, the solution of which calls on the united efforts of workers in a wide range of disciplines. In particular, many problems related to the quality of urban life hâve components which are related to weather and climate, some of which are of great importance. Indeed the basic problem of urban design planning is affected by thèse components. A great deal of study has therefore been devoted, in récent years, to such problems.

It is noteworthy also that the topic is of such gênerai interest that the United Nations has decided to convene a World Conférence on the subject (UN Conférence on Human Settlements (HABITAT), Vancouver, B.C., Canada, 31 May -11 June 1976) and this development has encouraged WMO to prépare a survey of the présent state of knowledge regarding the application of meteorology and climatology to meet urban seulement problems. To this end Professor T. J. Chandler, who has fruitfully collaborated with WMO for more than ten years and who is known as a pioneer-scientist in the field of urban climate, was invited to prépare this Technical Note.

I should like to express hère my gratitude to Professor Chandler for his continued co-operation and for the excellent report he has produced. It is also a pleasure to acknowledge the support that was provided by the United Nations Environment Programme (UNEP) to enable the survey to be carried out.

/ 5 CC-^S~2U> » * - p .

(D. A. Davies) Secretary-General

SUMMARY

Almost everywhere in the developed and developing world, centres of population are in fiood. Villages are expanding to become towns, towns are growing into cities and cities are merging into vast conurbations. By the end of this century, it is estimated that three out of every five people will live in towns. The urban environment provides the life framework for a rapidly growing number and proportion of the world's population. Atmo-spheric components are an essential part of this environment and it is therefore important to realize that modifications are made by buildings to the climates not only within their walls, but also in the streets, courtyards, parks and other areas around, between and above them.

Optimization of the indoor climate is a most important and long recognized rôle of the architect. In con-trast, although not entirely neglectful of climatic considérations, urban planners hâve, until comparatively recently, only rarely considered climate among the several constraints upon urban design. The layout of cities has in most cases been dictated, or almost accidentally created, by a séries of mainly political, social and économie decision-making processes, often over many centuries. The reason for the neglect of climatic considération has been partly the relative youthfulness of the science of urban climatology, and partly the relatively weak communication links that presently exist between climatology and planning. But faced with the exponential growth of the world's population and the accelerating pace of urbanization, it is clear that our cities must, where appropriate, be purposefully planned in order to optimize the environment of urban areas and avoid a séries of structural and functional design failures. Climate is an essential élément in this planning.

When concrète, brick, stone, glass and macadam are substituted for field, farm and forest, the physical and chemical properties of the boundary layer are also changed. Each élément of climate — airflow, air chemistry, températures, humidities, précipitation, visibility and so on — will differ in urban as compared with rural areas. Strong winds are decelerated and light winds are accelerated as they move into towns ; the air above cities is infused with a miasma of solid, liquid and gaseous pollutants; températures are generally higher in cities than in their rural surrounds; relative humidities are most frequently lower though absolute humidities can sometimes be higher in urban than in rural areas; précipitation tends to be enhanced within or to the lee of cities; visibilities are poorer and radiation receipts are also lowered by the pail of smoke and other pollutants which lie over most cities.

As a guide to planners, Chapter 1 of this Technical Note considers the various mass, moisture, heat and momentum exchanges within the urban boundary layer; Chapter 2 describes the conséquences of thèse as they affect the distribution patterns of each of the meteorological éléments; and Chapter 3 considers the implications of urban climates to successful urban design, together with the logistics of the climatic input into the planning process.

RÉSUMÉ

Presque partout dans le monde, que ce soit dans les régions développées ou en voie de développement, les centres de population connaissent une extension sans précédent. Les villages s'agrandissent et deviennent des villes, les villes s'étendent et deviennent des agglomérations qui, à leur tour, se transforment en vastes conurbations. On estime qu'à la fin de ce siècle, sur cinq personnes, trois vivront dans des villes. L'environnement urbain constitue le cadre de vie d'une proportion rapidement croissante de la population mondiale. Les éléments atmosphériques étant une partie essentielle de cet environnement, il est important de comprendre que les immeubles modifient le climat non seulement à l'intérieur de leurs murs, mais aussi dans les rues, les cours, les parcs, ainsi qu'autour et au-dessus d'eux.

La réalisation d'un climat optimal à l'intérieur des bâtiments est une tâche très importante de l'architecte et elle est reconnue depuis longtemps en tant que telle. Par opposition, bien que ne négligeant pas entièrement les facteurs climatiques, les urbanistes n'ont que rarement envisagé le climat, et ce jusqu'à une date relativement récente, comme l'une des nombreuses contraintes dont il faut tenir compte lors de la conception des villes. Le tracé de celles-ci a le plus souvent été dicté, ou presque accidentellement créé, par une série de décisions d'ordre principalement politique, économique et social, portant souvent sur plusieurs siècles. Si les facteurs climatiques ont été négligés, c'est en partie parce que la science de la climatologie urbaine est relativement jeune, et en partie parce que les relations existant actuellement entre climatologistes et planificateurs sont relativement faibles. Mais, face à la croissance exponentielle de la population du globe et au rythme accéléré de l'urbanisation, il est évident que la planification de nos villes doit être, le cas échéant, mûrement réfléchie, de manière à parvenir à optimaliser l'environnement des zones urbaines et à éviter toute une série d'échecs sur le plan des structures et du fonctionnement. Le climat est un élément essentiel de cette planification.

Lorsque le béton, la brique, la pierre, le verre et le macadam remplacent les champs, les fermes et les forêts, les propriétés physiques et chimiques de la couche limite subissent également des changements. Chaque élément du climat — circulation de l'air, chimie de l'air, température, humidité, précipitations, visibilité, etc. — sera différent dans la zone urbaine par rapport à la zone rurale. Les vents forts sont ralentis et les vents faibles accélérés lorsqu'ils pénètrent dans les villes; l'air au-dessus des villes est un miasme de polluants solides, liquides et gazeux; la température est généralement plus élevée dans les villes que dans les campagnes environnantes; l'humidité relative est le plus souvent inférieure, bien que l'humidité absolue puisse parfois être supérieure dans les zones urbaines par rapport aux zones rurales; les précipitations tendent à augmenter au-dessus et sous le vent des villes; la visibilité est moins bonne et le rayonnement reçu est également affaibli par le voile de fumée et les autres polluants qui recouvrent la plupart des agglomérations.

Le rapport, qui peut servir de guide aux planificateurs, passe en revue, dans le chapitre 1, les divers échanges de masse, d'humidité, de chaleur et de quantité de mouvement dans la couche limite urbaine. Le chapitre 2 décrit les conséquences de ces échanges dans la mesure où ils modifient la distribution de chacun des éléments météorologiques. Le chapitre 3 examine l'importance des climats urbains pour une bonne urbanisation, ainsi que la manière dont il faut tenir compte des données climatiques à tous les stades de la planification.

PE3I0ME

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RESUMEN

Prâcticamente en todos los lugares del mundo, tanto desarrollados como en desarrollo, asistimos a una expansion sin précédentes en los centros de poblaciôn. Los pueblos se desarrollan y se hacen ciudades, las ciudades crecen y se convierten en grandes capitales y estas a su vez se transforman en vastas aglomeraciones. Se estima que a finales de este siglo, de cada cinco personas très vivirân en ciudades. El medio ambiente urbano facilita el marco de vida para un numéro y proportion de la poblaciôn del mundo en râpido crecimiento. Los factores atmosféricos son parte fundamental de este medio ambiente, siendo por lo tanto importante saber que las modificaciones que los edificios introducen en el clima no solo actûan dentro de los muros de los mismos, sino que también influyen en las calles, en los patios, en los parques y en otras zonas en sus proximidades y por encima de ellos.

El logro de las condiciones climâticas ôptimas en el interior de los edificios es funciôn importantisima y bien reconocida de los arquitectos. Por el contrario, los planificadores urbanos, aunque sin olvidar por completo las consideraciones climâticas, hasta hace relativamente muy poco tiempo solamente en raras ocasiones tuvieron en cuenta el clima como uno de los factores que intervienen en la planification urbana. La ordenaciôn de las ciudades ha sido dictada, o incluso casi artificialmente creada, en muchos casos, por una série de procesos principalmente de orden politico y sotioeconômico a través de los siglos. La razôn de este olvido de las consideraciones climâticas se ha debido, en parte, a la relativa juventud de la ciencia de la climatologia urbana y, en parte, a los contactes mâs bien déficientes que actualmente existen entre la climatologia y la planification. Sin embargo, enfrentados con el problema del crecimiento exponencial de la poblaciôn del mundo y con el ritmo acelerado de urbanizaciôn, es évidente que nuestras ciudades deben ser planificadas para conseguir el medio ambiente ôptimo de las zonas urbanas y evitar una série de fracasos estructurales y funcionales. El clima es un elemento esencial en esta tarea de planification.

Cuando los campos, los bosques y las granjas desaparecen para dejar paso al cemento, al ladrillo, a la piedra, al cristal y al alquitrân, también cambian las propiedades fisicas y quimicas de la capa limite. Cada elemento del clima, es decir el flujo de aire, la quimica del aire, la temperatura, la humedad, la précipitation, la visibilidad, etc., sera diferente en las zonas urbanas si se compara con las rurales. Los vientos duros sufren una deceleraciôn y los débiles se aceleran a medida que se dirigen hacia las ciudades. El aire de las ciudades esta Ueno de un miasma de contaminantes sôlidos, liquidos y gaseosos. Las temperaturas son generalmente superiores en las ciudades que en los alrededores rurales; las humedades relativas son con gran frecuencia inferiores, aunque las absolutas pueden algunas veces ser superiores en las zonas urbanas que en las rurales; la précipitation tiende a reforzarse dentro o en el limite de las ciudades; la visibilidad es peor y la réception de radiaciôn también se ve disminuida por la capa de humo y por otros contaminantes existentes en la mayor parte de nuestras ciudades.

Como guia para los planificadores, el Capitulo I del informe examina los diversos intercambios de masa, de humedad, de cal or y de cantidad de movimiento dentro de la capa limite urbana; en el Capitulo II se describen las consecuencias de estos intercambios, ya que afectan a la distribution de cada uno de los elementos meteoro-lôgicos; en el Capitulo III se estudian las consecuencias que tienen los climas urbanos paralograr una planification urbana satisfactoria, asi como la logïstica de la aportaciôn climâtica al proceso de planification.

PREFACE

This Technical Note is neither intended to be a comprehensive survey and analysis of the enormous literature on urban climatology, since this has recently been most successfully undertaken by Oke (1974), nor is it designed as a stage-by-stage guide to the city planner in his task of incorporating the climatologîcal variable within his design process. What it sets out to do is to provide a summary of those aspects of urban climatology relevant to the design of new towns, urban expansion and urban renewal. Its object is to make the planner aware of the approaches and principles of urban climatology; it includes key références for those who wish to study the subject more deeply or extensively. Secondly, the Note tries to suggest lines along which this knowledge might be applied in individual cases. It is divided into three sections: Chapter 1 deals with the various energy and mass exchange processes operating in urban areas; Chapter 2 describes the characteristic spatial and time distributions in cities of the major climatological éléments; and Chapter 3 suggests simple applications of thèse distributions to the better design and climatic optimization of the urban environment.

Because each chapter is intended to be capable of being read and separately understood, there is some unavoidable overlap and répétition between chapters, but this has been kept to a minimum consistent with the objectives.

Urban climatology and urban planning hâve both witnessed major advances in knowledge and under-standing during the last 25 years. The time is now ripe for thèse two areas of learning to study their common ground and interdependence, though with the speed of advance in both subjects, there will obviously be the need for further periodic reviews of their common area of interest.

INTRODUCTION

Population growth, urbanization and meteorology

It took several hundred thousand years for the world's population to reach its first billion (109) around 1800, but only 130 years were needed to add the second billion, and less than 30 more for the third, around 1960. Now, in the mid-1970s, the fourth billion has already been reached with the expectation of well over two billions more being added in the last quarter of the century.

Such increases cannot help but extend and intensif/ meteorological and climatological changes in the atmospheric boundary layer, the latter being defined as the lower 600 m or so of the atmosphère where motions and other properties of the air are closely controlled by the nature of the Earth's surface. Three-quarters of the atmosphere's heat input and ail its moisture cornes via or from the Earth's surface, and surface friction accounts for the dissipation of about two-fifths of the atmosphere's kinetic energy. Clearly, surface conditions, natural or man-made, are of paramount importance in the atmosphere's various energy budgets and by changing thèse conditions, man has inadvertently affected atmospheric properties, more especially in the Earth-atmosphere interface or planetary boundary layer. In draining the marsh, clearing the wood, cultivating the fields and flooding the valleys, man has accidentally changed the thermal, hydrological and roughness parameters of the Earth's surface and the chemistry of the overlying air. Conditions hâve been indiscriminately changed over very substantial areas of the Earth, nearly always, until recently, with unknown and even undesirable conséquences. With the increase in world population, there are indeed.very few "natural" boundary-layer climates remaining, not only in the settled parts of our planet, but in the world as a whole.

In some areas of change, once local problems are now régional or even global in extent and their investigation and solution often dépend upon urgent and concerted international effort through, or assisted by, the various United Nations agencies and other international organizations.

It must of course be remembered that for the half million years or so that man has inhabited the Earth as an active agent of change he has, of course, been only one of several forces modifying the atmospheric sector of his environment; and modifications of climate stemming from man's activities are, and are likely to remain, for the most part completely dwarfed by those resulting from natural processes. But man's influence upon the atmosphère stems less from a direct energy input into the Earth-atmosphere System than from a modification of those surface conditions relevant to natural energy exchanges at the Earth-atmosphere interface. AH surface changes must alter the properties of the overlying air, but many of the modifications in rural areas are of limited environ-mental importance. They hâve not, in gênerai, created physical hazards or disamenity problems. The same cannot be said for urban areas.

One of the primary purposes of man's shelter is protection against adverse or undesirable climatic environ-ments, but each building changes the climate not only inside but outside its walls; when buildings are congregated in villages, towns, and conurbations, the resuit is a degree of modification of local boundary-layer conditions which amounts to the création of distinctive and often far from pleasant climates unless steps are taken purpose-fully to plan the building and urban environment using a positive climatic input.

Ail over the world, urban areas are expanding by migration and natural population increase. Already 30 per cent of the world's population live in towns of 5 000 or more persons and 18 per cent in cities of more than

XTV URBAN CUMATOLOGY AND ITS RELEVANCE TO URBAN DESIGN

100 000. What is more, the proportions are increasing fast and it has been estimated that by the year A.D. 2000, three out of every five people will live in towns. In almost every country, towns are expanding to consume more and more land. Almost everywhere in the developed and developing world, brick, stone, concrète and macadam are inexorably replacing field, farm and forest. Already, for instance, 12 per cent of the surface area of England and Wales is built upon and it has been estimated (Best, 1968) that by the year A.D. 2000 this will hâve increased to 15.4 per cent.

It is clear that the urban environment provides the setting for the life framework of a large and growing proportion of the world's population and it is therefore important to realize that because of this, urban dwellers spend much of their lives in a quite distinctive type of man-modified climate, indoors and out. Some of the more undesirable attributes can be avoided or their importance diminished by purposeful design and planning, while others can be used to advantage.

The study of building climatology and urban climatology is clearly of more than académie interest, being of practical importance in the proper design of buildings and cities to secure maximum comfort as well as optimum socio-economic performance.

The climaiic input into building and urban design

Many constraints control the location and design of individual buildings and building groups. Some are broadly social, others économie, some aesthetic and some physical. Each has to be carefully assessed and then balanced against the others in the decision-making process. Too often, however, the climatic input among the physical controls has been undervalued or completely overlooked. There are many reasons for this.

Optimization of the indoor climate is clearly a most important architectural rôle. The exercise obviously dépends upon an accurate knowledge of the external climate as it affects the behaviour of the building fabric and the provision of internai environmental controls. But in calculating the links between the external and internai conditions of buildings, the architect immediately faces the difficulties and uncertainties of a complex feed-back phenomenon inhérent in building and urban climatology. The very act of constructing a new building or group of buildings will change the external climate, which acts as the baseline for the internai and fabric design. The interaction also means that the architect becomes responsible, through the détails of his design, not only for internai conditions but also for the external climatic environment. In gênerai, the architect has to take responsibility for the successful design of conditions in the immédiate vicinity of the buildings, that is the micro-climatic envelope of structures, and the town planner controls the meso-climate of broad zones of the city. The twin sciences of building and urban climatology hâve grown up to study thèse two areas and it is to thèse that the architect and planner must turn for advice on how to assess the impact of their designs upon external conditions and the external conditions upon their design.

The building climatologyfurban climatology continuum

Each city is made up of a mosaic of individual building and other land-use units, the form and disposition of which are highly complex and, in détail, unique. No two buildings, no two cities, are exactly alike. But the physical and socio-economic factors determining urban land-use forms are sufficiently strong and sufficiently alike in comparably developed and organized societies to create broadly similar patterns of design which enable generalizations to be made in building and urban climatology.

Each urban unit, each wall, each roof, each pavement, courtyard, street or park créâtes above it a climato-logical sheath with which it interacts. Building climatology aims to study this interaction. Urban climatology differs from building climatology primarily in scale (and thereby to some degree in its methods): its objectives are to analyse the way in which groups of buildings and whole cities affect the overlying air. Building and urban

INTRODUCTION XV

climatology are clearly part of a meteorological continuum which, though roughly divisible in scale, in reality hâve no clear methodological or sharp scale boundaries between them.

Scales of décision making

The meteorological input into décision making in building and urban design has to be made at a number of scales, each demanding a différent set of data.

The climate of a site may be regarded as the intégration of a séries of controls, differing in scale. In séquence thèse are the régional climate, determined by synoptic factors; the modifying effects of the local orography; and the self-induced modifications of the buildings and building groups themselves.

At the synoptic scale, the détails of régional climate relate to the broad meteorological properties of the boundary layer, more particularly as thèse affect human comfort and stress as measured by various biometeoro-logical indices (see Sargent and Tromp, 1964 ; Landsberg, 1972(6)). Also important, at this scale, is the pattern of pollution concentrations and, hère, cognizance will hâve to be taken of the effect of the city upon the dispersai of not only its own effluents but also those generated outside the urban area.

The modifications exercised by the local orography upon the régional climate are of fundamental importance and hâve to be carefully considered, for substantial différences in radiation receipts and losses, température, précipitation, airflow and humidity can be generated by qui te small changes in altitude, slope and aspect (Geiger, 1969; Quitt, 1972; GoPtsberg, 1967; MacHattie and Schnelle, 1974). Such différences are particularly important in high latitudes and high altitudes where major différences can accrue from relatively small changes in slope and aspect and thèse différences need to be carefully considered along with other criteria in deciding such matters as land use zoning for residential, commercial and industrial uses.

The macro-scale climate, as modified by the meso-scale orographie factors, will essentially define the climate of the "green field site". To this hâve to be added the modifications to ail the meteorological variables exercised by building complexes and finally by individual buildings.

The changes by buildings and building groups are among the most radical induced by man upon the climate of the boundary layer. AU the éléments of weather are changed by amounts sufficient to rank many of them of major architectural and planning importance. The number of climatic design failures is évidence of this, and of the need for more careful considération of climatic building and urban design.

In built-up areas, the effects of the surface's complex geometry, the shape and orientation of individual buildings, the particular thermal and hydrological properties of building, road and other urban fabrics, the heat from metabolism and various combustion processes taking place in the city, and the pollution released to change the chemistry of the city's air ail combine to create a climate which is quite distinct from that of extra-urban areas. Ail the meteorological éléments are changed. The vector characteristics of each élément therefore hâve to be considered as they exist, or are expected to develop. The controls exercised by thèse climatic éléments then hâve to be matched against other constraints over a wide variety of factors, ranging from social to économie, that will collectively détermine the final form of the development, be it a new building or area development in an existing town or an entirely new city.

Organizational aspects and information transfer problems

Many of the most pressing problems facing mankind are ecological in nature; that is, they involve a number of interrelated, interactive components and a reciprocity between the living and inanimate parts of the System. The environment affects the organisms of the System, including man, and is, in its tura, affected by them. The

XVI URBAN CLIMATOLOGY AND ITS RELEVANCE TO URBAN DESIGN

study of thèse problems brings together workers from a wide variety of classical disciplines in order to cultivate the rich border territories that once divided but now link them. Urban studies is one such area. It unités engi-neers of ail types, economists, chemists, demographers, botanists, hydrologists, geographers, lawyers, socio-logists, historians, architects, planners and many others, including meteorologists and climatologists, brought together in interrelated inquiry into the ecology of urbanized man.

But although the real world, in this case the city, is a complex ecological entity, scholarship, or rather scholars, are not. And so one faces the problems of communication and undèrstanding between the disciplines. Even if and when one has recognized, for instance, the necessity for a climatological input into ail urban planning décision makmg, then there reinain the difficulties of data availability and of scientific communications between specialists, more particularly between meteorologists and climatologists on the one hand and architects and planners on the other. It is unfortunately true that, even in the developed countries, climatological data are generally too déficient in coverage and in the length and nature of the records to be used easily by building and urban climatologists, architects and urban planners. This means that great care has to be taken when interpret-ing the available data for building purposes. It will nearly always be necessary to extrapolate the surface values and vertical gradients at the required site from the records of a climatological station elsewhere. The nearest station to the site is not necessarily the best for thèse purposes. Although more distant, a climatological station having a topographie setting comparable to that of the building site is likely to be much more représentative of conditions at the site than records from a closer, but topographically dissimilar, recording station. For example, if a building is to be constructed in a city suburb lying in the bottom of a valley, then it would be oflittle value to use the records of a rural hill-top station, even though this were close by.

Very few climatological stations, and even fewer with long records, lie within urban areas because the moni-toring networks of most countries hâve been established primarily for synoptic purposes. For this reason, the recording stations hâve shunned àny sites such as hillsides, valley bottoms or urban areas which would exert a powerful local influence upon its records, thereby making its readings difficult to compare with others in a différent topographie setting. The attempt has been to achieve comparability of site conditions, theoretically an infinitely large, level grass-covered area. Another problem is that most of the data collected by national Meteoro-logical Services are not intended for the use of designers and are unlikely to be of the type most suitable for building design and urban planning purposes. Wind-speeds, for instance, are normally recorded at a height of ten mètres though some anemometers are located above the roofs of buildings. In either case, one needs to dérive représentative values in the boundary layer for a séries of levels from the ground to well above the top of the building in order to calculate wind pressures on its walls and the pattera of airflow around it. Thèse are not easy tasks, for our models of airflow ht the boundary layer are still fairly rudimentary and hâve, in any case, to be modified by the complex aerodynamics of urban areas. But it serves to illustrate the cautious and carefully informed use one has to make of the standard meteorological éléments, when available, in urban design. ( Thèse points will be taken up again in the body of this Technical Note.)

The applicability of what we might call traditional meteorology and climatology, with their standardized monitoring techniques, to building and urban climatology is far from straightforward. But in récent years, many authorities hâve commented upon the substantial benefits to be derived from the application of weather forecasting and climatological advice to agriculture and industry (Maspn, 1966; Maunder, 1970; Taylor, 1972, 1974). The need to channel more effort into the direct application of meteorological knowledge, more espe-cially of the boundary layer, into the better design of buildings and cities is fairly widely recognized as an important part of this re-thinking of the directions of meteorological effort in the light of changing human technologies and objectives.

Many countries, recognizing this need, hâve set up building research stations as a part of government advis-ory services to the building industry. Thèse stations provide a focus of expertise and advice made available to architects and planners. Universities hâve also played a part, usually by research into the fundamental processes operating in the urban atmosphère and the design implications of thèse findings.

INTRODUCTION XVII

Conclusion

Increases in world population and in the proportion of the world's peoples living in towns introduce many problems to the socio-economic organizations and stabilities of many parts of our planet. The accelerated rate of growth of our cities in ail parts of the world must be purposefully planned if we are to avoid a mass of structural and functional design failures. Climate is an essential élément in this planning.

The remainder of this Technical Note will describe the nature and application of building and urban climato-logy to the optimization of climate in and around individual structures and in city régions as a whole.

CHAPTER 1

CLIMATOLOGICAL PROCESSES

1.1 Introduction

Studies of the interrelationships between individual buildings and the external climate are the province of building climatology. The présent and following chapters are concerned with groups of buildings as they affect the meso-climate of major urban districts or even whole cities. In particular, the discussions are concerned, in Chapter 3, with the optimization of the climatic environment in urban areas by successful zonal and city planning in which the climatic implications of urban development hâve been considered along with physical and socio-economic factors influencing urban design.

As mentioned in the introduction to this Technical Note, building climatology and urban climatology are not separate, discrète disciplines; they are part of a continuous spectrum of both climate and urban form. Each building reacts with its atmospheric envelope and thèse microclimatic effects are then integrated into meso-climatic zones which commonly mirror the form of urban development and major land-use units. But in their turn, urban climates provide the background conditions for the proper design of individual buildings and so the circle of scale interactions of climate and buildings in the urban environment is complète.

stances: In practice, the application of urban climatology to city design is likely to arise in a number of circum-

(a) To provide information on ambient atmospheric conditions as a basis for building climatology design-decisions, as outlined in Chapter 1 ;

(b) To provide climatological information as a basis for the proper design of areas of urban renewal or expansion including housing estâtes, commercial districts, parks, roads, etc.;

(c) To provide an information input to the sélection of site and design of whole new cities and, at this scale as well as those above, to achieve optimum land-use zoning, bearing in mind the différent functional activities in relation, for instance, to patterns of températures, airflow and air pollution. The hydrology of urban areas is also important to proper urban design, for cities affect runoff and thereby the proper design of storm-water drainage Systems.

Thèse and other climatically influenced design décisions are taken up again in Chapter 3.

1.2 Scales of climate

The motions of the atmosphère exist at scales varying from thousands of kilomètres down to fractions of a centimètre, that is, from global to molecular or from macro- to micro-climatological. Thèse motions reflect the breakdown and conversion of énergies starting with a short-wave solar radiation input into the Earth-atmosphere system which is then converted into heat, potential and kinetic énergies in circulation Systems varying in size from mid-latitude dépressions, 2000 km in diameter, to meso-scale Systems, such as thunder-storms, with horizontal dimensions of 50-200 km, local Systems, such as mountain and valley winds moving over distances of 5-50 km, and micro-scale Systems with dimensions of perhaps 1 cm to 100 m.

URBAN CLIMATOLOGY AND ITS RELEVANCE TO URBAN DESIGN

Reflecting this cascade of énergies are various scales of climate. At one end of the spectrum the macro-scale, régional climates, each covering a major part of a continent and possessing a broad unity of climatic properties. Thèse régional characteristics are found repeated in broadly similar latitudes and continental positions and they establish the first-order attributes of an area's climate, such as seasonal température and rainfall régimes. One of the most famous définitions of thèse broad climatic régions was that proposed by Kôppen (1931).

Within each régional climate there are mesoscale variations from place to place because of the climatic controls exercised by the local topography, including altitude, surface morphology, proximity to water, etc. Finally, within each meso-climate, there are variations over distances up to a few tens of mètres because of changes such as those in soil type, végétation and aspect.

Surface terrain and cover exercise a very close control upon patterns of airflow, températures, précipitation and humidities, more particularly within the lower few thousand mètres of the atmosphère, thus very materially changing local détails of the broad, régional climate and the meteorological properties of the "green field" site (Geiger, 1969; Gol'tsberg, 1967; Quitt, 1972). But surface controls upon the properties of the over-lying air may be anthropogenic as well as natural. There are very few land areas outside the polar ice-caps which hâve not been changed at some time by man, but perhaps the most radical change to natural, physical conditions has been made in cities where the efiects of individual walls and groups of buildings lie at the small-scale end of the spectrum of controls and climates which begins with the macro-scale patterns of global winds.

Macro-climates exercise a first-order control upon indoor climates and influence those aspects of design intended to restore and/or maintain the interior climate at agreeable comfort levels — whatever thèse might be, and they certainly vary from one person and one community to another (Sargent and Tromp, 1964; Terjung, 1967; Landsberg, 1972(a)). But even individual buildings react with their envelope of air and small groups of buildings exercise an appréciable effect upon the exteraal climate (Landsberg, 1970(a)). The climate of city streets, parks and other open spaces stand in even more marked contrast to their rural envelopes.

The meteorological environment of a given site, be it the whole or part of a city, is therefore the intégration of the various scales of physical controls determining the climate of the "green field site", together with the changes induced by the urban development per se. Controls therefore exist over a very broad range of scales, each relevant to a particular type and scale of planning. Thèse relationships hâve been summarized by Lacy as shown in Table I.

TABLE I

The relative importance of différent climatological scales on stages of decision-making within architecture and construction (after Lacy, 1972)

Régional planning

Town planning

Site sélection and building design

Macro-scale (régional)

Dominant

Important

Important

Mesoscale (including urban)

Important

Dominant

Important

Micro-scale

Less important

Important

Dominant

The interrelated scales of climate and urban décision making are considered in more détail in Chapter 3.

CLIMATOLOGICÀL PROCESSES

Because of différences in régional climates and consequential contracts in the effect of cities upon the properties of the air between and immediately above theîr buildings, one would expect both building and city design to respond to thèse différences and to vary between the major régional climates. Unfortunately, although indigenate, traditional architecture and seulement design, normally to be found in essentially rural areas, were well suited to the properties of their local atmospheric environment; cities in most countries, even in the tropics, hâve increasingly aped mid-latitude, mainly European, styles of buildings and urban design, with most unfor-tunate conséquences.

And yet, although there is a widely recognized need for urban forms specifically adapted to the local climate so as to optimize atmospheric conditions as far as possible, given other constraints upon design, there hâve been astonishingly few studies of urban climates outside Europe, North America, Australia, New Zealand and Japan. The vast majority of studies hâve been made in mid-latitude, so-called temperate climates. There is a great need for studies of the sizes of the physical ternis involved in the urban climates of low-Iatitude cities (WMO, 1970 (è)), though some meteorological parameters such as airflow and pollution hâve known constraints which are common to ail climates. The behaviour of other meteorological parameters, such as température, in low-latitude cities is less well understood and Iess easily predicted from existing, mainly mid-latitude, mathe-matical and physical models.

1.3 The influence of urban areas upon the physics and chemistry of the boundary layer

In substituting "artificial" urban units for natural shapes and surfaces, man has changed the physical and chemical properties and processes taking place at the Earth-air interface. In replacing field, farm and forest by brick, concrète, glass and macadam, man has affected the aero-dynamic, thermal, hydrological and mass-exchange processes taking place in the atmospheric boundary layer. In conséquence, the meteorological properties of the air within and immediately above urban areas are profoundly changed to create a distinctive local climatic type, the urban climate.

Each élément of climate — airflow, air chemistry, températures, humidities, and so on — will be sepa-rately considered in Chapter 2, and in Chapter 3 considération will be given to the implication of particular meteorological parameters to successful urban design. For the présent, the discussion will be centred on the effect of towns upon the energy- and mass-exchange processes operating in the planetary boundary layer, treated as a physical System. The discussion is brief, since an excellent summary was recently published by Oke (1974) reviewing récent work within the field of urban climatology.

1.3.1 AIRFLOW IN URBAN AREAS

The patterns of airflow within and immediately above cities are fundamental to nearly ail the meteorological processes taking place in urban areas and thereby to the levels of pollution, température, humidity, etc., which collectively define the outdoor as well as indoor climate of the urban environment (Morgan and Rogers, 1972). And yet the complex geometry of built-up areas constituting the roughest of the Earth's surfaces, either natural or man-made, leads to enormous complexities in the patterns of airflow which vary very sharply in both space and time. Accélérations and décélérations, gusts and lulls, eddies and jet-like flows are to be found closely juxtaposed from one place to another in the city and from one second to the next. But the extrême complexities and gradients of wind speed and direction to be found in urban areas are but intensifications of conditions which everywhere characterize the atmospheric boundary layer. The roughness of the urban surface merely accentuâtes universal features of the lower 600 m or so of air.


1.3.1.1 Veîocity profile

One of the most fondamental effects of the frictional drag of the Earth upon the overlying air is to reduce mean horizontal wind speeds with maximum réductions immediately above the Earth's surface and zéro réduction, by définition, at the top of the boundary layer. The use of power law or other mathematical définitions of the shape of resulting veîocity profile has been the subject of intense study (WMO, 1970(è); Oke, 1974). But there are considérable problems in deriving représentative urban values for several of the terms in the pro-posed équations (Davenport, 1965 ; De Marrais, 1959; Munn, 1970; Pasquill, 1970(a); Yamamoto and Shima-nuki, 1964; Panofsky, 1971 ; Jones et al., 1971) such as roughness length, which is a measure of surface roughness and usually based on wind profile analysis. It is, of course, variable within cities (Peschier, 1973; Oke, 1974). Also, the veîocity profile concept is often applicable only above gênerai roof level, thereby necessitating the use of a zéro plane displacement technique (Shellard, 1968) with élévations measured from a height roughly corres-ponding to the gênerai level of the buildings or other roughness éléments. Beneath thèse heights, wind speeds and directions are extremely variable with areas of calm at the bottom of courtyards and other confined spaces but with strong winds channelled along streets orientated roughly in the direction of the wind, and eddies across streets running at right-angles to the wind (Georgii, 1970; Wise, 1970).

1.3.1.2 Wind speed

Studies in many cities hâve shown that average wind speeds are generally lower in built-up areas than over rural areas (Chandler, 1965 ; Landsberg, 1956; Munn, 1970), but Chandler (1965) and Bornstein (1969, 1972) hâve analysed data for London and New York respectively to show that the différence in strength between town and country is a function of the régional near-surface wind speed and wind profile. When winds are light, speeds are often greater in the built-up area than outside, whereas the reverse relationship exists when winds are strong. Chandler demonstrated that, on average, night-time wind speeds in central London were 14 per cent stronger than at Heathrow (London Airport), whilst day-time winds were 24 per cent weaker. On individual occasions, wind-speed increases of up to 80 per cent are qui te common, with the greatest accélération over the downwind half of the city as predicted by Bornstein's (1972) numerical model, and also by Estoque and Bhumral-kar (1968) for a heat island. The sizes of the urban/rural contrasts in wind speed are dépendent in part upon urban températures and stabilities which are studied elsewhere; but one of the most important effects occurs when the formation of the nocturnal rural inversion damps down near-surface rural wind speeds while the urban atmosphère remains relatively unstable, thus allowing faster urban than rural wind speeds to persist, often for several hours into the night. The mechanisms responsible for the urban accélérations are probably a combination of country breezes associated with heat-island circulations, and a momentum flux to the surface as a resuit of increased thermal and mechanical turbulence. (The heat-island circulations are analysed below). Increased thermal and mechanical turbulence results in a downward transport of momentum and in airflows having a sharp increase of wind speeds with height; this is then more than sufncient to compensate for increased frictional drag by the city fabric.

1.3.1.3 Wind direction

Within the city, wind direction, as already indicated, is very closely controlled by the form of the buildings, and the pattern of streets and open spaces, as well as, of course, by the topography upon which the city is built.

Above the buildings, winds blow at an appréciable angle to the isobars because of the powerful surface friction. Measurements of this angle in a number of cities vary from 15° at times of strong instability to more than 40° during température inversions. The wind fields above a number of cities hâve been studied, including New York (Hass et al., 1967; Angell et al., 1968; Johnson and Bornstein, 1974), Los Angeles (Angell et al., 1971(a) and 1972), Oklahoma City (Angell and Pack, 1972), Fort Collins, Colorado (Fosberg et al, 1972) and


St. Louis (Ackermann, 1972, 1974(a) and 1974(6)). Most studies show a tendency to anticyclonic turning over the central urban area, followed by a cyclonic curvature to the trajectories in the downwind suburbs and up to 30 to 60 km in the lee of the city. This type of effect is particularly strong by day. By night the eflfect, when it exists, is usuaily restricted to below 300 m and commonly disappears altogether, in which case the city may act as an obstacle to flow, diverting the wind around either side and possibly over the top of the warm air or heat island. The day-time bending is almost certainly the conséquence of changing surface friction.

Many field experiments hâve also shown rising air over cities, no doubt owing to the warmth of their surfaces and the rising limb of a cellular circulation linking the cool air of country areas with the city's heat island.

1.3.1.4 Local air flows

Studies in a number of cities (Fosberg et ai, 1972; Pooler, 1963; Emonds, 1954; Georgii, 1970; Chandler, 1961) hâve shown that on calm, clear nights, when a strong heat island develops over a city, there is a surface inflow towards the centre(s) of warm air. Thèse centripetal cool winds are no doubt linked to rising air over the city and a return flow (Bach, 1970) from city to country at a higher level, though there hâve been few observations of thèse Hnking flows. The in-blowing winds near the surface are very light, normally observed to be less than 4 m s-1 and for this reason they are quickly decelerated by surface friction in the suburban areas of many large cities (Chandler, 1961; Hass et al., 1967; Pooler, 1963). Hère, they occur as pulsating flows across the margins of the warm air of the heat island.

Numerous mathematical and physical models hâve been devised to study two- or three-dimensional flows over heated and/or rough surfaces corresponding to cities. Thèse hâve been carefully summarized by Oke (1974).

1.3.2 THE WATER BALANCE

Great theoretical and practical interest centres on the modifications induced by urban areas upon the atmospheric, surface and subsurface parts of the hydrological cycle because of the partial replacement of vegetation-covered soils by various urban fabrics. Much of the surface is sealed by concrète, asphalt and other impervious and semi-pervious materials, al though the still fairly large amount of open ground has often been overlooked in the analysis of urban hydrology. But the physical contrasts with rural areas are big enough to affect ail the hydrological processes operating in cities above, at and below the surface. Thèse changes are often, however, equal and opposite, so the total effect upon the water balance of cities may be qui te small (Oke, 1974).

The water balance of urban areas may be summarized as follows:

P + D + A + W = E+Re + S

where P is précipitation, i.e. rainfall, snowfall, hail, etc.; D is dew and hoar frost; A is the water released from anthropogenic sources, more particularly combustion; W is the piped, surface and subsurface water brought into the city; E is evaporation (including transpiration); Rt is the natural and piped surface and subsurface flow out of the city; and S is the change in water storage in the fabric of the city.

1.3.2.1 Précipitation

In 1970, Atkinson stated that "in contrast to analyses of [other] near-surface phenomena, much of the literature on urban précipitation is primarily concerned with establishing whether or not the urban area has any effect at ail on the précipitation distribution".


It seems, at first sight, plausible to postulate that because of the increased pollution from stationary sources and from vehicle exhausts, more particularly particles of lead which hâve combined with iodine or bro-mine, there should be increased cloud and précipitation above and downwind of cities (Changnon, 1972; Schaefer, 1968). But too large numbers of freezing and condensation nuclei might actually inhibit rainfall (Gunn and Phillips, 1957) and the évidence suggests additional nuclei to be a less important influence on précipitation in urban areas than increased mechanical and thermal turbulence. Landsberg (1956, 1962, 1970(c)) and Changnon (1968,1969,1972) hâve summarized the théories and field évidence for a number of cities in the U.S.A. and Europe, and Atkinson (1968, 1969, 1970, 1971, 1975) has produced several synoptic analyses for London.

Simple comparisons of précipitation inside and outside urban areas prove very little, because of the well-known fickleness of the élément, the relatively poor sampling in normal raingauge measurements, the drift of city pollutants into rural areas (Band, 1969) and the important orographie controls which apply in many cities. Also, more récent studies hâve emphasized the time élément involved in précipitation processes and hâve pointed to the common displacement of positive anomalies downwind of their initiation (Changnon, 1972).

Many studies hâve been made in a variety of cities, primarily in the U.S.A. (Oke, 1974), but the most concerted fîeld and theoretical effort has been made of St. Louis in a study known as Project METROMEX (Changnon, 1972; Beebe and Morgan, 1972).

Measured increases of précipitation in and downwind of urban areas vary but generally fall within the range of 5-15 per cent in annual rainfall amounts with generally larger increases in the winter than in the summer months. The number of thunderstorm days is generally 5 to 10 per cent higher in cities, with 15 to 20 per cent more in summer and at night when the city is generally warmer.

Cities are unlikely to make very substantial différences in the frequency of snowfall, for snow faits at times of the year and in meteorological conditions generally inhibitive to strong heat islands which would otherwise act both as a trigger to instability and as a means of melting the flakes. Nevertheless, one or two studies (Changnon, 1969) hâve shown small increases of snowfall in urban areas.

Dewfall and hoar frost are likely to be greater in suburban and rural areas than in city centres because of the higher températures there, but there are no measures of thèse différences.

1.3.2.2 Anthropogenic sources of water

Enormous amounts of water are released during the combustion of fossil fuels within most cities, more particularly those of middle and higher latitudes and those with a strong industrial base. But apart from studies of the effect of cooling towers upon fog frequencies in their vicinity (Hewson, 1970; Hage, 1972), little is presently known about the quantities of water released or its proportional effect upon vapour levels. Some récent estimâtes of anthropogenic moisture émissions hâve, however, recently been made in New York City (Bornstein and Tarn, 1975), where it was shown that the maximum annual émission rate from a particular square mile in Manhattan was equal to about 25 per cent of the annual evaporation rate in the coastal Atlantic off New York City.

1.3.2.3 Evaporation

Evaporation from cities has long been assumed to be drastically less than that from neighbouring rural areas because of the supposedly major contrasts between the hydrological properties of urban building materials and vegetation-covered soils. But evaporation is in any case one of the notoriously difficult meteorological processes to monitor in the fîeld, and the difficulties are compounded in urban areas. We therefore hâve to


rely on theoretical estimâtes of the effect of cities upon evaporation rates (Lull and Sopper, 1969; Oke, 1972(6)), although the quoted figures (e.g. a 59 per cent réduction with a 75 per cent impervious cover) might be rather unrepresentative since most cities retain large areas of open ground (Detwyler and Marcus, 1972) and the absorp-tive capacities of bricks, tiles and other building fabrics are much greater than has sometimes been assumed. A study in central London (Watkins, 1963) for instance, showed that for an area which was 95 percent developed, only 50 per cent of the précipitation in a given storm eventually flowed through the drains, the remaining 50 per cent presumably being absorbed into the urban fabric. Much more research is clearly needed into the absorbency and storage capacities of urban building materials.

1.3.2.4 Runoff

Considérable interest has been taken by both climatologists and engineers (see Chapter 3) in the effects of urban areasupon runoff (Muller, 1968; Detwyler and Marcus, 1972; Landsberg, 1970(c)). In gênerai, runoff is increased in urban areas in comparison with rural environments because of the extensive areas of impervious and semi-pervjous surfaces quickly leading to storm-water drains. This contrasts with the absorbent soils and groundwater flow of rural areas.

Leopold (1968) has analysed the effect of urbanization and expressed the effect of différent percentages of impervious cover and per cent of area served by storm sewers upon increased urban discharge. With 60 per cent of the area impervious and 60 per cent served by storm sewers, the ratio of discharge after urbanization to discharge before urbanization was about 3.5, and with 80 per cent, about 5.0. Similar increases in runoff and discharge hâve been reported for many urban areas (Carter, 1961 ; Carter and Thomas, 1968). The effect upon runoff is likely to be most marked with heavy storms and, as many cities are built in the bottom of valleys, the danger of flooding is obviously enhanced.

1.3.3 HEAT BALANCE

Again, Oke (1974) has provided an excellent review of récent work on the various heat-exchange processes operating in urban areas including surface and boundary-layer modelling. But the complexity of the processes, their sensitivity to synoptic and other controls (Chandler, 1965; Ludwig, 1970; Oke and Hannell, 1970; Clarke and Peterson, 1973 ; Parry, 1956) and the paucity of knowledge of the sizes of the tenus involved in the energy balance, more especially in low-latitude situations, presently prevent ail but the crudest models of the energy balance in urban areas. In its very simplest form, the energy balance can be stated as:

Rn + F = H+G + A + LE

where Rn is the net all-wavelength radiation; F the artificial or anthropogenic heat generated within the city by combustion and metabolism; H is the convective sensible heat transfer; G is the net heat storage within the urban fabric (buildings, roads, soils, etc.); A is the net advected energy; and LE is the latent heat transfer (Figure 1). There are relatively minor tenus including the energy transfers by photosynthesis, précipitation, snowmelt and surface friction, but thèse can be neglected.

1.3.3.1 Radiation

In towns, ail the terms of the radiation balance are changed because of the physical properties of urban as compared with rural surfaces. Though varying with the seasons, incoming short-wave radiation receipts in urban areas are generally less, perhaps by 10-20 per cent, than in the sunounding countryside, mainly owing to atténuation by pollution (Nader, 1967; East 1968; Yamashita, 1970; Detwiller, 1970; Probald, 1972; Bach, 1970, 1971(a)).


DAY

Rural Urban-

Wind Rn

<y ffife^fe^ 0 ^ Q

NIGHT

Rural-

Wind

2 M LE Rn

a <^ gain

Urban

A °ît*

H and LE

a G

<ttff<?r Fuf75te o/jrf Oke, 1970)

Figure 1 — Two dimension scheme of heat exchanges in rural and urban areas. Rn = radiation; H = sensible heat; LE = latent heat; Ph — Photosynthesis; G = heat storage in buildings and ground; A = advection


The surface albedo (reflectivity) of most urban surfaces lies within the range from 0.10 to 0.30 with most observational values occurring around 0.15. The lower ranges are équivalent to those for water and coni-ferous and tropical rain forests and the higher ones to those for sandy and stone déserts. Cities such as those in désert or semi-desert climates, where building materials are often light coloured, tend to hâve rather high albedos (Kung et al., 1964). The albedo in Ibadan (Nigeria), one of the very few tropical cities for which there are measurements, has a recorded value of 0.12 to 0.15 (Oguntoyinbo, 1970) which is comparable to those of mid-latitude cities; a not surprising resuit given the similarity of many of the building materials. The effective out-going short-wave radiation is also of course reduced from the bottom of city streets following the trapping and absorption of reflected solar radiation by the sides of buildings (Craig and Lowry, 1972). Thus, although there is considérable variation between cities, depending upon their building materials and urban design and also upon the nature of the surface cover of végétation in the surrounding région, it is generally true that urban albedos are likely to be less than for rural areas.

Surface long-wave radiation losses from urban areas are likely to be greater than from rural areas because of the higher températures and différences in surface emissivities. There are very few direct measurements of surface long-wave émissions, but thèse generally support the overall generalization.

Long-wave counter-radiation from a pollution haze above the city has long been quoted as an important factor in the genesis of heat islands. Récent observations in a number of mainly North American cities (Peterson, 1969; Bach and Patterson 1969; Terjung, 1970(a), 1970(6), 1971) hâve confirmed slightly greater long-wave counter-radiation in urban areas but Oke and Fuggle's detailed study in Montréal (1972) using an infra-red pyrometer mounted on a vehicle, showed that the six per cent increase in nocturnal counter-radiation over the city could be explained almost entirely as a conséquence of higher air températures.

Probald (1972), in a study of Budapest, also reported a very small (less than one per cent) higher long-wave counter-radiation in the urban area and Rouse and McCutcheon's (1972) study of Hamilton (Canada) recorded a similarly small increase in night-time counter-radiation, but this was much larger by day when it was of roughly the same size as the incoming short-wave solar radiation. What is more, the depletion in the latter by air pollution was almost exactly balanced by the increase in the former.

Net infra-red radiative cooling from elevated heavily polluted layers above cities may, however, over a period of a few hours and in the absence of advective and mixing effects help to produce elevated inversions (Atwater, 1971(a), 1971(6)). Thèse findings were apparently confirmed by Fuggle (1971) in his studies of radiative flux divergence over Montréal where radiative cooling rates were found to be almost three times as great as the measured nocturnal cooiing. Atwater's numerical simulation analysis does, however, contrast somewhat with the measurements of Oke and Fuggle; but the explanation may be Atwater's use of rather large values of pollution concentration and his simplifying assumptions about the effects due to various gaseous and particulate components having overlapping absorption and émission spectra. In a later paper, Atwater (1972) showed that infra-red radiative cooling by aérosols has relatively little effect upon the température profile of an atmospheric column which incorporâtes air motions.

To the présent, there hâve been no comprehensive field studies of ail the terms in the radiation balance, but some qualitative indication of the net balance can be gained by noting that the effect upon short-wave radiation receipts of the pollution haze (Atwater, 1972) and the albedo changes in urban areas are opposite in sign so that, knowing roughly the sizes of the terms, it is reasonable to deduce that différences in net short-wave receipts in urban and rural areas are likely to be quite small, confirming the findings of Probald (1972) in Budapest. There is a similar balancing by cities, as already noted, in their effect upon long-wave radiation exchanges. Both long-wave radiation from the surface and counter-radiation from the overlying air are increased, though the former change is likely to be greater than the latter (Oke and Fuggle, 1972) because of température différences between urban surfaces and the air. Net long-wave radiation is therefore likely to be greater from urban than from neighbouring rural areas; because of instrumental problems, however, which may be partly solved by the use of satellite sensors (Rao, 1972), there are no observations of this.

10 URBAN CLIMATOLOGY AND ITS RELEVANCE TO URBAN DESIGN

Net short-wave radiation is therefore likely to be much the same in urban as in rural areas, but net long-wave radiation is likely to be greater in cities in comparison with rural areas. Urban areas are therefore likely to hâve smaller net radiation gains by day, and larger losses by night. Such a resuit has obvious and important implications to the understanding of the physical processes operating to produce urban température excesses known as heat islands.

1.3.3.2 Anthropogenic heat production

Once again, there is a very limited literature, theoretical or observational, on the amount of heat libe-rated by combustion processes, air-conditioning units and metabolism in urban areas. And what there is, is entirely limited to North American and European cities (Jaske et al., 1970). In many of thèse cases, anthropogenic sources of heat are substantially greater in winter than the net radiation balance. In Sheffield (population 500 000), for instance, Garnett and Bach (1965) found that combustion processes produced a heat flux equal to 35 per cent of the net annual radiation and 20 per cent of the solar radiation receipt in 1952. Similar figures hâve been reported for Berlin and Vienna (Kratzer, 1956) and New York City (Bornstein, 1968).

In winter, in New York City, the amount of heat from combustion is more than twice that reaching the ground from the sun, but in summer the ratio drops to 1: 6. In Hamburg the heat from combustion has been estimated as 40 cal/cm2, compared with only 42 cal/cm2 from global solar radiation on a winter day (Kratzer, 1956). But there are clearly very substantial spatial variations in artificial heat production within cities which are, indeed, far from homogeneous in any of the heat-balance terms. Major industrial complexes, for instance, often constitute important heat sources (Oke and Hannell, 1970).

The relevance of thèse figures to the genesis of heat islands has to be considered in terms of diffusion theory (Craddock, 1965), Although, in winter, anthropogenic heat sources may be important in explaining the urban heat island, on an annual basis it may be of much less importance (East, 1971) as evidenced by the intense summer heat islands in many cities (Chandler, 1965).

Thus, in spite of the tremendous amount of heat released from the various combustion processes in a city and more particularly from low domestic chimneys of mid-latitude settlements in winter, most authors agrée that it is rapidly diffused and, in gênerai, not of major importance in most cities in explaining urban heat islands (Sundborg, 1950; Chandler, 1965; Craddock, 1965; Myrup, 1969).

1.3.3.3 Sensible heat transfer

Measurement of the sensible heat transfers within and from cities and comparisons with those in rural areas are made almost impossible by the complexity of the physics of active surfaces within the city and of the thermal and mechanical turbulence and other air circulation Systems which link the surfaces and effect the heat fluxes. Most of our quantitative information is therefore derived from mathematical and physical models of heat-exchange processes in urban areas (Terjung, 1970(a), 1970(6); Probald, 1972; Oke et a/., 1972(a), 1972(6)). The few observations we hâve suggest that the sensible heat transfer from cities is slightly larger than from rural areas by day. After sunset the sensible heat flux from cities does not décline as rapidly as from rural areas and clearly contributes to the warming of the overlying air and the nocturnal heat island commonly found above cities.

1.3.3.4 Heat storage

Because of the mass and nature of the urban fabric, cities possess a higher thermal capacity than rural areas. The enormous surface area of buildings also means a more rapid sensible heat flux between the buildings and their air envelope. As a resuit, the urban fabric is able to store large amounts of solar and anthropogenic

CLIMATOLOGICAL PROCESSES 11

heat which it then releases to heat the night-time air. By day, in contrast, the high thermal capacity of urban areas means that they warm more slowly than the surrounding vegetation-covered soils. The thermal lag thereby induced into the urban thermal system as compared with that in rural areas has been widely quoted as fundamental to the genesis of heat islands (Chandler 1965, 1967(6), 1970; Oke et al., 1972 (a)). The close correspondence found between local heat-island intensifies and close-by urban land uses and building densities, more especially on calm, clear nights, is added évidence of the importance of urban heat capacities and conductivities and of the heat-storage term in the génération of heat islands (Chandler, 1965; Clarke and Peterson, 1972).

1.3.3.5 Advected energy

Although the inverse relationship between wind-speed and heat-island intensity has received a good deal of statistical treatment (Chandler, 1965), few studies hâve been made of the advection of energy between cities and their surrounding countryside. Observational studies show, however, that country air normally invades the windward suburbs of cities, and city air, often warmer than that of the country, spreads beyond the leeward urban fringe into and above the surrounding country (Chandler, 1965; Clarke, 1969). Most analyses of heat islands hâve, in fact, been made at times of light or zéro advection when their intensifies are normally greatest.

1.3.3.6 Latent heat transfers

As emphasized in 1.3.2.3, we are presently very uncertain regarding the size of the terms involved in evaporation and surface condensation in urban areas. It seems likely that in gênerai, there is more evaporative heat loss from rural than from urban areas and more dew formation in rural than in urban districts (Nappo, 1972), but thèse différences are likely to be smaller than has sometimes been assumed (Oke, 1974). More hydro-logical and meteorological studies are needed to résolve thèse uncertainties.

1.3.4 AIR POLLUTION: EMISSION AND ABSTRACTION

Man has always treated the atmosphère as a freely available open sewer, commonly loading it beyond its natural self-cleansing capacities. But in recognizing this important and émotive circumstance, it is important to remember that man, and much less urbanized man, is not solely responsible for the contamination of his atmospheric environment. In the sparsely settled parts of the globe, most air pollutants are the products of natural processes, comprising such substances as pollen, minerai dust, and the smoke from forest and bush lires. Even so, the concentrations of Aitken nuclei (having a radius of less than 0.2 micromètre) in cities are, on average, sixteen times greater than over inland rural areas and 160 times greater than over the océans (Landsberg, 1938). The increasing pace of urbanization serves to highlight the importance of the high concentrations of pollutants in urban areas. Air pollutants embrace an enormous number and variety of substances. One American hand-book (Sherry et al., 1969) lists 260 potential atmospheric contaminants. It must also be remembered that ail or nearly ail pollutants (the exceptions are mainly rather specialized radioactive substances) exist naturally in the atmosphère. Man's rôle has been to increase concentrations locally or through time.

Emissions, transport and abstraction processes are complex and the subject of intensive and urgent study in most countries by a variety of scientific disciplines. The results are of obvious importance in urban design (see Chapter 3).

1.3.4.1 Nature of pollutants

The pollutants of greatest concern in urban areas are those released as waste products from combustion processes of one sort or another. Among those which hâve attracted the greatest attention are the following:


participâtes including various metals, oxides of sulphur, carbon monoxide, carbon dioxide, oxides of nitrogen and hydrocarbons. Thèse are emitted from a variety of sources in urban areas including tall stacks, industrial "process" émissions, and low-level area and mobile sources. Released in differing amounts and at various speeds and températures from a multiplicity of fixed and moving sources, it is extremely difficult to compile detailed inventories of the patterns of émission in urban areas. Add to this the complexity of the meteorological controls upon diffusion, chemical changes and abstraction rates, and one begins to understand the limitations which exist upon présent knowledge, though the enormity of the national, régional and international scientinc effort within the field of air pollution in récent years has undoubtedly greatly improved our understanding of the problem.

In 1969, estimâtes were made of the contribution of various sources to the total weight of air-pollutant émissions in the United States: fuel consumption from stationary sources contributed 16 per cent of the total; transportation, 51 per cent; industrial processes, 14 per cent; garbage incinération, 4 per cent and other sources, 14 per cent (U.S. Council on Environmental Quality, 1971) *. Différent countries hâve developed différent attitudes to each of the différent pollutants and set varying priorities for their émission control. In most European countries, smoke and sulphur oxides hâve been accorded first priority (O.E.C.D., 1972); in the United States, sulphur oxides were rated only eighth in order of priority, oxides of nitrogen and hydrocarbons coming first and second, respectively. Low-latitude cities, with very différent combustion patterns, would presumably generate différent air-quality priorities again.

Air pollution is not of course a uniquely urban problem although the greatest mass of pollutants is generated in towns. Thèse pollutants are then carried distances varying from a few mètres to thousands of kilomètres before being returned to the Earth's surface, depending upon the height and other physical properties of émission, the character of airflow and températures in the atmospheric boundary layer, précipitation and the aerodynamic and chemical properties of the pollutants (WMO, 1972). Though cities are clearly the most polluted areas, nowhere entirely escapes the problem.

TABLE II

Typical concentrations of air pollution in United States towns, 1969/1970

Size oftown

Less than 10,000 10,000 25,000 50,000

100,000 400,000 700,000

1,000,000 3,000,000 Rural

Particulates (mg m - 3 )

57 81 87

118 95

100 101 134 120 25

so2

35 18 14 29 26 28 29 69 85 10

N02

116 64 63

127 114 127 146 163 153 33

(Source: The Mitre Corp., MTR-6013; cited in Berry and Horton, 1974)

Table II shows that the larger the city, the higher in gênerai are the concentrations of the main pollutants although the relationship between concentration and city size is by no means simple or constant. Much dépends upon the précise location of the monitor(s) in relation to sources of émission. The gradients of concentra-

CLIMATOLOGICÀL PROCESSES 13

tion away, for instance, from individual stacks or main highways are often steep (see Chapter 2) and for this reason the records of individual or small numbers of monitors (as used in the compilation of several of the averages quoted in Table II) must be treated with caution.

1.3.4.2 Urban diffusion modeîs

The complexées and uncertainties of émission patterns and source strengths in cities, allied to the intricacies of the meteorological controls upon vertical and latéral diffusion, again inadequately monitored and imperfectly understood, hâve mitigated against anything but the simplest diffusion models for urban areas. Not only do weneed to improve ourknowledge of diffusion processes and coefficients in gênerai, and more partic-ularly as they occur in urban areas (Pasquill, 1970(6)), but it is also important to clarify the détails of chemical reactions and removal processes such as impact, fallout, washout and rainout, which are presently understood only partially, even in level rural "green field" situations. Much dépends, however, upon the émission charac-teristics: in particular the height, speed and température of the plume as it leaves the stack. For this reason, and upon thèse bases, urban émissions can be divided into a number of types:

Individual, tall-stack sources

Many modem industries, such as power stations, count as discrète, point sources for enormous quanti-ties of pollution. For thèse a source-orientated model, of which there are several, (WMO, 1972) is appropriate, provided the urban effects can be incorporated. The source-orientated model is partic-ularly useful, in theory at least, for forecasting the effect of potential new sources in a city. Unfortun-ately, the relatively simple models that can be used in level rural environments, many of them based upon Sutton's (1947) pioneer work, are not easily adapted to the very différent and more complex meteorological conditions found in urban areas. If thèse complexities are incorporated into the model (Moore, 1975), then very considérable assumptions hâve to be made about source strength, plume rise, the mixing depth of the layer into which the pollution is diffused and the effect of cross and vertical components of winds upon the behaviour of the plumes.

In adapting point-source, non-urban models to the urban situation, new complexities and uncertainties are added to already intricate controls and thèse further diversify the temporal and spatial patterns of pollution concentrations near the ground. In particular, the aerodynamically rougher surface of the city and the urban heat island both change the behaviour of plumes. Much will dépend upon the stabil-ity of the boundary layer (Hanna, 1971; Gifford and Hanna, 1970; CONCAWE, 1966; WMO, 1970, 1972) and time- and space-dependent patterns of airflow affecting diffusion and removal processes.

Area sources

A multitude of individually small, low-level residential and industrial sources of pollution often constitue the main source of near-surface concentrations of pollution in urban areas, but inventories of such émissions are rarely available and the patterns of diffusion in the immédiate vicinity of buildings are exceedingly complex (Pasquill, 1970(Z>)). Many attempts hâve been made to produce simplified mathematical models (WMO, 1972; Moore, 1975; Duprey, 1968) andphysical models (Robins, 1975). Différences of approach hâve focused upon the means of integrating the individual émissions and the sélection and relative rôle of the meteorological parameters. Thereafter, several attempts hâve been made to integrate the diffusion équations directly and systematically across the urban area (Lamb and Neiburger, 1971; Marsh and Withers, 1969; United States Environmental Protection Agency, 1970). But, given the inévitable complexities and uncertainties in the émission, diffusion and removal terms of any area source model, several attempts hâve been made to simplify the spatial terms in the équations through the use of the so-called "box-model" équations for heat, moisture, momentum and pollutants (Reiquam, 1970).

14 URBAN CLIMATOLOGY AND ITS RELBVANCE TO URBAN DESIGN

Lettau, (1970) used the "box-model" approach to dérive what he termed the "flushing-frequency" of an urban area, being the air replacement frequency per unit time. Given a "box" of width w and height h, a constant wind speed u, concentration of pollution a function of height only, the eddy flux of conservation parameters out of the box at any height being zéro, the "flushing frequency", / , is given by:

f=ufw

This emphasizes the importance of high wind speeds in ventilating the city and reducing pollution, and of city size in increasing concentrations.

Isolated Une sources

The most obvious and important example of a line source is a highway carrying heavy and fairly contin-uous traffic. In cities, latéral diffusion is often severely constricted, except along streets and through open spaces flanking the highway. The vertical co-ordinate is then the only one left to provide dilution.

Apart from the simple monitoring of représentative concentrations of pollution from motor vehicles (see Chapter 2), there hâve been few attempts to model the diffusion from line sources (Johnson et a/., 1969).

In récent years, what might be called the traditional pollutants of smoke and sulphur dioxide from the burning of coal and heavy (sulphur-rich) oil hâve been joined in almost ail cities by a great variety of pollutants from the carburettors, petrol tanks and exhausts of the vehicles which increasingly congest the streets of most modem cities. Thèse include various oxides of nitrogen, polycyclic hydrocarbons (including 3:4 benz-pyrene), carbon monoxide and inorganic lead, lead bromides and chlorides. In cities such as Los Angeles, Mexico City, and Perth, having strong radiation receipts, there are complex photo-chemical reactions upon the enormous quantities of thèse gases released into the air, and one of the products is ozone which, contrary to widespread popular opinion, is toxic, not tonic. In cities such as Paris, Chicago, and London, having stronger winds and street ventilation and weaker sun-shine, such photo-chemical reactions are less important, but even hère, in the bottom of deep street chasms congested by slow moving traffic, concentrations of carbon monoxide can rëach 200 p.p.m. for short periods (Reed, 1966). The diesel engine produces virtually no carbon monoxide, although when overloaded or badly maintained, it does generate clouds of malodorous black smoke.

CHAPTER 2

OBSERVATIONAL STUDIES

2.1 Introduction

In this section, an attempt is made to abstract from the dozens of particular studies in individual cities those features of the temporal and spatial distribution of meteorological éléments which are common to most towns.

The séries of studies should provide sufficient broad information for many purposes or, more likely, guidelines for a new monitoring network whereby to establish the main framework of the existing or developing urban climate in a particular city by either a network of fixed climatological stations or a programme of traversing using instrumentée! vehicles.

Each élément is considered in turn.

2.2 Airflow

As explained in Chapter 1, airflow in urban areas differs in nearly ail respects from that above the aero-dynamically smoother and generally cooler surfaces of rural areas. Because of thèse différences a number of fundamental changes ensue: the shape of the mean horizontal wind profile in the boundary layer above urban areas is altered; wind speeds and directions are changed in space and time and the turbulence spectrum is quite différent. And because the wind responds in such an intimate way to the detailed geometry and thermal distributions of urban areas, each city's pattern of airflow is, to a large degree, unique.

In conséquence of the complexities and uniqueness, it is extremely difficult to find anything worthy of the term "représentative site" for urban wind observations, and the extrapolation of measurements in one city to conditions in another has to be done with care, remembering the individual complexities. But although the variations from one place and one time to another may be considérable, this does not mean that generaliza-tions hâve no value and that models of airflow hâve no rôle in building and urban design. Provided the size of the variance is realized, generalizations or models of atmospheric behaviour in cities hâve an important part to play in proper urban design, just as they do in many other fields of environmental science.

Page (1964) recognized three layers of urban airflow:

(a) A surface boundary layer close to the ground, where the highly turbulent airflow is controlled largely by the aerodynamic shape of nearby buildings ;

(b) A somewhat less turbulent planetary boundary layer, which extends from roof level to the level of the gradient wind which is more-or-less free from the influence of surface friction. The depth of this layer increases as air flows over the city;

(c) Surface-friction-free, streamline flow above the boundary layer.


2.2.1 WlND PROHLE

Wind speeds in the lower 40 m or so of the atmosphère, known as the surface boundary layer, are extremely turbulent, but the wind profile in a neutrally stratified atmosphère over a uniform, extensive surface (the latter conditions being unlikely to be properly met in an urban area) is normally assumed to approximate to a logarithmic profile:

u z ^ = k XoKQ

where ûg is the mean horizontal wind speed at height z ; u is the friction velocity ; k is von Karman's constant and z0 is the roughness length. But in addition to the other limitations upon the use of this formula in urban areas, Munn (1970) has pointed out that it assumes a balance between the production and dissipation of turbulent kinetic energy which is again unlikely in a built-up area.

Above about 40 m, the logarithmic law no longer seems to represent conditions fairly, even in rural areas, and the increase of speeds from this level to the top of the boundary layer is more accurately given by a power relationship:

%='ui\i where « is the mean horizontal wind speed at height zx and a is an empirical power exponent. Panofsky (1971), quoting a relation suggested by Blackadar (1962), and Davenport (1965) hâve both proposed other formulae, but neither seems to offer any greater practical accuracy or ease of use. The power-law relationship seems to be as valid and easily used as any other and is the one most commonly used in calculating winds at a séries of heights in the atmospheric boundary layer. Shellard (1965) has however suggested that heights should be calculated not from the ground, but from a level approximating to gênerai roof height in the surrounding area, a principle known as zero-plane displacement. Below this height, that is in the streets, squares and courtyards or urban areas, winds are highly turbulent and are often organized in eddies (see p. 4) so that the wind profile has little meaning.

Investigations in a number of cities hâve yielded a range of values for the exponent a in the power-law relationship. Thèse are listed in Table III.

TABLE m

Estimâtes of power-law exponents in différent cities

City

Paris Leningrad New York City Copenbagen London (U.K.) London (Ontario) Kiev Tokyo Tokyo

Upper îimit of investigation

(mètres)

305 149 381 73

183 40

179 61

250

Exponent

0.45 0.41 0.39 0.38 0.36 0.36 0.35 0.34 0.33

Référence

Eiffel (1900) Ariel and Kliuchnikova (1960) Rathbun (1940) Jensen (1958) Shellard (1967) Davenport (1967) Ariel and Kliuchnikova (1960) Shiotani and Yamamoto (1950) Soma (1964)

OBSERVATIONAL STUDIES 17

Thus of ail the formulae proposed for modelling the increase in mean horizontal wind speed with height above an urban area, the power law seems to give as reasonable and certainly as easily obtainable results as others, though both Pasquill (1970(a)) and Helliwell (1973) hâve noted the difficulties in deriving reasonable values for the zéro plane displacement.

Davenport (1965) has shown that the power-law relationship gives, height for height in the boundary layer measured above the ground, lower wind speeds over a city than above rural surfaces and lower speeds hère than above a low friction sea surface. The rate of increase of wind velocity with height (wind shear) is however greatest above the city. With an exponent of 0.40, winds at 170 m above ground for instance, are equal to 90 per cent of the gradient wind (blowing at the top of the boundary layer) over the sea, 75 per cent of the gradient wind over rural areas, but only 60 per cent of the gradient wind over a city (Figure 2).

The velocity profile must of course take some time to adjust to changes in surface roughness conditions. Taylor (1962) has estimated that for winds blowing from rural to urban areas, the profile does not become adjusted to the increased surface roughness for 1.2 km at a height of 30 m and 8 km at a height of 100 m down-wind of the rural/urban boundary. With winds blowing from urban to rural areas, the distances are 3.2 km and 14 km respectively so that it appears to take twice the distance for adjustment in winds blowing out of an urban area as for those blowing in.

There are several important implications. First, the "surface of adjustment" defined as the top of the adjusted layer, slopes upward from the rural/urban boundary with a slope which is initially about 1: 32 and then shallows to about 1: 80, which roughly equals the slope of the deepening boundary layer as a whole (Jones et al., 1971). Secondly, the adjustment distance at 100 m for winds blowing from country to city is greater than the width of many towns which means thatin thèse cases, winds will never achieve the true urban wind speed for this height. Thèse features hâve obvious implications for the design of tall buildings in urban areas.

The depth of the boundary layer also responds to the degree of surface roughness and is much deeper over urban than over rural areas. Représentative depths are as follows :

Surface

Fiat, open country Woodland Towns and city suburbs City centres

Depth ofboundary layer

(mètres)

270 400 400 420

(After Davenport, 1965)

What is interesting and, indeed, highly relevant to building engineering, is the question of how rapidly the boundary layer adjusts itself to changes in surface roughness. How quickly does it deepen when crossing the rural/urban boundary and how rapidly does it shallow again when leaving the lee side of the city? There is, unfortunately, only limited theoretical and observational évidence by which to answer thèse questions. Panofsky and Townsend's (1964) theoretical study of airflow across a marked change in surface roughness provides some guide but its value to the urban situation is limited by the failure of the model to include a température as well as roughness change at the boundary.

It has been suggested by Kratzer (1956) that on nights with a well-developed température inversion at the top of the atmospheric boundary layer, the airflow aloft might become "decoupled" (Munn, 1970) from that below and, rising over the generally domed upper surface of the heat island and boundary layer over the

(S) Ui

oc

2

600 -

500 -

400 -

300 -

200 -

100 -

A

GRADIENT WIND-100

.040

rGRADIENT WIND —100

Ml

r-GRADIENT WIND —100

Uocz

- 1 5 0 0

- 2 0 0 0

LU U.

- 1 0 0 0 N

- 500

(Afier Davenport 1965)

> Z P

3 o 3

i > z o ta

3

> Z 0

§

Figure 2 — Diagrammatic mean horizontal wind velocity profiles above urban, rural and sea surfaces respectively. In each case the mean wind speed u is related to the height z using représentative power-law indices. The numbers are percentages of the gradient wind speed at the top of the boundary layer

0BSERVÀ1Ï0NAL STUDIES 19

city, the winds might accelerate as they do over topographie obstructions in their path. It is therefore interesting to note that in a study of winds above Minneapolis, U.S.A., in summer (when heat islands would be strongest), Deland and Binkowski (1966) found cases of supergeostrophic winds at heights of 150 m above the city.

Because of différences in the depth and intensity of turbulence and in the consequential vertical transport of momentum, winds near the ground tend to be strongest by day and weakest by night whilst, at higher levels, the reverse is generally true. Such différences are likely to be important over the height range of many modem buildings, for in Leningrad and Kiev (Ariel and Kliuchnikova, 1960) the reversai of the diurnal wind régime was found to occur at heights of 100-150 m and 50-80 m respectively, and in Philadelphia at 171 m.

2.2.2 WIND SPEED

Because of increased surface friction, average wind speeds are reduced in cities in comparison with those in country areas (Landsberg, 1962; Steinhauser et ah, 1959; Frederick, 1964; Chandler, 1965; Munn and Stewart, 1967; Munn, 1970; Graham, 1968). In autumn, winter and spring, for instance, when régional winds tend to be strong, speeds in central London are reduced by eight, six and eight per cent respectively whilst in summer, with lighter winds, there is little or no différence in mean urban and rural speeds. The overall annual réduction in wind speed in central London, for ail winds, is six per cent, but for winds of more than 1.5 m s-1, the réduction is 13 per cent (Chandler, 1965).

But although average wind speeds are less in urban than in nearly rural areas, on individual occasions the relationship may be reversed. Chandler (1965), analysing London data, and Borastein (1969; 1972), working on measurements in and around New York, both showed quite independently that the rural/urban wind-speed différence was a function of the régional near-surface wind speed and wind profile. More significantly, when winds were light, near-surface speeds were greater in the built-up areas than outside, whereas the reverse relationship existed when winds were strong. The critical threshold speed at which the sign of the urban/rural speed différence was reversed in the London case was 3 to 5 m s"1 as recorded at London Airport on the western fringe of the city. Bornstein et al. (1972) recorded a threshold of 4 m s-1. Chandler (1965) found the relationship between speeds at London Airport and Kingsway in central London to dépend on season, time of day and régional wind speed (see Table IV).

TABLE IV

Average wind speed at London Airport (on the western fringe of the city) and its excess above that at Kingsway (in central London) in m s-1

Season

December-February March-May June-August September-November Year

0700 GMT

Mean speed

2.5 2.2 2.0 2.1 2.2

Excess speed

- 0 . 4 - 0 . 1 - 0 . 6 - 0 . 2 - 0 . 3

1300 GMT

Mean speed

3.1 3.1 2.7 2.8 2.9

Excess speed

0.4 1.2 0.7 0.6 0.7

The accélération of speeds in central London was mainly a night-time phenomenon, brought about by the strong downward transport of momentum induced by mechanical turbulence above the city, which at times of light winds and a sharp wind speed profile (fairly common night-time occurrences) more than compensâtes for increased surface friction by the buildings.


Bornstein et al. (1972) showed that, in winds of less than 4 m s-1, there was a 20 per cent increase in speeds over the city, the greatest increases occurring in winds of less than 1.3 m s-1.

Wind accélérations hâve also been monitored by Angell et al. (1971 (a)) using tetroon flights above Columbus, Ohio. They found the size of the accélération to be dépendent upon the strength of the nocturnal inversion and registered décélération downwind of the city centre, as had Bornstein et al. (1972) in New York. Lee (1975(a)), on the other hand, was unable to find convincing proof of this suburban décélération in his studies in London.

2.2.3 TURBULENCE

The spatial and temporal variations in wind speed and direction are much stronger in city than county areas but very few detailed studies hâve been made of turbulence in urban areas and the results in any case would be very particular to the detailed surface geometry of individual cities (Marsh et al., 1967, 1969).

One of the few studies available is that by Graham (1968) of Fort Wayne, Indiana. He showed that above this city, set in an almost perfectly flat, featureless plain, the intensity of turbulence, as measured by the standard déviations of the azimuth angle of the wind, was closely related to the density and form of development. The pattern of turbulence was concentric to the city centre where standard déviations were more than twice as large as in the surrounding country.

Observations taken on tall meteorological observation towers show gustiness to decrease fairly rapidly with height. But the increase of "instantaneous" gust speeds with height is slower than the increase in mean velocities, so that whilst gust speeds in the surface boundary layer are generally in excess of mean wind speeds, at higher levels they are more similar.

2.2.4 WIND DIRECTION

In the lowest part of the boundary layer, between and immediately above the buildings of a city, airflow is very turbulent and wind direction is variable in both space and time. The surface orography upon which the city is built also exercises a unique control in each case. But in spite of thèse complexities there are patteras to changes in the three-dimensional airflow in urban areas which are sufficiently consistent from one city to another as to constitute a real urban model.

Above the turbulent eddies which characterize airflow around buildings, winds blow at an appréciable angle to the isobars because of the powerful surface friction. Measurements of this angle in a number of cities vary from 15° at times of strong instability to more than 40° during température inversions (Ariel and Kliuch-nikova, 1960). Several investigators (Hass et al., 1967; Angell et al., 1968, 1971(a), 1971(6), 1972; Ackerman, 1972, 1974(a), 1974(6); Fosberg et al., 1972; Johnson and Bornstein, 1974) hâve shown the tendency for anti-cyclonic turning of the streamlines of airflow in the lowest levels of the air over cities by day, followed by a cyclo-nic recovery downwind, and explained this as a conséquence of frictional turning, vertical mixing and a meso-scale high-pressure distribution at the top of the heat dôme. Johnson and Bornstein (1974) found évidence of cyclonic curvature over New York in strong winds and anticyclonic in light.

At times of strong stability and a marked heat island, however, the heat dôme over a city may act as an obstacle diverting airflow around either side and over the top, with the consequential accélérations hère (Munn, 1970; Deland and Binkowski, 1966).

Many tetroon flights over American cities hâve traced strong, rising and sinking air currents over built-up areas by day. Over New York City for instance, a mid-afternoon flight showed a rise of over 800 m above


the warm, physical obstacle of Manhattan with almost vertical sinking again over East River (Hass et al., 1967). Studies by Angell et al. (1971(û)) showed that the strength of the upcurrents and down draughtsover Colurabus, Ohio, were very dépendent upon the stability of the urban air.

2.2.5 LOCAL AIRFLOWS

At times of calm or very weak régional winds, urban heat island s will set up a country-city airflow System in many ways similar to the sea-breeze. Studies in London (Chandler, 1960, 1965), Leicester (Chandler, 1961), Frankfurt (Georgii, 1970), Bonn (Emonds, 1954), Asahikawa, Japan (Okita, 1960), Louisville (Pooler, 1963), New York (Davidson, 1967) hâve ail provided field évidence for the surface link in this type of airflow. This consists of a cool inflow of a few mètres per second towards the centre(s) of warm air. Thèse shallow centri-petal flows are rapidly decelerated by surface friction in the suburban fringe and, for this reason, they rarely pene-trate more than a few kilomètres into the city. Hère they rise, probably in a rather diffuse zone of uplift, to join an outblowing, centrifugal flow from city to country again at a higher level. The cellular system is then completed by sinking over the country areas around the city (Spangler and Dirks, 1972).

The surface inflows are pulsating in character, for in blowing across the urban margins they erode the shape thermal gradients of the heat island which were their cause. They therefore die down until a new cliff-like edge to the heat island is built up which once more générâtes an antitryptic, thermal country-wind across it.

Thèse country breezes, blowing across the margins of the heat island, help to sharpen up the edge of the urban pollution dôme at times of calm or light winds.

2.3 Humidities

There hâve been relatively few comparative studies of the moisture content of urban and rural atmosphères and what observational évidence there is, is not completely conclusive.

Much of the urban surface is sealed by concrète, asphalt and other impervious and semi-impervious materials, although the fairly large amount of open ground that still remains in most cities has sometimes been overlooked. Certainly a lot of rainwater is led quickly underground but the amount of moisture absorbed by bricks and tiles has frequently been underestimated. Nevertheless, in spite of ail thèse qualifications, there is no doubt that evaporation rates in cities are substantially less than from vegetation-covered soils. On the other hand, amounts of dewfall are almost certainly greater in rural areas and the added moisture from combustion is greater in the city.

Contrasts in evaporation rates are likely to manifest themselves in différences in absolute humidity only when the atmosphère is calm. For this reason, average annual or monthly vapour-pressure contrasts between urban and rural areas are likely to be very small (Ludwig and Kealoha, 1968; Ackerman, 1971; Auer and Dirks, 1974; Kopec, 1973; Goldreich, 1974; Bornstein and Tarn, Yam-Tong, 1975). Chandler (1965), for instance, found that the mean annual vapour pressure in London was only 0.2 mb lower, on average, than at a nearby rural climatological station, and Kratzer (1956) reports an urban/rural déficit of 0.2 to 0.5 mm. On calm nights, however, the moisture content of the air above London and above another English town, Leicester (pop. 270 000), was highest in the centre of the city (Figure 3), being 1.5 to 2.0 mb higher there than above the surrounding country. This was explained in terms of the low rate of diffusion of air "trapped" between tall buildings so that it retained its characteristically high day-time humidities. Similar day-time results were reported by Ludwig and Kealoha (1968) for Dallas, Texas.


VAPOUR PRESSURE (mb) 2315 G.M.T.

19th August 1966 M i l e

(After Chandler, 1967a)

Figure 3 — Vapour pressure (in millibars) in Leicester, England, at 23.45 G.M.T. on 23 August 1966. This was a clear night except for a one-eighth cover of stratocumulus and there was little or no wind. The built-up districts are clearly more humid than the surrounding rural areas


Relative humidities, being a function of prevailing températures, are normally found to be inversely related in towns to the local intensity of the urban heat island and average annual urban/rural différences of about five per cent hâve been widely reported (Kratzer, 1956; Chandler, 1965, 1967(a); Sasakura, 1965). On individual calm, clear nights the patterns of relative humidities in cities are closely related to the detailed form of their heat islands which in turn dépend upon the distribution of urban building densities (Chandler, 1967(6)). Relative humidity différences between town and country can, on thèse occasions, amount to between 20 and 30 per cent.

Few studies hâve been made of the vertical gradients of humidities over urban areas, one of the small number being that by Bornstein, Lorenz and Johnson (1972) who reported a night-time excess of absolute humidity of about four per cent to heights of 500-700 m above the built-up area of New York City, the greatest différences occurring during the early morning when vertical mixing was strong.

2.4 Précipitation

It has been said that in contrast to analyses of (other) near-surface meteorological phenomena, much of the literature on urban précipitation is primarily concerned with establishing whether or not the urban area has any effect at ail on the précipitation distribution (Atkinson, 1970). It seems at first sight plausible to postulate that because of increased pollution with active condensation and freezing nuclei (Schaefer, 1968; Squires, 1966) as well as more active thermal and mechanical turbulence, and the water vapour from combustion processes, there will not only be increased cloud but also more précipitation above cities. Landsberg (1956, 1962), Huff and Changnon (1972), Changnon (1968, 1969, 1970, 1971, 1972) and Changnon et al (1971 and 1972) hâve summarized the théories and field évidence for a number of cities in Europe and the United States, while Atkinson (1968, 1969, 1970, 1971, 1975) has produced a number of detailed synoptic analyses of storms crossing London.

Simple comparisons of précipitation inside and outside cities prove very Iittle, because of the well-known temporal and spatial fickleness of the élément, the relatively poor sampling inhérent in normal rain-gauge measurements, the drift of city pollutants into rural areas and the important orographie controls upon précipitation exercised by the complex underlying terrain of most cities. Also, many récent studies hâve empha-sized the time élément in précipitation processes and hâve drawn attention to the common displacement of positive anomalies downwind of city centres. Changnon (1972), for instance, demonstrates the major enhancement of summer rainfall in the St. Louis région to occur downwind of the city where it is most likely affected by the urban heat plume. Whether there is enhancement or not also clearly dépends on the synoptic situation (Landsberg, 1962), the greatest enhancement over St. Louis occurring in air masses free from synoptic disturbances.

Apart from the highly individual meteorological circumstances behind the famous La Porte anomaly, east of Chicago-Gary (Changnon, 1968; Holzmann and Thom, 1970; Ogden, 1969; Harman and Elton, 1971), most increases of précipitation over urban areas in the United States average between five and eight per cent in annual total s (being greater in the cold than the warm season) and between 17 and 21 per cent in the number of summer thunderstorms. Similar increases hâve been reported for Munich (Schmauss, 1927), Nuremberg (Kratzer, 1956), Budapest (Berkes, 1947), London (Atkinson, 1968, 1969) and Bombay (Khemani and Murty, 1973).

Cities are unlikely to make very substantial différences to the frequency of snowfall, for snow falls at times of the year and in meteorological conditions generally inhibitive to strong heat islands which would other-wise act as both a trigger to instability and a means of melting the flakes. Nevertheless, Landsberg (1956) cites a few instances where snowfall over an urban area was lighter than at nearby rural locations and Potter (1961) obtained similar results for Montréal and Toronto. But in the absence of orographie controls, fallen snow will melt more quickly in central urban streets and parks than in suburban gardens where it will often disappear several days before that covering farmlands around the city.

2 4 URBAN CLIMATOLOGY AND ITS RELEVANCE TO URBAN DESIGN

Because of the higher températures and decreased stability in air over cities, hail occurrence as well as heavy rainfall is likely to be enhanced, though the only concrète évidence for this cornes from the very spécial La Porte situation (Changnon, 1968).

2.5 Hydrology of cities

Quite apart from any effect cities may hâve upon précipitation, Leopold (1968) has identified three main physical effects of urban land use upon hydrology. Thèse are: changes in total runoff, changes in peak flow characteristics, and changes in water quality.

The two principal controls upon runoff and régime are the percentage of impervious surfaces and the rate at which water is carried across the land. Urban areas will obviously affect both variables. Not ail urban fabrics are impervious, of course; indeed, many, such as bricks and tiles, are highly absorbent. Also, the percentage of the total urban area which is built upon is very variable, both within and between cities (Lull and Soper, 1969; Antoine, 1964; Bach, 1972; Crippen and Waananen, 1969) and over time (Harris and Rantz, 1964). But in gênerai, runoff from urban areas can be expected to be greater than from orographically similar rural areas (Brater and Sangal, 1969; Carter, 1961; Anderson, 1967; Espey et al., 1966; Kinosita and Sonda, 1969; Martens, 1966; Mills, 1968; Reagan et al, 1971; Sawyer, 1963; Skelton, 1972; Saule, 1971; Thorpe, 1973; Unesco/FAO, 1973; Waananen, 1969; Wilson, 1966; Wiitala, 1961.

Results in the many independent inquiries, mostly in the United States, vary of course from one to another, but they range, in gênerai, from a 50 per cent increase in mean annual flood discharge for a one square mile area which is 20 per cent impervious and is 20 per cent covered by storm sewerage, to a 180 per cent increase with 50 per cent impervious and 50 per cent coverage by storm sewerage, and a 400 per cent increase with 80 per cent impervious and 80 per cent coverage by storm sewerage. For unsewered areas, the différence between 0 and 100 per cent impervious will increase peak discharge on average 2.5 times; for areas that are 100 per cent sewered, the ratio of discharge after urbanization to that before will rise from about 1.7 with zéro impervious, to roughly 7.0 with 100 per cent impervious (Berry and Horton, 1974).

But as runoff is increased by urbanization so, for the same reasons, soil- and groundwater storage and replenishment are reduced. In their turn, thèse will resuit in decreased low flows (Miller, 1966). Urbanization therefore has the effect of intensifying the extrêmes of flow (Hammer, 1973). This means that the récurrence interval in years of a given discharge is also sharply reduced by urbanization (Berry and Horton, 1974; Hollis, 1975).

The influence of urbanization upon the chemical composition of surface and groundwater has attracted much attention in récent years as one of the most serious aspects of environmental pollution.

The typical city dweller uses daily 150 gallons of water, four pounds of food and 19 pounds of fossil fuels (Berry and Horton, 1974). This is eventually converted into roughly 120 gallons of sewerage (assuming an 80 per cent recovery of the water input) and four pounds of solid waste. Though only occasionally a problem in the developed world, water-borne diseases in polluted water supplies are among the main causes of morbidity and mortality in cities of the underdeveloped countries.

Water pollution per se is not within the declared ambit of this Note but one aspect of water quality is worth mentioning. This is pollution accruing from so-called scrubbing and other processes designed to remove particulate and gaseous pollutants from chimney plumes, thus converting an air- to a water-pollution problem. The net benefit from such a change has to be judged separately in each individual case. What is important is that it is not ignored.


2.6 Radiation and sunshine

Because of the blanket of pollution which, to a greater or lesser extent, shrouds ail urban areas, radiation receipts at the ground are reduced, often significantly, more especially at times of low solar élévation. It is likely that most of the atténuation is by absorption, since most scattered radiation will be directed forwards (Sheppard, 1958). Roach (1961) estimated that over heavily polluted areas absorption was sufficient to cause température rises of 10°C per day. For this reason the urban effect is particularly noticeable in high-latitude cities during the early morning and late evening in winter.

An atténuation of solar radiation by between 10 and 30 per cent has been reported from many cities (Hand, 1943; Steinhauser et al., 1955; Sheppard, 1958; Monteith, 1962; deBoer, 1966; McCormickand Kurfis, 1966; East, 1968; Lettau and Lettau, 1969; Terjung, 1969; Rouse and McCutcheon, 1972). De Boer (1966), using two years of global solar radiation measurements at six stations in and around Rotterdam, showed that the centre of the city received three to six per cent less radiation than the suburbs and 13 to 17 per cent less than the country. From November to March, and before the réductions in air turbidity following the U.K. Clean Air Acts, many smoky British cities received between 25 and 55 per cent less radiation than nearby rural areas (Chandler, 1965). In central London the loss of sunshine amounted to about 270 hours per year, there being a réduction of more than 50 per cent in December (Chandler, 1965). But with the more récent improvements in air quality, winter sunshine receipts in London hâve increased by 50 per cent, being as much as 70 per cent in January (Jenkins, 1969). On a shorter time scale, investigations in many cities hâve shown a weekly cycle of radiation with higher amounts at weekends when industrial émissions of pollution are generally less (Chandler, 1965; McCormick and Kurfis, 1966). Mateer (1961) showed that in Toronto, Canada, solar radiation receipts were 2.8 per cent greater on Sundays than the average for other days. Furthermore, from October to April the Sunday excess was 6.0 per cent but during the remaining months it was only 0.8 per cent.

It is also important to note that the atténuation of radiation by pollution varies significantly with wavelength. Maurain (1947) reported a 100 per cent decrease in the ultra-violet wavelengths between the centre and outskirts of Paris. In Leicester (England), Meetham (1945) demonstrated that ultra-violet radiation in the range 0.314 to 0.355 microns was reduced by 30 per cent in winter but by only six per cent in summer. Clear-day atténuation of ultra-violet radiation between Mt. Wilson (5350 ft) and central Los Angeles has been measured at 14 per cent on clear days, rising to 58 per cent on smoggy days (Nader, 1967) and 90 per cent or more under extrême conditions (Stair, 1966).

In the infra-red wavebands, Roach (1961) found that up to 35 per cent of solar radiation over southern England was absorbed by pollution in the lower 1000 ft of atmosphère.

There hâve been very few comparative studies of urban and rural long-wave radiation amounts, but an eight per cent higher day-time value has been recorded over Cincinnati (Bach and Patterson, 1969), a six per cent higher nocturnal value over Montréal (Oke and Fuggle, 1972) and a six per cent increase over Johannesburg (Fuggle, 1972).

As with other éléments, only a small number of studies hâve been made of the variation of radiation with height over urban areas. One of the few was of Cincinnati by McCormick and Baulch (1962) and McCormick and Kurfis (1966), who used aircraft measurements to identify a layered structure of température and pollution over the city which significantly reduced the amount of radiation reaching the surface.

2.7 Température

Within the field of urban climatology, more attention has been paid to the study of températures within cities than to any other meteorological élément with the possible exception of pollution. Oke (1974) has listed most of the récent studies.


The warm air which, more frequently than not, and particularly by night, covers built-up areas is known as a heat island. Thèse hâve been charted in a large number of cities, most of them in mid-latitudes, and the extensive literature on urban heat islands has been documented and summarized in several publications, more especially by Kratzer, 1956; Landsberg, 1956, 1962; Chandler, 1968; Peterson, 1969; Bryson and Ross, 1972; Tyson et al., 1973; Munn, 1973; and Oke, 1974.

2.7.1 HORIZONTAL TEMPéRATURE FIELD

Urban/rural température différences stem from the integrated contrasts between town and country in each of the terms of the energy balance described in Chapter 1. Many of thèse éléments undergo characteristic diurnal, seasonal and synoptic changes in their values so that the overall urban/rural température différence varies not only from one city to another and through time (Oujezdsky, 1973), but also from one part of a city to another. This is because local air températures respond to the make-up of the internai land-use éléments so that areas of similar urban development give rise to fairly uniform air températures separated from other discrète development areas by often very sharp thermal gradients.

In gênerai, urban heat islands are most intense by night when the urban/rural température différence has frequently been measured as 5°C and may reach 11°C (Chandler, 1965; Bornstein, 1968; Oke and East, 1971 ; Peterson, 1969).

Maximum heat-island intensities are usually attained a few hours after sunset, mainly as a resuit of strong rural cooling (Oke et a/., 1972(a). Later in the night, urban cooling rates often slightly exceed those in rural areas and the urban/rural température différence grows less. After dawn, the vegetation-covered soils of rural areas, with a relatively low thermal capacity and fully exposed to solar radiation, warm faster than air in city streets, so that the urban/rural température différence further decreases or even reverses to form an urban "cold island".

Table V gives the mean températures in and around London for the period 1931-1960.

TABLE V

Average annual températures in and around London, 1931-1960

Average height (m)

Surrounding country 87.5 Suburbs 61.9 Central districts 26.3

Heat-island intensity

Central districts Suburbs

Maximum (°C)

13.7 14.2 14.6

0.9 0.5

Minimum (°C)

5.5 6.4 7.4

1.9 0.9

Mean (°C)

9.6 10.3 11.0

1.4 0.7

Thèse average values are probably fairly typical of most médium to large towns in mid-latitudes. Much more work needs to be done before we can generalize about settlements in other climatic régimes, although such studies are now becoming more common. (Nakamura, 1966; Nieuwolt, 1966; Goldreich, 1970; Raman and Kelkar, 1972.)


*->? -«i- . ^•"•-- -* ' -£

MINIMUM TEMPERATURE

4 JUNE 1959 S miles

w

(After Chantier. 1965)

Figure 4 — Minimum température in London, 4 June 1959. This was a clear night except for a one-eighth cover of high cirrus and there was little or no wind


TEMPERATURE 23-45 GMT.

23 rd AUGUST 1966

?^g^ (After Chandler, 1967b)

Figure 5 — Température in Leicester, England, at 23.45 G.M.T. on 23 August 1966. Isotherms are labelled in °C with "F in brackets. The built-up districts are divided, rather subjectively, into areas of différent building densities. The high densities of mainly central districts are indicated by dark shading and the lower densities of mainly outer suburban areas, by light shading


By day, heat islands are much less intense; indeed, cold islands with lower city than rural températures are quite common. In central London, for instance, day-time cold islands occur on about one day in three on average and one day in two from February to April : by night the frequency is less than one night in five with a clear winter peak. Part of the explanation of lower day-time températures within cities than outside is the thermal lag effect of areas such as city centres having a high thermal capacity (Chandler, 1965), but also important in this respect is the shading of city streets, gardens and courtyards by tall buildings. For this reason, some inves-tigators (Ludwig and Kealoha, 1968) hâve shown the highest day-time températures to occur in a ring around the central business district (downtown) area of cities.

Detailed studies in many cities hâve demonstrated the complex, vaguely concentric pattern of iso-therms in cities, but with local peaks and hollows in the température surfaces reflecting changes in urban building forms and densities (Figures 4, 5 and 6).

In spite of individual différences, certain features are common to most heat islands. One is the close correspondence found, more especially on calm, clear nights, between the pattern of températures and the urban

TEMPERATURE °C 0 7 0 0 E.S.T.

23rd January 1969 6 Miles

(AfterOke and East,1971)

Figure 6 — Température in Montréal at 0700 E.S.T. on 23 January 1969. Isotherms are labellcd in °C. The built-up area is shadeci. This was a clear night, except for urban smog, with a wind of less than 1.5 m s - 1 from NE


morphology. Highest night-time températures are generally found in the densely built-up areas of city centres, where the surface areas and thermal capacities of the buildings are greatest and diffusion is weakest in the street and courtyard canyons and cavities between buildings (Figure 7).

The absolute values and pattern of températures in a city are profoundly influenced by site conditions (Norwine, 1972; Chopra and Pritchard, 1972) and by the prevailing weather conditions (Chandler, 1961, 1965; Clarke and Peterson, 1972, 1973; Oke and Hannell, 1970). Tn gênerai, although the usual pattern of isotherms is roughly concentric refiecting the annular arrangement of most urban forms (Figures 4 and 5), on individual occasions other than during absolute calms, the peak températures will be displaced somewhat downwind of the areas of densest building development by the advection of cold air into the windward side of cities (Chandler, 1965).

Refiecting the close correspondence between heat-island intensifies and local building densities, more particularly at times of clear skies, light winds and an inversion of température in the lower 100 to 300 m of the atmosphère, that is at times when heat islands are strongest (Chandler, 1965; Lee, 1975(6)), is a characteristi-cally steep margin to the heat island which roughly parallels the limit of continuous urban development. Hère, température gradients between the outer ring of suburban development and the open countryside may reach around 4°C km-1 (Oke and Hannell, 1970; Chandler, 1965) on some nights.

Variations in the real-time intensifies of day-time and nocturnal heat islands are related to changes in each of the terms in the gênerai urban heat-balance équation (see Chapter 1) as affected by synoptic meteoro-logical controls, including those upon diffusion. Studies in many cities hâve shown that among ail the mete-orological parameters, those of greatest importance are stability, wind-speed and cloud amount (Sundborg, 1950; Chandler, 1965; Peterson, 1969; Tyson et al., 1973). As a resuit of changes in thèse controlling factors, heat islands will constantly change their form and maximum intensity, even within an individual seulement (Clarke and Peterson, 1973). As a resuit, mean urban/rural température anomalies, which often average between 1.5 and 2°C for night-time conditions over a broad spectrum of seulement form and size, cover a very wide range of individual values. In central London, for instance, night-time heat islands during the period 1931-1960 averaged 1.9°C, but in the décade 1951-1960, 17 per cent of occasions had values in excess of 2.8°C. The average day-time heat-island intensity for 1931-1960 was 0.9CC, two per cent of occasions in 1951-1960 having values in excess of 2.8°C. In central London, the 1951-1960 modal night-time and day-time heat-island intensities were both 0.6°C. In the suburbs, heat islands were weaker, averaging 0.9°C by night and 0.5°C by day. The modal values hère were 0°C by both night and day.

Différences between cities are probably due not only to contrasts in their urban morphology and the relative importance of the liberated heat of combustion, but also to différences in the gênerai climates of their régions, more particularly in relation to seasonal changes in wind-speed, cloud amounts and the incidence of inversions. Thèse hâve also been held responsible for allegedly more intense heat islands in continental than in maritime climates (Holland, 1972).

Although night-time peaks of heat-island intensity are almost universal, there are large and largely unexplained régional variations in the seasonal pattern of values (Chandler, 1970). Many cities such as those in the U.K. and western North America record a clear summer maximum in mean intensities; others, such as many in Japan, show a winter peak; while others, such as several in central Europe, Scandinavia and central America, record no clear seasonal rhythm in values (Mitchell, 1961; Sundborg, 1950; Landsberg, 1956; Stein-hauser et al., 1955, 1959).

Because of the dependence of heat-island intensity upon a séries of variable parameters, including cloud amount and wind speed, year-to-year changes are also apparent in mean intensities (Chandler, 1965).

But what has emerged in récent studies of températures (and other éléments) in urban areas is the close correspondence between local values and nearby urban development forms and densities. Chandler (1967(6)) first commented upon this in a comparison of simultaneous urban/rural température anomalies in Leicester

TEMPERATURE TRAVERSE

London 11-12 October 1961 (Night)

ACTUALAND MEAN TEMPERATURES Inward Traverse Outward Traverse Mean

""!"" Intensity ôf Building

TRAVERSE PROFILE

M M I g, g, .

ARBITRARY UNITS

r 7 5

3 h 50 §

25 i

flve miles

U U) l*> Q 0 V

fivekms

§

I r 3 5

Figure 7 — Température traverse across London; mean températures at 23.30 G.M.T. on 11 October 1961. Also shown are indices of the intensity of urban development within 500 m radius of each of a séries of stations along the line of traverse. Thèse indices are a product of the percentage of the total area which is built upon and the mean height of the buildings


and London, and it was developed in more détail by Clarke and Peterson (1972). Chandler showed that on individual nights of stability, clear skies and calm, areas of similar urban development in Leicester (pop. 270 000) and London (pop. 8.25 million) recorded the same excesses of température above those of nearby rural areas.

City size thus seems to be relatively unimportant at times conducive to the strongest heat islands (Chandler, 1964), the intensity of the immédiate urban development then being the more important déterminant of the strength of the heat island. But at other times and over a long time average, there does seem to be a close relationship between city size and heat island intensity. Mitchell (1953, 1961, 1962) has shown that during the first half of this century, city températures increased faster than those of rural areas, though it is possible that this was because of trends in global climate rather than city growth. Dronia (1967) made similar observations using 67 paired urban/rural stations around the world, but Chandler (1965) showed that there was no such consistent trend in London's températures, which responded to year-to-year fluctuations in the primary meteoro-logical controls upon heat-island intensifies.

Oke (1972(a)), on the other hand, has demonstrated a close logarithmic relationship between the average urban température excess and city size, measured as population, for North American and European cities. The size of the city may be relevant to several parameters of the urban heat balance, but two of its main effects are likely to be first, through the roughness and fetch influences upon airflow and thereby upon wind-speed and turbulence at the city centre (see Chapter 1), and secondly, through the relationship that commonly exists between city size and the intensity of development in the central business district. Large cities are likely to hâve massive central building developments, unlike smaller towns, and this alone will affect the peak intensity of their heat islands.

City size also seems relevant, through its control upon critical wind-speeds, for the élimination of the heat island. Oke and Hannell (1970) found a statistical relationship (corrélation coefficient + 0.97) between this critical wind speed U and the logarithm of the population, P (used as an index of the spatial extent of the city) in the form of the régression équation:

U = - 1 1 . 6 + 3.4 P

For London (population 8.25 million) the critical speed for the élimination of the heat island was 12 m s-1; for Montréal (population 2 million), 11 m s-1; for Reading, England (population 120 000), 4.7 m s-1; and for Palo Alto, California (population 33 000), 3.5 m s-1.

The authors then used this équation to estimate the smallest seulement that would form a heat island. When £7 = 0, P = 2500, but other évidence (Chandler, 1965; Landsberg, 1970(a), Landsberg and Maisel, 1972; Chopra and Pritchard, 1972; Norwine, 1972) shows that small building groups and even individual buildings can produce measurable effects upon local températures (see Chapter 3). In much the same way, Chandler (1970) has noted a strong corrélation between local heat island intensifies and the amount (roughly volume) of urban development within 500 m of the points of measurement (Figure 7).

Several authors (Chandler, 1965; Kopec, 1970; Lindqvist, 1968; Pool, 1970; Fonda et al, 1971; Sekiguti, 1972; Sekiguti et al, 1972; Sharan and Koplowitz, 1972; Oke, 1972(a)) hâve noted the surprisingly large heat islands that can sometimes develop over small settlements provided their building densities are high. Others hâve studied the effects of parks and squares (Chandler, 1965; Clarke and Bach, 1971 ; Conrads and van der Hage, 1971; Herrington et al., 1972; Oke, 1972(6); Jauregui, 1972), noting the lower températures that are usually found in open spaces within cities, a feature of some importance in urban planning (see Chapter 3).

2.7.2 VERTICAL STRUCTURE OF TEMPéRATURE

Much less is known about the vertical than about the horizontal pattern of températures in heat islands. Meteorological towers, balloons and tall buildings carrying température recorders hâve been used in

23 MAY 1967

Wind Direction

40-

Downtown Cincinnati Suburban 1 mile.

! 1

(After Clarke, 1969)

Figure 8 — Cross-section of the vertical température structure above Cincinnati, Ohio, 23 May 1967. The heat plume rising downwind of the city, underneath a région inversion, is shaded

o a ta

I O


several cities, but perhaps the most detailed results hâve corne more recently with the use of instrumented heli-copters. Among the more detailed observations of températures in the boundary layer above cities are those in San Francisco (Duckworth and Sandberg, 1954); Louisville (De Marrais, 1961); Cincinnati (McCormick and Baulch, 1962; McCormick and Kurfis, 1966; Clarke, 1969); New York (Davidson, 1967; Bornstein, 1968); Minneapolis-St. Paul (Baker tt al., 1969); Fort Wayne (Yap et al., 1969); Columbus (Angell et al., 1971(a)); St. Louis (Spangler and Dirks, 1972); Montréal (Yap et al., 1969; Oke and East, 1971); various sites in southern Ontario (Munn and Stewart, 1967); Liverpool, England (Jones et al., 1971); Frankfurt (Georgii, 1970), and various cities in Japan (Shitara, 1959; Yamamoto and Shimanuki, 1964; Sekiguti, 1970).

AH of the studies show that, because of increased thermal and mechanical convection, night-time inversions of température are less fréquent, weaker and higher over the city than over the country, although there is often little différence between the two environments by day. By night, the well-mixed mixing layer shrinks in depth but, except in the lower 50 m or so, still exhibits a weak stability structure.

In his study of New York City, Bornstein (1968) recorded multiple elevated inversions over the sur-rounding country. Between the ground and heights of up to 300 m over the city, températures were generally higher than over the country and the lapse rate was pseudo-adiabatic. Above about 400 m, températures over the city were generally lower than those above the surrounding country. This "crossover" of urban and rural température profiles with colder air above the city than the country at a level of about 300 to 400 m above ground has not been fully explained, but it is suggested that it may be related to long-wave radiation from a high-level pollution haze over the city. Other suggested causes are the adiabatic effects of vertical mixing and urban-induced cellular circulations linking city and country. The elevated cool layer above Montréal is at higher levels than generally found elsewhere and seems to occur only with wind speeds of less than 3.0 m s-1.

Clarke (1969), measuring températures above Cincinnati at times of clear skies and light winds, found évidence of an "urban heat plume" rising above the city and then spreading several miles downwind at inter-mediate heights. Upwind of the urban area, there was a strong, deep radiation inversion whilst over the built-up area, température lapse conditions prevailed up to about 60 m above the ground. Downwind of the urban area there was a strong but shallow inversion at the surface, above which there was a weak lapse condition in the urban heat plume, followed at higher levels again by the upper part of the régional nocturnal radiation inversion (Figure 8). Similar results hâve been obtained from investigations in New York (Bornstein, 1968); Montréal (Oke and East, 1971) and Johannesburg (Tyson et al., 1973), though with weak rural stability and moderate winds the model works less well.

With calm air, the envisaged model of the boundary layer is of an urban dôme rather than heat plume rising downwind from the city. Within the dôme, both heat and pollutants are well mixed with steep thermal and concentration gradients near the upper limit of the dôme (Roberts et al., 1970; Leahey, 1969 ; Bach, 1971(a), 1971(ô); Oke and East, 1971).

It is interesting to observe that many of the more récent studies of the vertical structure of heat islands hâve confirmed one of the earliest models, that by Summers (1965). Summers suggested that the increased thermal and mechanical convection over cities would produce an adiabatic layer over the city which would deepen downwind of the windward rural/urban boundary in response to the urban heat input. By night, stable rural air moving across an assumed uniformly heated city would produce a modification proportional to the square root of the urban fetch. This would mean that the intensity of the heat island and its depth would be related to the rural lapse rate, city size and heat input.

2.7.3 HEAT-ISLAND MODELLING

If it were possible to generalize the meteorological and topographical controls upon heat island intensives in a simple and realistic model, capable of wide application, this would provide a most important tool for

OBSERVATIONAL STUDŒS 35

further research and planning. Many attempts hâve been made to model heat islands and the problem has been approached from a number of différent directions, recently summarized by Tyson et al. (1973) and Oke (1974).

The most straightforward approach is to attempt simple multivariate statistical relationship between heat-island intensities and a séries of controlling meteorological parameters. This was adopted by Sundborg (1950) in Uppsala and Chandler (1965) in London. Similarly, Ludwig and Kealoha (1968, 1970) showed a close régression relationship between the intensity of heat islands and the strength of the prevailing rural température lapse. Three équations were derived, the constants varying with the size of the population:

AT = 2.6—14.80y for cities of more than 2.0 million (root mean square error, ±0.96)

AT = 1.7— 7.24y for cities of 0.5 to 2.0 million (root mean square error, ± 1.00)

AT = 1.3— 6.78y for cities of less than 0.5 million (root mean square error, ±0.66) where y is the measured environmental lapse rate in CC mb-1.

For a number of Canadian settlements, Oke (1972) demonstrated that the urban/rural température différence was correlated directly with population and inversely with wind speed through the following équation:

T=pK/4ûK

where T is the strength of the heat island, p is the city population and u is a représentative rural wind speed.

One of the earliest heat-island models, upon which several more récent attempts hâve been based, was that of Summers (1965), who showed that:

AQ = (Fw(~ pcpU

where 0 is potential température, Fis the anthropogenic heat production, Wis city width, p is air density, cp

is spécifie heat of air at constant pressure and C/is a représentative rural wind speed.

A combination of topographie (land-use) and meteorological parameters was used by Clarke and Petersen (1972) to express the size of the heat island over various parts of St. Louis. Their analysis showed that land-use controls were the most important déterminants of overall heat-island intensity though stability and wind-speed greatly affected the spatial patterns of individual heat islands.

An energy balance approach to heat-island modelling was suggested by Fuggle and Oke (1970), who spelt out the primary ternis of the balance. Using a simple Earth/atmosphere heat exchange approach for urban and rural areas, Myrup (1969, 1970) produced a model which appeared to show the dominance of differential evaporation rates, aerodynamic roughness, thermal properties of the surface and wind speed, in heat-island génération. The model has been criticized and refined by Miller, Johnson and Lowry (1972), who conclude that although nocturnal heat islands are likely to be well developed in low and mid-latitude vegetated environ-ments, they are likely to be weak in low and mid-latitude désert envjronments. Day-time heat islands, they deduce, will be weakly developed in summer in low-latitude vegetated environments and in ail seasons in mid-latitude vegetated environments. Their model indicates that cold islands will be well developed by day in low-and mid-latitude désert environments.

A more refined energy-balance model was developed by Myrup and Morgan (1972) differentiating the physical properties and energy budgets of small areas of metropolitan Sacramento. Terjung (1969, 1971) has conducted a similar study in metropolitan Los Angeles, though his conclusions are made more tentative by the limited data and the calculation of the sensible heat flux as an energy-balance residual.


A somewhat différent, thermodynamic approach to heat-island modelling was first adopted by Summers (1965), who studied the advection of stable, rural air over a rough, warm city surface. He was able to show that an adiabatically mixed urban boundary layer with a constant wind profile would develop from the windward edge of the city and that the growth of the layer was a function of the rural stability, the heat sources within the city and the wind-speed in the mixed boundary layer. The heat-island intensity Arwas shown to be given by:

&T = ph(x)

where J? is the différence between the dry adiabatic and the rural lapse rate and h is the depth of the mixing layer at a distance x downwind of the upwind edge of the city.

Summers' model was later modified by Leahey (1969) and despite the considérable assumptions he made, more especially about wind and température profiles, surface roughness, latent heat exchanges and heat storage, the model was found to give good prédictions of mixing depths and urban température excesses over New York (Leahey and Friend, 1971).

Finally, there are the dynamic models which attempt to solve équations of motion, continuity, state and thermodynamics. The various attempts to use this approach hâve recently been summarized by Tyson et al. (1973) and Oke (1974).

Among the most notable attempts to model the dynamics of urban atmosphères was McElroy's (1972(a), 1972(6)) modification of the Estoque (1963) model of the planetary boundary layer to include the effect of urban form and land use, and Atwater's (1972) model.

2.8 Air pollution

Man has, until recently, largely ignored the fact that locally, and perhaps globally, he has overloaded the natural self-cleansing capacity of the atmosphère so as to produce very high and perhaps dangerously bigh concentrations of air pollutants, many emanating from combustion processes of one type or another. Because man's socio-economic organizations hâve concentrated his waste-forming activities in towns, it is hère that the most serious problems are generally, though not exclusively, to be found.

Before the advent of individual and collective environmental consciousness leading to the graduai improvement in many aspects of air quality, man had wantonly discharged vast quantities of waste products into the air from house and factory chimneys and plant, and from quarries, cars, lorries, trains and ships. Thèse still plague many cities but, because local concentrations are derived in large part from nearby sources (Marsh and Forster, 1967; Williams, 1960), the pattern of pollution and more particularly of smoke and vehicle émissions in cities is closely related to the individual pattern of émissions. This is particularly true at times of stability and light winds when concentrations in urban (unlike rural) areas are highest. For this reason, it is less easy to make meaningful generalizations about the distribution of pollutants in cities than about most other meteoro-logical éléments, concentrations varying over very small distances and very short periods of rime so as to produce complicated spatial and temporal patterns of pollution as individual to each city as the distribution of its émissions (Chandler, 1965; Marsh and Forster, 1967). Bach (1969), for instance, studied the turbidity coefficient in various land-use districts of Cincinnati and showed the frequency of polluted air (having a turbidity coefficient of 0.21 or greater) to increase from a city park to the suburbs and then in ascending order of percentage frequency, through city residential areas and the city centre to an industrial area. The industrial site experienced almost exclusively "polluted air" whilst the park had predominantly "clean air" (turbidity coefficient of 0.10 or less) over the same summer period.


Land use also affects the form of the diurnal variation of aérosol concentrations (Bach, 1969), residential areas showing a typical morning and evening peak at times of heavy émissions and moderate or strong stability. This same diurnal form was also demonstrated by Chandler in his studies of pollution in London where domestic sources are the dominant emitters of smoke.

Patterns of sulphur dioxide, the major gaseous pollutant of most mid- and bigh-latitude cities, are much less complicated than those of smoke (Chandler, 1965). This is because the height of émissions is generally greater and the aerodynamic properties of a gas are différent from those of an aérosol. In conséquence, sulphur dioxide is capable of drifting farther than smoke and does not show the latter's sharp gradients within cities or precipitous fall-offin concentrations near the margin of the urban area (Chandler, 1965). For the same reasons, S02 can drift far farther away from its source than can smoke. Much industrial émission is from tall stacks which help efficiently to diffuse the pollutant so as to produce low ground-level concentrations, more especially in their immédiate vicinity. This is because ground-level concentrations of pollution are, on average, inversely proportional to the square of the effective height of the émission, with the greatest concentrations some distance from the stack. Domestic smoke concentrations, on the other hand, are released (mainly in winter) at lower températures and from much lower levels than industrial pollutants and are caught in the eddies around buildings and across streets so as to pollute most severely those areas in the immédiate vicinity of the sources. This close relationship between émissions and concentrations (Williams, 1960) is fundamental to the success of local smoke control orders in residential areas, allowing considérable local improvements to resuit from actions to reduce émissions.

The closeness of the relationship will, of course, vary from one city to another and through time, and will dépend upon prevailing meteorological conditions (Peterson, 1970, 1972), high ground-level concentrations of smoke and sulphur dioxide being associated in most towns with light winds and a stable atmosphère within the mixing Iayer up to the effective height of émissions (Stern, 1968; Singer and Smith, 1970).

The latéral drift of smoke across cities by the prevailing wind is important, but much less so than has often been supposed. Except at times of light winds and stable atmosphères, smoke tends to spread vertically more than laterally (Chandler, 1965), the reasons being the wide-angled diffusion of smoke from low-level sources, including downwash in the wake of buildings, and the often turbulent nature of prevailing winds. (This feature has important implications for planning which are discussed in the final chapter of this Technical Note.) The latéral drift of sulphur dioxide across a city is much greater than that of smoke, with substantial rises in ground-level concentrations in winds that hâve crossed the city (Chandler, 1965).

But although the greatest contribution to ground-level concentrations will generally come from nearby émissions, additions can sometimes be made by sources far beyond the city boundary. Smoke and sulphur dioxide can drift hundreds, sometimes thousands of miles from their source (Barnes, 1975). It often does so several hundred mètres above the Earth, frequently trapped and concentrated under a température inversion, before being brought down to the ground again by the increased turbulence of a city lying in the path of a plume (Marsh and Forster, 1967; Parry, 1970). Concentrations of smoke and sulphur dioxide in eastern Reading, 64 km west of the centre of London, for instance, are often significantly enhanced by downwash from a high-level drift from London at times of light easterly winds.

The diffusion of pollution from mobile sources is mainly controlled by airflow around buildings. Ail that need be said hère is that concentrations fall offvery rapidly in streets leading away from the main trame highways (Reed, 1966; Motto, 1970); that concentrations in streets orientated at right-angles to the wind are affected by eddies which form in thèse situations with a downdraught of relatively clean air on the lee side and the highest concentration of vehicular pollutants occurring in the lower windward side of the street chasm (Georgii, 1970); and finally, that the extra turbulence produced by cars on major urban highways may help to keep concentrations of pollution down since there is some évidence of moderate increases in stability occurring soon after, rather than during, the morning and evening peak traffic periods (Munn, 1970).


2.9 Vfeibility

Because of the high aérosol concentrations in urban areas, visibilities are generally lower, and because of the particulates, fog droplets form more readily and evaporate more slowly than in rural areas (Byers, 1965). The densest fogs are often found in the suburbs rather than in city centres which are warmer and hâve lower absolute humidities. Central London, for instance, had, before the Clean Air Act of 1956, nearly twice as many hours of fog (visibilities of less than 1000 m) per year than rural areas around the city but only about the same number of hours of dense fog (visibilities of less than 40 m). The London suburbs, on the other hand, had only between 28 and 32 per cent more hours of fog than rural areas beyond the city but up to four times as many dense fogs (Chandler, 1965).

The warmer air and gentler winds of city centres also delay the formation of evening fogs and their dispersai the following morning respectively so that in the evening the fog lies like an annulus around a gradually disappearing clear centre ; in the morning the fog clears in the stronger winds and more rapidly rising températures of rural and suburban areas to leave fog in only the centre of the city.

Since the Clean Air Acts (1956 and 1968) in England, there has been a dramatic improvement in visibilities in London (Brazell, 1964; Freeman, 1968), and similar improvements hâve been recorded in Manchester (Atkins, 1968) and elsewhere though some of the improvement might be because of stronger winds (Schmidt and Velds, 1969) and is certainly not universal as Green and Battan (1967) hâve pointed out for Tuscon, Arizona, and McNulty (1968) for New York City.

CHAPTER 3

URBAN CLIMATES AND URBAN PLANNING

3.1 Introduction

Traditional building architecture, more especially in areas of climatic extrêmes, has, over the centuries, yielded a séries of almost classical designs, each by various shapes and constructions, ameliorating the less welcome and sharpening the more désirable attributes of the local outdoor climate. In contrast, although not completely oblivious of climatic controls and objectives, city designers hâve until comparatively recently only rarely considered climate among the several constraints upon urban design. The layout of cities has nearly always been dictated, or almost accidentally created, by a séries of mainly political, social and économie decision-making processes.

But more recently the principles of urban climates hâve been applied in an increasing number of urban growth, renewal, and new city design programmes, although still, it must be admitted, in a generally less rigo-rous, more empirical manner than in building climatology, which, in spite of its shortcomings, offers a more scientific approach to building architecture than urban climatology presently brings to city design. But even a pragmatic approach is better than total neglect of the climatic implications of urban plans and the more récent Systems approach in urban climatology offers hope of a more scientific application of climatological principles to purposeful urban design.

Landsberg (1970) has charted the slow progress made during the first four décades of the présent century in the formulation of climatological principles and guidelines for use by architects and town planners, culminat-ing, as far as urban design is concerned, in the first comprehensive monograph on urban climates, by Kratzer, in 1937. This not only brought together in a single volume the existing literature on how buildings and building groups affect the climate outside their walls, but included a study of the application of urban climatology to town planning.

But the writings of Kratzer (1956) and other early pioneers of applied urban climatology such as Schmauss (1927), and Brezina and Schmidt (1937) were largely ignored by the practising city planners of the inter-war years or, what is more likely, the lack of communication between planners and climatologists meant that most of the former worked in ignorance of the latter's knowledge, limited though this was at the time. Another difficulty was that the structure of a city is extraordinarily conservative. Though individual buildings and whole areas are periodically replaced, the skeleton of the urban communications networks and major land-use régions frequently remain unchanged through the centuries. Many ancient towns of Europe and else-where still retain their basic médiéval plan in their central areas and this résistance to fondamental change, in spite of urban renewal, imposes a severe constraint upon the successful application of urban climatological principles to city design.

Building climatology is relevant to conditions inside buildings, to the fabric itself and to environmental quality in the immédiate air envelope of structures. It is therefore of importance in the design and construction of ail new buildings. Urban climatology, on the other hand, is relevant only in cases of major expansion, urban redevelopment or, most obviously, in new town design. Only in thèse circumstances is there real scope for correct-ing the failures of faulty urban land-use designs in existing or expanding towns or for creating, ab initio, attractive


atmospheric environments in entirely new towns (Taesler, 1973(a), 1973(6)). But simply because thèse opportuni-ties are few in number, it is even more vital to take advantage of them whenever and wherever they materialize. Because buildings, individually and collectively, always modify the original topoclimate of the area, it is logical to attempt, as far as is compatible with other influences on design, to use building development as a conscious instrument for the control of climate in the urban environment. This must be done by close collaboration between the building and urban climatologist for, as emphasized in the Introduction to this Technical Note, their work is part of a continuous spectrum of planning scales, with décisions about the location, orientation, shape and construction materials of individual buildings at one end of the spectrum and major land-use zoning at the other. No part of the spectrum acts independently of the other; ail are interrelated and the resuit is a com-plex mosaic of micro- and meso-climates in the city. As explained in the Introduction, the décision making also straddles a wide range of planning and meteorological scales.

In régional and town planning, a climatological input at the macro- and meso-scales will détermine the properties of the green field site in ail major respects such as insolation, température (including the frequency of extrêmes), airflow (relevant to human comfort and the diffusion of pollution), précipitation, etc. Thèse, in combination, will then define meso-scale climatic régions, some of which may be more préférable than others, and will be taken into account in the sélection of the site for a new town or major expansion.

Meso-scale urban influences upon town climates, as described in Chapters 1 and 2, should then beused, where the opportunity présents itself, to help détermine the major land-use patterns, such as the zoning of industrial and residential quarters (relevant, for instance, to the diffusion and human environmental impact of pollution), the orientation of major highways (relevant to air movement and pollution from vehicles), development densities and heights (significant for températures, airflow, the diffusion of pollution and radiation in streets, etc.), and the size and location of open spaces (a factor influencing most meteorological éléments).

3.2 The decision-making process

Several authors, but more particularly Page (1964, 1973(a), 1973(6)) hâve made a detailed analysis of the decision-making processes in complex Systems of, or related to, the design of individual buildings, building groups and new towns.

Figure 9 is an attempt to analyse the stages by which the principles of urban climatology, described earlier, are introduced as and where appropriate into the decision-making séquence by which the socio-economic and other constraints upon urban design are matched as successfully as possible to those producing optimum out-of-door climates.

There are essentially two lines of parallel analysis and décision making: one leading to the définition of the required climatological performance standards for each urban activity in its spatial context; and the second, proceeding through the modifications of the régional climate by the local orography and land uses to the définition of the resulting urban climate. The hope is that desired and expected city climates will be the same but, if not, the décision will hâve to be made whether to accept the disparity or to modify the design so as to achieve the required external climates. Thèse will, of course, provide the background data for the proper design of individual buildings.

There is also a subsidiary séquence in the Systems analysis, relating climate to each stage of construction, thus leading to the définition of an optimum séquence. It may, for instance be préférable, other things being equal, first to develop the more elevated parts of a city being constructed in an area of marked relief. In this way the danger of frost pockets aflecting early developments in the valley bottoms will be diminished. When valley bottoms, hillsides and hill crests are equally covered, cold air drainage into lower parts of the city will be inhibited by the increased friction of the urban surface, and températures will also be moderated by the heat

URBAN CLIMATES AND URBAN PLANNING 41

Urban Land Use Functions

Désirable Urban Land Use Pattern

Required Environmental Performance

Standards for each activity

" ••

Choose Alternative Form

Range of Broad

Régional Climate

• Unsatisfactory-

Modifications by local topography -Greenfield site

Modify External Factors

Modifications by Land Uses (rural and/or

urban)

I Data for I i Individual | •Building Décision.' ! Process J

Calculated impact of final

Urban Development

on Mesoclimate

7 Comparison of Required and Anticipated

Urban Climate

Satisfactory

Désirable Séquence of Construction

Proceed with

Construction

Calculated impact Stage

by Stage during Construction

Comparison of Required

Séquence and Climatologîcally

Preferred Séquence

Optimum

Séquence

Figure 9 — The séquence of urban planning décision making, incorporating a climatic input


island. In certain areas, of course, it may be préférable never to settle the flood-plains of deeper valleys (Lands-berg, 1970(è)) since the paving of town surfaces and possible felling of trees may add to an already dangerous flood risk near to a river (see 3.3). The définition of the désirable climatic standards in each of the main land-use areas of the city cannot of course be made independently since, to varying degrees, the climate of one area will influence that of its near and even distant neighbours. The most obvious example of this is the drift of pollution, often over considérable distances, from its point of émission. The définition of standards is of course dépendent upon the fast developing sciences of environmental physiology and psychology for fixing its levels of comfort, and on physical and mental stimulus (Ryd, 1973).

Calculating the climatological impact of each facet of the design upon the urban climate is not, as already explained, as yet a précise science, but some indication of the likely range of values has been given in Chapter 2 and the approach thereafter will hâve to be either to estimate the impact of a given development by analogy with similar circumstances already documented elsewhere; to use the available theoretical or empirical équations described in Chapter 1 ; in the case of sunshine patterns to use the well-known shade diagrams or, for the study of airflow, to make use of models in wind tunnel experiments (Ryd, 1973).

There now follows a séries of studies of the relevance of each of a number of éléments of urban climates to urban design. The treatment is gênerai since each city is unique, but it is hoped that the discussion will provide gênerai guidelines of use to the urban planner. It must of course be remembered that though each climatic élément is hère treated separately, the final décision will hâve to be made using an integrated assessment of several éléments (Hill, 1968) which may sometimes work in sympathy with each other but are often in conflict. The improvement in airflow conditions may, for instance, worsen the thermal environment and priorities or compromises hâve frequently to be defined, more especially where différent human activities are involved in a given area.

It is of course important to monitor the changing values of the various meteorological parameters as a new city is built and grows (Landsberg, 1970(6)) so as to provide the background, climatological information for major schemes of expansion or redevelopment. The difliculties of obtaining suflîciently long records for a représentative group of urban sites was referred to in the Introduction, but several authors hâve addressed them-selves to the problem and hâve suggested the types of data that are necessary, and the format in which they are best presented (Caspar, 1973; Ryd, 1973; Taèsler, 1973).

3.3 Hydrology

The particular attributes of urban hydrology, described in Chapter 2, are clearly relevant to the efficacy of natural drainage channels within cities, and the proper design of storm-water drains in order to cope with the magnitude and frequency of peak flows (Unesco, 1972; Unesco/FAO, 1973; Watkins, 1962, 1963). Thèse aspects are made more critical where, as is often the case, the city lies astride a flood-plain in the bottom of a valley (CIRA, 1973; Landsberg, 1970(6)).

The storm-water sewerage system needs to be planned in great détail, bearing in mind the considérable control exercised by the local percentages of impervious surfaces etc. upon peak discharge amounts and fre-quencies (see Chapter 2). Care must also be taken not to allow the unnecessarily extensive sealing of the surface by urban materials but, for hydrological as well as other reasons, to retain or create as many parks, gardens and other areas of vegetation-covered soils as possible. Storage réservoirs or ponds in drainage channels upstream of, within and below cities, also help to smooth out the flows.

3.4 Températures

Several authors hâve attempted to include man within the complex energy-exchange System of the urban environment, with intricate radiation and sensible heat transfers between the human body and the kalei-


doscope of the differently orientated surfaces of buildings, roads and other units, each with its own spécial physical properties. Notable among thèse studies hâve been those by Myrup and Morgan (1972), by Terjung and bis collaborators (1969; 1970(a); 1970(6); 1973; 1974), who attempted to measure and model the various heat-exchange processes operating in the streets of Los Angeles, and Clarke and Bach's (1971) measurement of air températures, humidities, wind speeds and radiation above paved and grassed surfaces in central and sub-urban Cincinnati and their bearing upon human comfort.

Environmental températures, indoors and out, obviously affect human well-being and, in extrême circumstances, human health, so that in addition to the many socio-economic constraints operating in town planning décision making, the efifect of the city upon outdoor températures may often warrant serious considération (Clarke, 1972(a), 1972(6); Garnier and La Fleur, 1972; Beuchley et al, 1972).

High environmental températures are obviously deleterious to health and comfort and prolonged exposure can be instrumental in the incidence of strokes, heart disease and pulmonary disorders (Schuman et al., 1964). During a heat wave in the central U.S.A. in July 1966, for instance, there were over 500 excess deaths in the city of St. Louis (Henschel et al., 1968) and an excess of more than 1100 deaths in the state of Illinois (Bridger and Hefland, 1968). In the St. Louis case, Henschel et al. found that the death rate within the city was SVi times greater than outside and Clarke and Bach's (1971) re-analysis of the data shows a very clear, positive statistical relationship between air température above 32.3°C and the numbers of deaths attributed to the heat, which would indicate that the St. Louis heat-island température excess has itself a very large effect upon the num-ber of recorded deaths. The raising of nocturnal températures, giving no night-time relief from high day-time values, seems to be important in this respect. Garnier and La Fleur (1972) carried out traverses across Montréal to détermine the patterns of température, wind, relative humidity and (very roughly) radiation and used thèse to calculate local thermal stress indices which they found were determined mainly by variations in wind speed at colder times and by radiation and heat loads under hotter conditions. Davenport (1973), amongst many others, has also commented upon the rôle of wind in human comfort conditions in urban areas.

The implication for the proper urban design of cities in high-temperature climates is to plan the buildings and their spatial arrangement so as to weaken the heat island, and particularly street températures, as much as possible. This may be done by the use of sufficient and properly spaced parks and the use of high-albedo, low thermal-capacity, low thermal-conductivity building materials (Bach, 1971 (c)). An aerodynamically rough urban surface would aid turbulent heat (and pollution) diffusion between buildings and a radial street plan, allowing country air to penetrate as deeply as possible into the city centre, would also help to ameliorate conditions there (Givoni, 1974).

In the design of coastal towns, having high summer températures, care should be taken to avoid creat-ing a high and continuous barder of tall buildings along the sea front, since this would présent a severe obstacle to the daily pénétration of the cool sea-breeze inland to the centre of the heat island. Haifa is an example of a city where thèse considérations hâve been carefully borne in mind in its urban design.

Well-distributed parks, gardens, lakes and other open spaces, particularly when located close to the downtown or other areas of high development densities, play an important rôle not only in providing the population with récréation and nearby relief from the oppressively high températures and weak ventilation of street environments in many city centres (Herrington et ah, 1972; McElroy, 1972(6)), but also, because cellular circulation Systems are commonly established in thèse circumstances, in linking the cool parks and other vegetated open spaces with heat island peaks in the surrounding densely built-up areas (Sekiguti, 1972). Thèse airflows serve to ventilate the warmer parts of the city, preventing températures reaching as high as they would otherwise do (Givoni, 1973). Gold (1956), for instance, commented upon the part played by the parks, squares and gardens of central London in ventilating such hot-spots as Oxford Circus; and Chandler (1965) measured the lower températures in central London's major open space, Hyde Park.

4 4 URBAN CLIMATOLOGY AND ITS RELBVANCE TO URBAN DESIGN

Water bodies within or at the margins of cities also offer climatological as well as aesthetic and recrea-tional advantages, since températures will generally be lower around their shores at the warmest part of the day or year. In the case of large lakes, lake breezes may cool and ventilate the neighbouring areas, as happens to a depth of about two kilomètres inland from the waterfront of Lake Michigan in Chicago (Landsberg, 1970(6)).

In some high-latitude cities, where the problem in terms of human comfort is of thermal déficit rather than excess, the opposite urban design considérations might profitably be taken into account (Adamenko, 1970; Adamenko and Khairullin, 1972). Hère, development densities might be purposefuUy raised in areas such as shopping centres so as to maintain a strong heat island and keep street and precinct surface and air températures as high as possible and wind-speeds as low as possible whilst at the same time avoiding any uncomfortable eddies around buildings. The generated warmth will also help in maintaining a larger growing season in small central parks and gardens and reduce the period of snow lying in city streets and precincts (Chandler, 1965). The frost-free period in central London, for instance, is about two months longer than outside. In the suburbs it is about 1 Vi months longer.

Although empirical in approach and not capable of précise interprétation, it is common to regard the "active growing season" for végétation in a temperate climate as the period of the year during which mean températures exceed 5.6°C although some workers hâve used 6°C as their base température. Accepting this as a rough indication of a relevant parameter, we find that, for the period 1931-1960, the average annual growing season at Wisley, just to the south-west of London, was 259 days compared with 281 days in Camden Square Gardens near the centre of the city. Higher parts of London, such as Hampstead (137 m), lie above the main mass of most heat islands and so the local growing season is almost the same length as in low-lying country areas.

Another effect of the heat island is to reduce the space-heating requirements of buildings. In London, for instance, accumulated day-degrees below a heating threshold of 15.6°C are 15 per cent lower in central London than in the surrounding country or the more elevated parts of the city (Chandler, 1965). This is not to say that heating provisions and costs will be reduced by the same amount, since température is only one of several parameters relevant to space-heating design.

3.5 Pollution

As public awareness of air pollution has increased, so our understanding of it as an environmental hazard has improved. As our monitoring networks hâve been extended, so officiai and public demands hâve increased for urgent forthright and effective means of air-pollution control. There is not a single, unique solution to the problems of air-pollution control; there is not a discrète path to designated air-quality objectives (Fuller, 1973 ; Page, 1973 (a), 1973(6)) but even uncertainty what thèse objectives should be. Improved air quality can be achieved in a variety of ways : controls can, for instance, be exercised upon the chemistry and amount of émissions either through ail time, or in real time during times of high pollution concentrations — controls upon the number and height of chimneys, perhaps through district heating schemes. Restrictions to efficient diffusion can be removed and physical planning policies and économie controls can be exercised to achieve the optimum locations designed to bring about the minimum impact of pollution emitters upon sensitive receptor areas such as housing estâtes. Reduced émissions can be achieved in a variety of ways. Many of the solutions are technological, such as: raising the height of the chimney so that the plume rises above surface inversions and is sufficiently diluted by the time it reaches the ground (urban plumes hâve an average cône half-angle of about 15°) so as not to cons-titute a serious problem; making sure the chimney is at least VA times the height of nearby buildings so as to prevent the plume being caught up in their eddies; maintaining the plume at a high température so as to give it maximum thermal buoyancy and lift; abstracting pollutants from the fuel (this perhaps to be used merely during high-risk periods), during combustion or manufacture, or from the stack (Stem, 1968); imposing légal limitations locally or throughout the town upon émissions or ambient concentrations (Berry and Horton, 1974);


concentrating émissions, as in district heating schemes, at a smaller number of points; or planning controls upon development densities or the relative position of émission and receptor areas, that is through structure and development control.

Urban and rural planners, intentionally or not, exercise a very considérable control upon the pattern of pollution.

Zéro pollution concentrations, as a universal principle, is technically impossible and economically indefensible. There is, in theory at least, a defînable level of pollution beyond which, in a closed économie System, it would be uneconomic to reduce ambient levels since control costs (including equipment, administration, effects on patterns of development, unemployment, etc.) would then exceed the damage costs (including damage to health, materials, végétation, amenities, etc.). At présent, our knowledge and expertise is too inadéquate to allow us to quantify ail the many terms involved in cost-benefit analyses of pollution and pollution control (Berry and Horton, 1974; Programmes Analysis Unit, 1972). We are rarely certain what degree of damage will accrue from exposure to a given level of pollution for a specified period of time, more particularly from low concentrations over long-time periods. And even if our knowledge of damage functions were more complète and certain than it is, it would still remain to put a monetary value upon the damage, which is presently far from an exact science, even at the level of damage to living and inanimate matter. The cost to such aspects as amenity is presently so spéculative as to be of little real value.

But the search for a more précise methodology of pollution économies continues, spurred on by the increasing use of environmental impact statements designed, in theory at least, to assess the degree of pollution and other environmental damage in the compilation of structural and development plans (Council on Environ-mental Quality, 1973; Dickert and Domeny, 1974).

It is the planner's responsibility to weigh against each other the several planning criteria, including pollution, so as to optimize the total environmental quality, often by balancing widely conflicting interests. The adopted design may, of course, not be the one which achieves minimum levels of pollution damage, other partly irreconcilable objectives being more heavily weighted in achieving the inévitable compromise design having the highest score in the environmental goals achievement matrix (Hill, 1968). Unfortunateîy, and for the reasons explained elsewhere in this Technical Note, planning authorities often hâve to make décisions in the absence of désirable environmental standards and the methods by which they could be achieved. Owing to the complex meteorological conditions existing in urban areas, it is not possible accurately to détermine the relative contribution of each of the many sources responsible for the total burden of pollution at a given place, and thèse pro-portional contributions will in any case change from minute to minute. Nor is it possible to be certain about the local and more distant concentrations that will resuit from any change in the pattern of émissions though, as shown in Chapter 1, substantial advances hâve recently been made which allow us to deduce something of the broad features of the distributions.

Most of the literature on the use of planning as a tool in controlling air pollution is American (Bach, 1972; Burns, 1970; Hagevik et al., 1974; Pelle, 1964; Rydell and Schwarz, 1968; Schueneman, 1968; Taylor et al., 1961) and of the others, few are for low-latitude cities.

Using the principles of topoclimatology, it is possible to predict those parts of a green field landscape which, because of their high incidence of inversions or turbulent eddies, are more susceptible than others to the trapping of pollution (Singer and Smith, 1970) and should be avoided either as a whole (in the siting of new towns) or by those land uses constituting major sources of pollutants.

Many planning solutions rest heavily upon the séparation of industry from residential and other sensitive areas within towns by a distance of at least one kilomètre, although the degree of séparation required will dépend in meteorological terms upon prevailing émission and diffusion circumstances, with the effective


height of the stack exercising a fundamental control (Craxford and Weatherley, 1964; Scorer 1972). Though heavy grit and dust particles from industrial boilers, furnaces and quarries constitute very serious local problems close by the chimneys or works, many forms of pollution, more especially sulphur dioxide, can often spread considérable distances from their source (Barnes, 1975) and, having crossed open country, can sometimes be brought down the ground rather sharply by the increased surface friction of urban areas in its path (Parry, 1970; Marsh and Forster, 1967).

Open spaces within cities, quite apart from acting as zéro or very low-emission zones, also serve, when vegetated, to filter the air passing over them. Concentrations of pollutants at the centres of parks are normally much lower than at the margins (Meetham, 1945; Hill, 1971) because of the trapping and absorption by foliage. Narrow tree belts or even a single row of trees, particularly conifers (Neuberger et al., 1967), can abstract considérable amounts of particulate matter and appréciable quantities of gaseous pollutants (Saunders and Wood, 1974) providing, for instance, a kind of cordon sanitaire between industrial and residential areas.

It is sometimes too readily assumed that the greatest advantage is to be obtained by siting major low-level emitters, including noxious industry, on a town's leeward margins as defined by the prevailing wind. This is not necessarily true since the prevailing wind is often associated with strong, highly turbulent airflows which generally lead to the lowest average surface concentrations, though individual puffs of smoke will bring occasional high values. The worst pollution conditions over a long period often accompany calm conditions or very light winds from a quite différent direction from the prevailing wind, and it would therefore be more sensible to locate major polluters on the leeward margins of the city at times of température inversions and light airflows (Page, .1964).

At the more local scale of planning, often considered at the development control stage, the décisions about land use or about local restrictions upon émissions will normally bear quite heavily upon pollution levels in the immédiate vicinity, with the scope for very considérable improvements in air quality since plumes widen sharply in the intense mechanical and thermal turbulence characteristic of urban areas so that a close correspondance is often found between the patterns of émission and concentration, more particularly with respect to smoke (Williams, 1960; Chandler, 1965; Singer and Smith, 1970).

In détail, of course, the geometry and arrangement of buildings hâve an important bearing upon local concentrations of pollution (McCormick, 1971), which it is vital to consider as an élément of development control. Two aspects are involved: the effect of a new development upon the patterns of airflow and thereby upon the distribution of pollution from other sources; and secondly, the effect of airflow around a building upon pollution released from its own boiler or other outlets (Singer and Smith, 1970; McCormick, 1971). The replacement of areas of old, low, high-density housing, each unit acting as a multiple low-level pollution source, by modem high-rise residential and commercial buildings serves to concentrate and elevate the points of émission, both changes helping to improve dispersion. District heating plants behave in much the same way. But the resuit is often to intermingle high and low buildings with the inévitable danger of pollution from the latter sources spreading downwind at the level of the middle and upper floors of the taller ones.

Apart from controls upon fuel usage and pollution émissions, and those exercised by the physical séparation of important emitter and receptor areas, urban design can also make a contribution to improved air quality by suitable land-use zoning in relation to the local topography (Frenkiel, 1956). Important sources, especially low-level sources of pollution, should not be sited in low-lying areas characteristically subject to light winds, except those blowing along the valley, and to température inversions (Singer and Smith, 1970; Arnold and Edgerley, 1967). Exposed hilltop and upland sites are préférable (Craxford, 1973) although detailed local analyses will hâve to be made of the frequency, location, depth and intensity of température inversions in relation to the point of émission, to make sure the plumes are not brought down by eddies set up by the topography or trapped beneath an inversion (Scorer, 1972; Smith, 1968; Ministry of Housing and Local Government, 1955).


As regards air pollution from vehicles, this can be reduced in a number of ways. As with pollution from other urban sources, the first and most obvious, but not necessarily most économie, approach is to control émissions either through the fuel or through exhaust-abstraction Systems. But sensible planning can also help. Canyon-like city streets in high-density urban areas which severely inhibit the efficient diffusion of pollutants generated at their base are to be avoided (Georgii, 1970). In gênerai, the broader and more open the roadway, the greater the turbulence and the lower the levels of pollution, although even narrow streets orientated in the direction of the wind hâve enhanced wind speeds because of a venturi effect upon airflow (McCormick, 1971). Elevated roadways, well above the gênerai street level environment, are favourable for dispersion (Page, 1964). As with ail other éléments, the advantage of sensible planning is that it is always better and cheaper to achieve clean air from the beginning rather than having to take expensive remédiai steps at a later stage.

3.6 Humidifies

Several authors hâve drawn attention to the important bearing of humidities on man's health and comfort (see, for instance, Sargent and Tromp, 1964). In gênerai, the small différence in the water-vapour content of urban and rural atmosphères is unlikely strongly to differentiate them in terms of human biometeoro-logy, although the lower relative humidities in towns are likely to be something of an advantage, particularly in areas of high températures. But it is the secondary effect of evaporation rates in making urban parks and other vegetated open spaces cooler than they would otherwise be which is probably worth noting and bearing in mind along with ail the other advantages of such urban oases referred to elsewhere in this Technical Note.

Oke (1972(6)) has made a most important pilot study of évapotranspiration cooling rates in Montréal and the results are likely to be représentative of a variety of mid-latitude cities in summer. Oke used a numerical climate-simulation model to gain insight into the relative importance of each of a number of parameters in the urban energy System. The model allowed one to vary the relative percentages of the surface area covered by

E UI M _1

Il H x E 4"

«a O 3H «0 K C

I O 00

2

H

-1 O o Percent Surface Area

Evapotranspiring

(After Oke, 19 72 b)

Figure 10 — Simulated effect on cooling of varyiog the area of a city covered by evapotranspiring surfaces


various evaporating and transpiring surfaces such as water, soils, trees, lawns, etc. In addition to revealing interesting variations of the Bowen ratio (sensible to latent heat flux) which underlined the importance of evaporation as a means of heat transfer in the city, the model made clear the vital rôle of evaporation and transpiration in cooling a city having even a modest percentage of vegetated open space. The effect is most pronounced by day and is not linear, but with only 30 per cent of the city covered by végétation, 66 per cent of the possible cooling is achieved by évapotranspiration alone (Figure 10).

But because of the circulations that commonly link cool, moist park atmosphères with the hot, dry air of densely built-up areas at times of light airflow in summer, there will often be subsidence of hot, dry and possibly polluted air over their central areas (Chandler, 1965; Oke, 1972(6)) which will increase évapotranspiration rates and the associated cooling effect. The water demand of park végétation will also be increased. At the edge of the park, however, cooled, cleaned air will advect some distance down suitably orientated roads.

Végétation thus plays a very important rôle in basically hot climates in keeping urban parks as cool oases, as well as its function in filtering out much of the pollution drifting into the park from neighbouring areas and in reducing local noise levels.

REFERENCES

Ackerman, B., 1971 : Moisture content of city and country. In Preprints, Conférence on Air Pollution Meteorology. Amer. Met. Soc, 154-8.

Ackerman, B., 1972: Winds in the Ekman layer over St. Louis. In Preprints, Conférence on Urban Environment and Second Conférence on Blometeorology, Amer. Met. Soc, Philadelphia, 22-7.

Ackerman, B., 1974(a) : Wind field over the St. Louis metropolitan area. / . Air Poil. Control Ass., 24, 232-6.

Ackerman, B., 1974(6): METROMEX: Wind fields over St. Louis in undisturbed weather. Bull. Am. Met. Soc, 55, 93-5.

Adamenko, V. N., 1970: Possibilités of Improving Wtnter Micrometeorological Conditions in existing and Planned Urban Areas. Trudy GGO (MGO), 269, 56-72. Translated by A. Nurlik, 1972, Meteorol. Transi. No. 19, Canada. Atmos. Environ. Service, 30 pp.

Adamenko, V. N. and Khairullin, K. Sh., 1972 : Evaluation of conditions under which unprotected parts of the human body may freeze in urban air during winter. Boundary-Layer Met., 2, 510-18.

American Society of Mechanical Engineers, 1968: Recommended Guide for the Prédiction of the Dispersion of Airborne Effluents, 85 pp.

Anderson, D. G., 1967: Effects of Urban Development on Floods in Northern Virginia. U.S. Geological Survey open-file report.

Angell, J. K. and Pack, D. H., 1972: The Kinematics of the Urban Wind Field and Some Forecast Implications. Paper presented at Fourth Conf. on Weather Anal, and Forecasting, 17 pp.

Angell, J. K., Pack, D. H., Dickson, C. R. and Hoeker, W. H., 1971 (a): Urban influence on night-time airflow estimated from tetroon flights. Jour. App. Met., 10, 194-204.

Angell, J. K., Pack, D. H., Hass, W. A. and Hoeker, W. H., 1968: Tetroon flights over New York City. Weather, 23, 184-91.

Angell, J. K., Pack, D. H., Machta, L., Dickson, C. R. and Hoeker, W. H., 1972: Three-dimensional air trajectories determined from tetroon flights in the planetary boundary layer of the Los Angeles Basin. Jour. App. Met., 11,451-71.

Angell, J. K., Pack, D. H., Machta, L., Proulx, R. and Dickson, C. R., 1971(6): Mesoscale trajectories, helicopter oxidant measure-ments and surface wind fields. Conférence Air Poil. Meteorol., Amer. Meteorol. Soc, 119-20.

Antoine, L. H., 1964: Drainage and the best use of urban land. Public Works, 95, 88-90.

Ariel, N. Z. and Kliuchnikova, L. A., 1960: Veter v usloviuakh gorodd (Wind in cities). Glavnaia Geofizicheskaia Obseivatoriia, Trudy No. 94, 29-32. (Trans. in English: Irène A. Dohenoo, U.S. Weather Bureau, 1961).

Arnold, G. and Edgerley, E., 1967: Urban development in air pollution basins — an appeal to the planners for help. / . Air Poil. Cont. Ass., 17, 235-7.

Atkins, J. E., 1968: Changes in the visibility characteristics at Manchester/Ringway Airport. Met. Mag., 97, 172-4.

Atkinson, B. W., 1968: A preliminary examination of the possible effect of London's urban area on the distribution of thunder rainfall, 1951-60. Trans. Inst. British Geogr., 44, 97-118.

Atkinson, B. W., 1969: A further examination of the urban maximum of thunder rainfall in London. Trans. Inst. British Geogr., 44, 97-119.

Atkinson, B. W., 1970: The Reality of Urban Effect on Précipitation — a Case Study Approach. In Urban Climates, Technical Note No. 108, World Meteorological Organization, Geneva, 342-60.

Atkinson, B. W., 1971 : The effect of an urban area on the précipitation from a moving thunderstorm. Jour. App. Met., 10, 47-55.

Atkinson, B. W., 1975: The Mechanical Effect of an Urban Area on Convective Précipitation. Occasional Papers No. 3, Dept. of Geog., Queen Mary Collège, London University, 27 pp.

Atwater, M. A., 1971(o): The radiative budget for parallel layers of the urban environment. Jour. App. Met., 10, 205-14.

Atwater, M. A., 1971(6): Radiative effects of pollutants in the atmospheric boundary layer. Jour. Atmos. Sci., 28,1367-73.

Atwater, M. A., 1972: Thermal changes induced by urbanization and pollutants. In Preprints, Conférence on Urban Environment and Second Conférence on Biometeorology. American Meteorological Society, Philadelphia, 153-8.


Auer, A. H. and Dirks, R. A., 1974: Contributions to an urban meteorological study: METROMEX. Bull. Amer. Meteor. Soc, 55(2), 106-10.

Bach, W., 1969: Solar Radiation and Atmospheric Turbidity. Report prepared for the Kettering Laboratory. University of Cincinnati, Cincinnati.

Bach, W., 1970: An urban circulation model. Arch. Met. Geoph. Biokl. Ser. B, 18, 155-68.

Bach, W., 1971(cr): Variation of solar atténuation with height over an urbanized area. / . Air Poil. Control Assoc, 21, 621-8.

Bach, W., 1971(é): Atmospheric turbidity and air pollution in Greater Cincinnati. Geog. Review, 61, 573-94.

Bach, W., 1971(c): Seven steps to better living in the urban heat island. Landscape Architecture, 61(2), 136-8, 141.

Bach, W., 1972: Urban climate air pollution and planning. In Urbanization and Environment (Ed. Detwyler, T. R. and Marcus, M. G.), Duxbury.

Bach, W. and Patterson, W. A., 1969: Heat budget studies in Greater Cincinnati. Proc. Assoc. Amer. Geographers, 1,7-11.

Baker, D. G., Erz, J. W. and Paulus, H. J., 1969: Frequency, duration, commencement time and intensity of température inversion at St. Paul-Minneapolis. / . App. Meteorol. 8, 747-53.

Band, G., 1969: Der Einfluss der Siedlung auf das Freilandklima. Mitt. Inst. Geophys. Meteorol. Univ. Ko'ln, 9.

Barnes, R. A., 1975 : The Long Distance Transportation of Atmospheric Smoke and Sulphur Dioxide In and Around the United Kingdom. Unpublished Ph. D. Thesis, University of London.

Beebe, R. C. and Morgan, G., 1972: Synoptic analyses of summer rainfall periods exhibiting urban effects. In Preprints, Conférence on Urban Environment and Second Conférence on Biometeorology, Amer. Meteorol. Soc, Philadelphia, 173-6.

Berkes, Z., 1947: A Csapadek Closzlasa Budapest teruleten. Idojaras, 51, 105-11.

Berry, B. J. L., and Horton, F. E., 1974: Urban Environmental Management. Prentice Hall, New Jersey, 425 pp.

Best, R. H., 1968: Extent of urban growth and agriculture displacement in post-war Britain. Urban Studies, 5, 1-23.

Beuchley, R. W., van Bruggen, J. and Truppi, L. E., 1972: Heat island == death island? Environ. Res., 5, 85-92.

Blackadar, A. K., 1962: The vertical distribution of wind and turbulent exchange in a neutral atmosphère. / . Geophys. Res., 67, 3095-102.

Bomstein, R. D., 1968: Observation of the urban heat-island effect in New York City. Jour. App. Met., 7, 575-82.

Bomstein, R. D., 1969: Observed Urban-Rural Wind Speed Différences in New York City. Paper presented at the Am. Geophys. Union Meeting, San Francisco, California. Dec. 1969.

Bomstein, R. D., 1972: Two dimensional non-steady numerical simulations of night-time flow of a stable planetary boundary layer over a rough warm city. In Preprints, Conférence on Urban Environment and Second Conférence on Biometeorology, American Meteorological Society, Philadelphia, 89-94.

Bomstein, R. D., Lorenz, A. and Johnson, D. S., 1972: Récent observations on urban effects on winds and températures in and around New York. In Preprints, Conférence on Urban Environment and Second Conférence on Biometeorology, American Meteorological Society, Philadelphia, 28-33.

Bomstein, R. D. and Tam, Yam-Tong, 1975: Anthropogenic Moîsture Production and its Effect on Boundary Layer Circulations over New York City. Preprint, Conférence on the Urban Physical Environment, Syracuse, New York, August 1975. Lawrence Livermore Laboratory, University of California.

Bowne, N. E., and Bail, J. T., 1970: Observational comparison of rural and urban boundary layer turbulence. Jour. App. Met., 9, 862-73.

Brater, E. F. and Sangal, S., 1969: Effects of urbanization on peak flows. In Effects of Watershed Changes on Streamfiow (Ed. Moore, W. L. and Morgan, C. W.). Texas University Press, pp. 201-214.

Brazell, J. H., 1964: Frequency of dense and thick fog in central London as compared with frequency in outer London. Met. Mag., 93, 129-35.

Brezina, A. and Schmidt, W., 1937: Das kiinstliche Klima in der Umgebung des Menschen. Stuttgart. 212 pp.

Bridger, C. A. and Hefland, L. A., 1968: Mortality from heat during July, 1966, in Illinois. Int. Jour. Biometeorol., 12, 51-70.

REFERENCES 51

Bryson, R. A. and Ross, J. E., 1972: The climate of the city. In Urbanization and Environment (Ed. Detwyler, R. and Marcus, M. G.), Dunbury Press, Belmont, California, 52-68.

Burns, L. S., 1970: Urban planning aspects of air pollution abatement. Project Clean Air II, University of California, Berkeley.

Byers, H. R., 1965: Eléments of cloudphysics. University of Chicago Press, 191 pp.

Carter, R. W., 1961: Magnitude and Frequency of Floods in Suburban Areas. U.S. Geological Survey Prof. Paper 424-B, 9-11.

Carter, R. W. and Thomas, D. M., 1968: Flood Frequency in Metropolitan Areas. In Proc. 4th Amer. Water Resources Conf., New York, 56-67.

Caspar, W., 1973: Introduction to the discussion on climatological data for régional and town planning. In Teaching the Teachers on Building Climatology. Proceedings No. 25, Document D 20, National Swedish Institute for Building Research, 44-8.

Chandler, T. J., 1960: Wind as a factor of urban températures — a survey in north-east London. Weather, 15, 204-13.

Chandler, T. J., 1961 : Surface breeze effects of Leicester's heat island. East Midland Geographer, 15, 32-8.

Chandler, T. J., 1964: City growth and urban climates. Weather, 19, 170-1.

Chandler, T. J., 1965: The Climate of London. Hutchinson, London, 292 pp.

Chandler, T. J., 1967(a): Absolute and relative humidities in towns. Bull. Am. Met. Soc, 48, 394-9.

Chandler, T. J., 1967(6): Night-time températures in relation to Leicester's urban form. Meteor. Mag., 96, 244-50.

Chandler, T. J., 1968: Selected Bibliography on Urban Climate. World Meteorological Organization, Publ. No. 276, T.P. 155, 383 pp.

Chandler, T. J., 1970: Urban climates: Inventory and prospect. In Urban Climates, Technical Note No. 108, World Meteorological Organization, Geneva, 2-14.

Changnon, S. A., 1968: The LaPorte weather anomaly — fact or fiction? Bull. Am. Met. Soc, 49, 4-11.

Changnon, S. A., 1969: Récent studies of urban effects on précipitation in the United States. Bull. Am. Met. Soc, 50, 411-21.

Changnon, S. A., 1970: Récent studies of urban effects on précipitation in the United States. In Urban Climates, Technical Note No. 108, World Meteorological Organization, Geneva, 325-41.

Changnon, S. A., 1971 : Comments on the effect on rainfall of a large steelworks. / . Appl. Meteorol., 10, 165-8.

Changnon, S.A., 1972: Urban effects on thunderstorm and hailstorm frequencies. In Preprints, Conférence on Urban Climates and Second Conférence on Biometeorology, American Meteorological Society, Philadelphia, 177-85.

Changnon, S. A., Huff, F. A. and Semonin, R. G., 1971 : METROMEX: an investigation of inadvertent weather modification. Bull. Am. Meteorol. Soc, 52, 958-67.

Changnon, S. A., Semonin, R. G. and Lowry, W. P., 1972: Results from Metromex. In Preprints, Conférence on Urban Environment and Second Conférence on Biometeorology, American Meteorological Society, Philadelphia, 191-7.

Chopra, K. P. and Pritchard, W. M., 1972: Urban shopping centres as heated islands. In Preprints, Conférence on Urban Environment and Second Conférence on Biometeorology, Amer. Meteorol. Soc, Philadelphia, 310-16.

CIRA (Construction Industry Research and Information Association), 1973: Research CoUoquium on Rainfall, Runoff and Surface Water Drainage of Urban Catchments. Mimeo. papers for Bristol (U.K.) symposium.

Clarke, J. F., 1969: Nocturnal urban boundary layer over Cincinnati, Ohio. Mon. Wea. Rev., 97, 582-9.

Clarke, J. F., 1972(o): Some climatological aspects of heat waves in the contiguous United States. Environ. Res., 5, 76-84.

Clarke, J. F., 1972(6): Some effects of the urban structure on heat mortality. Environ. Res., 5, 93-104.

Clarke, J. F. and Bach, W., 1971 : Comparison of the comfort conditions in différent urban and suburban micro-environments. Int. J. Biometeorol., 15, 41-54.

Clarke, J. F. and Peterson, J. T., 1972: The Effect of Régional Climate and Land Use on the Nocturnal Heat Island. In Preprints, Conférence on Urban Environment and Second Conférence on Biometeorology; Am. Meteorol. Soc, Philadelphia, 147-52.

Clarke, J. F. and Peterson, J. T., 1973: An empirical model using eigenvalues to calculate the temporal and spatial variations of the St Louis heat island. Jour. App. Met., 12, 195-210.


Conrads, L. A. and van der Hage, J. C. H., 1971 : A new method of air-temperature measurement in urban climatological studies. Atmos. Environ., 5, 629-35.

Craddock, J. M., 1965: Domestic fuel consumption and winter températures in London, Weather, 20, 257-8.

Craig, C. D. and Lowry, W. P., 1972: Reflections on the urban albedo. In Preprints, Conférence on Urban Environment and Second Conférence on Biometeorology, Amer. Meteorol. Soc, Philadelphia, 159-64.

Craxford, S. R., 1973 : Air pollution. In Teaching the Teachers on Building Climatology. C.I.B. Proceedings No. 25, Document D 20, The National Swedish Institute for Building Research, pp. 234-39.

Craxford, S. R. and Weatherley, M.-L. P. M., 1964: Air pollution and town planning. In Proc. Clean Air Conf, Harrogate. National Society for Clean Air, London, 54-70.

Crippen, J. R. and Waananen, A. C , 1969 : Hydrologie Effects ofSuburban Development Near Palo Alto, California, U.S. Geological Survey open-file report.

Davenport, A. G., 1965 : The relationship of wind structure to wind loading. In Proc. Conf. Wind Effects on Buildings and Struct., Nat. Phys. Lab., HMSO, London, 53-102.

Davenport, A. G., 1967: Instrumentation and measurements of wind speed spectra in a city. Proc. Ist Canad. Conf. Micromet. Pt. II, Toronto, 361-9.

Davenport, A. G., 1968: The dependence of wind loads on meteorological parameters. In Proc. Symp. Wind Effects on Structures, Univ. Toronto Press, 19-82.

Davenport, A. G., 1973: An approach to human comfort criteria for environmental wind conditions. In Preprint, WMO-CIB Colloquium on Teaching the Teachers on Building Climatology, Vol. 1, Statens Inst. fôr Byggnadsforskning, Stockholm.

Davidson, B., 1967: A summary of the New York urban air pollution dynamic research program. Jour. Air Poil. Control Assoc, 17, 154-8.

De Boer, H. J., 1966: Atténuation ofSolar Radiation Due to Air Pollution in Rotterdam and ils Surroundings. Koninklijk Nederlands Meteorologisch Institute, Wetenschappelijk Rapport W. R. 66-1, de Bilt, Nederlands, 36 pp.

Deland, R. J. and Binkowski, F. S., 1966: Comparison of wind at 500 feet over Minneapolis and Louisville with geostrophic wind. J. Air Poil. Control Assoc, 16, 407-11.

De Marrais, G. A., 1959: Wind-speed profiles at Brookhaven National Laboratory. / . Meteorol., 16, 181-90.

De Marrais, G. A., 1961: Vertical température différences observed over an urban area. Bull. Am. Met. Soc, 42, 548-54.

Detwiller, J., 1970: Incidence possible de l'activité industrielle sur les précipitations à Paris. In Urban Climates, Technical Note No. 108, World Meteorological Organization, Geneva, 361-2.

Detwyler, T. R. and Marcus, M. G. (Eds.), 1972: Urbanization and Environment. Duxbury Press, Belmont, California, 287 pp.

Dickert, T. G. and Domeny, K. R., 1974: Environmental Impact Assessment, Cuidelines and Commentary. University of California, Berkeley.

Dronia, H., 1967: Der Stadteinfluss auf den weltweiten Temperaturtrend. Meteorologische Abhandlungen, 74, 1.

Duckworth, F. S. and Sandberg, J. S., 1954: The effect of cities upon horizontal and vertical température gradients. Bull. Am. Met. Soc, 35, 198-207.

Duprey, R. L., 1968: Compilation of air pollutant émission factors. USPHS Pub. No. 999-AP-42.

East, C , 1968: Comparaison du rayonnement solaire en ville et à la campagne. Cahiers de Géog. de Québec, 12, 81-9.

East, C , 1971: Chaleur urbaine à Montréal. Atmosphère, 9, 112-22.

Emonds, H., 1954: Das Bonner Stadtklima. Bonn Universitât, Geographisches Institut, Arbeiten zur Rheinischen Landeskunde, 7, 64 pp.

Espey, W. H., Morgan, C. W. and Masch, F. D., 1966: A Study ofSome Effects of Urbanization on Storm Runoff from a Small Watershed. Texas Water Development Board Report 23.

Estoque, M. A., 1963: A numerical model of the atmospheric boundary layer. Jour. Geophys. Res., 68, 1103-13.

Estoque, M. A. and Bhumralkar, C. M., 1968: Theoretical Studies ofthe Atmospheric Boundary Layer. Res. and Dev. Tech. Rep., ECOM-G7G2-F, 46 pp.

REFERENCES 53

Fonda, R. W., Dahms, J. E., Fralick, J. E. and Kendall, K. M., 1971 : Heat islands and frost pockets in Bellingham, Wash. Bull. Amer. Meteorol. Soc, 52, 552-5.

Fosberg, M. A., Rango, A. and Marlatt, W. E., 1972: Wind computations from the température field in an urban area. In Preprints, Conférence on Urban Environment and Second Conférence on Biometeorology, Amer. Meteorol. Soc, Philadelphia, 5-7.

Frederick, R. H., 1964: On the representativeness of surface wind observations using data from Nashville, Tennessee. Int. Jour. Air Water Poil., 8, 11-19.

Freeman, M. H., 1968: Visibility statistics for London/Heathrow Airport. Met. Mag., 97, 214-18.

Frenkiel, F. N., 1956: Atmospheric pollution and zoning in an urban area. Scientific Monthly, 82(4), 194-203.

Fuggle, R. F., 1971: Nocturnal Atmospheric Infra-Red Radiation in Montréal. Unpublished Ph.D. Thesis, McGill University.

Fuggle, R. F., 1972: Rural/urban variations in atmospheric counter radiation. South Afr. Geogr. Jour., 54, 138-44.

Fuggle, R. F. and Oke, T. R., 1970: Infra-red flux divergence and the urban heat island. In Urban Climates, Technical Note No. 108, World Meteorological Organization, Geneva, 70-8.

Fuller, H. I., 1973: Review of European air pollution control measures. In T. McManus (Ed.), Report of the Symposium on Air Pollution, Dublin, 1972, pp. 7-38. Technical Inf. Div., Institute for Industrial Research and Standards, Dublin.

Garnett, A. and Bach, W., 1965: An estimation of the ratio of artificial heat génération to natural radiation heat in Sheffield. Mon. Weather Rev., 93, 383-5.

Garnier, B. J. and La Fleur, 1972: A study of human comfort in Montréal, Canada. In Preprints, Conférence on Urban Environment and Second Conférence on Biometeorology, Amer. Meteorol. Soc, Philadelphia, 274-9.

Geiger, R., 1969: The Climate Near the Ground. Harvard, 611 pp.

Georgii, H. W., 1970: The Effects of Air Pollution on Urban Climates. In Urban Climates, Technical Note No. 108, World Meteorological Organization, Geneva, 214-37.

Gifford, F. A. and Hanna, S. R., 1970: Urban air pollution modelling. Proc. Inst. Air Poil. Conf. oflUAPPA, Washington.

Givoni, B., 1973: Effect of town planning on the urban climate. In Preprints, WHO-CIB CoUoquium on Teaching the Teachers on Building Climatology, Vol. 1, Statens Inst. fôr Byggnadsforskning, Stockholm.

Givoni, B., 1974: Architectural and urban planning in relation to weather and climate. In Progress in Bioclimatology. Tromp, S. W. and Boama, J. J. (Eds.), Swats and Zeitlinger, Amsterdam, 183-93.

Gold, E., 1956: Smog: the rate of influx of surrounding cleaner air. Weather, 11, 230-2.

Goldreich, Y., 1970: Computation of the magnitude of Johannesburg's heat island. Notos, 19, 95-106.

Goldreich, Y, 1974: Observations on the urban humidity island in Johannesburg. Israël J. ofEarth Sci., 23, 39-46.

Gol'tsberg, L. A., 1967: Microclimate of the U.S.S.R. Gidrometeorologicheskoe IzdateVstvo, Leningrad, 1967. Translated English Israël Programme for Scientific Translations, Jérusalem, 1969.

Graham, I. R., 1968: An analysis of turbulence statistics at Fort Wayne. Indiana. Jour. App. Met., 7, 90-3.

Green, C. R. and Battan, L. J., 1967: A study of visibility versus population growth in Arizona. J. Ariz. Acad. Sci., 4, 226.

Gunn, R. and Phillips, B. B., 1957: An expérimental investigation of the effect of air pollution on the initiation of rain. / . Meteor., 14, 272.

Hage, K. D., 1972: Nocturnal températures in Edmonton, Alberta. / . Appl. Meteorol., 11, 123-9.

Hagevik, G., Mandelkcr, D. and Brail, R., 1974: Air Quality Management and Land Use Planning. Praeger, London.

Hammer, T. R., 1973: Impact of Urbanization on Peak Streamflow. Régional Science Research Institute, Discussion Paper, 63, 77 pp.

Hand, I. F., 1943: Transmission of the total and the infra-red component of solar radiation through a smoky atmosphère. Bull. Am. Met. Soc, 24, 201-4.

Hanna, S. R., 1971 : Simple methods of calculating dispersion from urban area sources. Journal Air Pollution Control Association.

Harman, J. R. and Elton, W. M., 1971 : The La Porte, Indiana, précipitation anomaly. Annals Assoc. Amer. Geogr., 61, 468-80.


Harris, E. E. and Rantz, S. E., 1964: Effect of Urban Growth on Streamflow Régime of Permanerte Creek, Santa Clara County, California. U.S. Geological Survey Water Supply Paper, 1591-B.

Hass, W. A., Hœcker, W. H., Pack, D. H. and Angell, J. K., 1967: Analysis of low-level, constant volume balloon (tetroon) flights over New York City. Quart. J. R. Met. Soc, 93, 483-93.

Helliwell, N. C , 1973: Wind over London. In Preprints, WMO-CIB Colloquium on Teaching the Teachers on Building Climatology, Vol. 2, Statens Instit. fôr Byggnadsforskning, Stockholm, 32.1-32.10.

Henschel, A., Burton, L., Margolies, L. and Smith, J. E., 1968: An analysis of heat deaths in St. Louis during July 1966. Env. Contr. Admin., Cincinnati, 22 pp.

Herrington, L. P., Bertolin, G. E. and Léonard, R. E., 1972: Microclimate of a suburban park. In Preprints, Conférence on Urban Environment and Second Conférence on Biometeorology. Amer. Meteorol. Soc, Philadelphia, 43-4.

Hewson, E. W., 1970: Moisture pollution of the atmosphère by cooling towers and cooling ponds. Bull. Amer. Meteorol. Soc, 51, 21-2.

Hill, A. C , 1971 : Végétation: a sink for atmospheric pollutants. / . Air Poil. Cont. Ass., 21, 341-6.

Hill, M., 1968: A goals-achievement matrix for evaluating alternative plans. Journal American Institute of Planning, 34, 19-28.

Holland, D. T., 1972: EfFects of the urban heat island upon meteorological parameters. Aerospace Sciences Review, 72, 8-10.

Hollis, G. E., 1975: The effect or urbanization on floods of différent récurrence interval. Water Resources Research, 11(3), 431-5.

Holzman, B. G. and Thom, H. C. S., 1970: The La Porte précipitation anomaly. Bull. Am. Meteorol. Soc, 51, 335-7.

Huff, F. A. and Changnon, S. A., 1972: Climatological assessment of urban effects on précipitation at St. Louis. / . Appl. Meteorol., II, 823-42.

Jaske, R. T., Fletcher, J. F. and Wise, K. R., 1970: A National Estimate of Public and Industrial Heat Rejection Requirements by Décades Through the Year 2000 A.D. Paper presented at the American Institute of Chemical Engineers 67th National Meeting, Atlanta. Quoted by Landsberg, H. E., 1970, Man-Made Climatic Changes, Science, 170(3964), 1265-74.

Jaurcgui, E., 1972: Effect of a large greenarea on Mexico City's climate. AbstractS.G.S.inSuppl. toVol. 16 of Int. J. Biometeorol., 67-8.

Jenkins, L, 1969: Increase in averages of sunshine in central London. Weather, 24, 52-4.

Jensen, I. M., 1958: The model-law for phenomena in natural wind. International Edition, Copenhagen, 121-8.

Johnson, D. S. and Bornstein, R. D., 1974: Urban-rural wind velocity différences and their effects on computed pollution concentrations in New York City. In Preprints of Am. Met. Soc. Symp. on Atmos. Diff. and Air Poil., Santa Barbara, California, Sept. 1974.

Johnson, W. B., Ludwig, F. L. and Moon, A. E., 1969: Development of a practical, multipurpose urban diffusion model for carbon dioxide. Proc of Symp. on Multiple-Source Urban Diffusion Models, USEPA.

Jones, P. M., de Larringa, M. A. B. and Wilson, C. B., 1971 : The urban wind velocity profile. Atmos. Environ., 5, 89-102.

Kinosita, T. and Sonda, T., 1969: Change of runoff due to urbanization. In Floods and their Computation vol. 2, Proceedings of the Unesco Symposium in Leningrad, August, 1967, pp. 787-96.

Khemani, L. T. and Murty, B. R., 1973: Rainfall variations in an urban industrial région. / . App. Met., 12, 187-97.

Kopec, R. J., 1970: Further observations of the urban heat island in a small city. Bull. Amer. Meteorol. Soc, 51, 602-6.

Kopec, R. J., 1973: Daily spatial and secular variations of atmospheric humidity in a small city. / . Appl. Meteor., 12, 639-48.

KOppen, W., 1931 : Die Klimate der Erde. Berlin.

Kratzer, A., 1937: Das Stadtklima (lst éd.). Die Wissenschaft, 90, Braunschweig, 143 pp.

Kratzer, A., 1956: Das Stadtklima (2nd éd.). Friedrich Vieweg und Sohn, Braunschweig, 184 pp.

Kung, E. C , Bryson, R. A., Lenschow, D. H., 1964: Study of a continental surface albedo on the basis of flight measurements and structure of the Earth's surface cover over North America. Mon. Weather Rev., 92, 543-64.

Lacy, R. E., 1972: Survey of Meteorological Information for Architecture and Building. Current Paper CP 5/72, Bldg. Res. Stn., Department of the Environment, London, 22 pp.

REFERENCES 55

Lacy, R. E., 1973: General data. In Teaching the Teachers on Building Climatology, Proceedings No. 25, Document D20, National Swedish Institute for Building Research, 183-7.

Lamb, R. G. and Neiburger, M., 1971: An intérim version of a generalised urban air pollution model. Atmos. Env., 5, 239-64.

Landsberg, H. E., 1938: Atmospheric condensation nuclei. Erg. Kosm. Physik., 3, 155-252.

Landsberg, H. E., 1947: Micrometeorology. Architectural Forum, 86(3), 114-9.

Landsberg, H. E., 1956: The climate of towns. In Man's Rôle in Changing the Face of the Earth. (Ed. W. L. Thomas), University of Chicago Press, 584-606.

Landsberg, H. E., 1962: City Air — Better or Worse. In Symposium : Air Over Cities. U.S. Public Health Service, Taft Sanitary Eng. Center, Cincinnati, Ohio, Technical Rept., A62-5, 1-22.

Landsberg, H. E., 1970(o): Micrometeorological température differentiation through urbanization. In Urban Ctimates, Technical Note No. 108, World Meteorologicai Organization, Geneva, 129-36.

Landsberg, H. E., 1970(6): Climates and Urban Planning. In Urban Climates, Technical Note No. 108, World Meteorologicai Organization, Geneva, 364-71.

Landsberg, H. E., 1970(c): Man-made climatic changes. Science, 170(3964), 1265-74.

Landsberg, H. E., 1972(a): Inadvertent Atmospheric Modification Through Urbanization. Tech. Note No. BN 741, Institut. Fluid Dyn. Appl. Math., Univ. Maryland, 73 pp.

Landsberg, H. E., 1972(6): The Assessment ofHuman Bioclimat e: A LimitedReview ofPhysical Parameters. Technical Note No. 123, World Meteorologicai Organization, Geneva, 36 pp.

Landsberg, H. E. and Maisel, T. N., 1972: Micrometeorological observations in an area of urban growth. Boundary-Layer Meteorol., 2, 365-70.

Leahey, D. M., 1969: An Urban Heat-Island Model. Report TR-69-11, Department of Meteorology and Oceanography, New York University, 70 pp.

Leahey, D. M. and Friend, J. P., 1971 : A method for predicting the depth of the mixing layer over an urban heat island with application to New York City. Jour. App. Met., 10, 1162-73.

Lee, D. O., 1975(a): A Study ofthe Influence of Surface Roughness and Température of Cities on Urban Airflow. Unpublished Ph. D. Thesis, University of London.

Lee, D. O., 1975(6): Rural atmospheric stability and the intensity of London's heat island. Weather, 30(4), 102-8.

Leopold, L. B., 1968 : Hydrologyfor Urban Land Planning — A Guidebook on the Hydrologie Effects of Urban Land Use. Washington, D.C., Geological Survey Circular, 554.

Lettau, H. H., 1970: Physical and Meteorologicai Basis for Mathematical Models of Urban Diffusion Processes. In Proc. ofSymp. of Multiple-Source Urban Diffusion Models, USEPA, APCO Pub. No. AP-86.

Lettau, H. H. and Lettau, K., 1969: Shortwave radiation climatonomy. Tellus, 21, 208-28.

Lindqvist, S., 1968: Studies in the local climate in Lund and its environs. Geogr. Ann„ 50, Ser. A., 79-93.

Ludwig, F. L., 1970: Urban Température Fields. In Urban Climates, Technical Note No. 108, World Meteorologicai Organization, Geneva, 80-107.

Ludwig, F. L. and Kealoha, J. H. S., 1968: Urban Climatological Studies. Final Rept., Contract OCD-DANC-20-67-C-0136. Stanford Research Institute, Menlo Park, California.

Lull, H. W. and Sopper, WE., 1969: Hydrologie Effects from Urbanization of Forested Watersheds in the Northeast. U.S.D.A. Forest Service Research Paper NE-146, 31 pp.

McCormick, R. A., 1971: Air pollution in the locality of buildings. Phil. Trans. Roy. Soc. Land. A, 269, 515-26.

McCormick, R. A. and Baulch, D. M., 1962: The variation with height ofthe dust loading over a city as determined from the atmospheric turbidity. Bull. Air. Poil. Contr. Assoc, 12, 492-6.

McCormick, R. A. and Kurfis, K. R., 1966: Vertical diffusion of aérosols over a city. Quart. Jour. Roy. Met. Soc, 92 393-6.

McEIroy, J. L., 1972(o): A Numerical Model ofthe Nocturnal Urban Boundary Layer. Proceedings ofthe Symposium on Air Pollution, Turbulence and Diffusion, Las Cruces, New Mexico 1971.


McElroy, J. L., 1972(6): Effects of alternate land use stratégies on the structure of the nocturnal urban boundary layer. In Preprints, Conférence on Urban Environment and Second Conférence on Biometeorology, American Meteorological Society, Philadelphia, 185-90.

MacHattîe, L. B. and Schnelle, F., 1974: An Introduction to Agrotopoclimatology. Technical Note No. 133, World Meteorological Organization, Geneva, 131 pp.

McNulty, R. P., 1968: The effect of air pollutants on visibility in fog and haze at New York City. Atmos. Env., 2, 625.

Marsh, K. J. and Forster, M. D., 1967: An expérimental study of the dispersion émission fromchimneys in Reading—I. The study of long tenu average concentrations of sulphur dioxide. Atmos. Emir. 1, 527-50.

Marsh, K. J. and Withers, V. R. 1969: An expérimental study of the dispersion of the émissions from chimneys in Reading — III: the investigation of dispersion calculations. Atmos. Envir., 3, 281-302.

Martens, L. A., 1966: Flood Inundations and Effects of Urbanization in Metropolitan Charlotte (North Carolina). U.S. Geological Survey open file report.

Mason, B., 1966: The rôle of meteorology in the national economy. Weather, XXI, No. 11, 383-93.

Mateer, C. L., 1961 : Note on the effect of the weekly cycle of air pollution on solar radiation at Toronto. Int. Jour. Air Water Poil., 4, 52-4.

Maunder, W. J., 1970: The Value of the Weather, Methuen, London, 388 pp.

Maurain, C. H., 1947: Le Climat Parisien. Presses Universitaires, Paris, 163 pp.

Meetham, A. R., 1945: Atmospheric Pollution in Leicester. D.S.I.R. Technical Paper No. 1, H.M.S.O., London, 161 pp.

Miller, E. G., 1966: Effect of Urbanization on Low Flows. U.S. Geological Survey Prof. Paper 550-A., pp. 166.

Miller, E. L., Johnston, R. E. and Lowry, W. P., 1972: The case of the muddled metromodel. In Preprints, Conférence on Urban Environment and Second Conférence on Biometeorology. Am. Met. Soc, Philadelphia, 77-82.

Mills, W. B., 1968 : Effect of Urbanization and Runoff. U.S. Geological Survey Prof. Paper, 550-A.

Ministry of Housing and Local Government, 1955: The Use of Land for Industry. Technical Mémorandum 2, H.M.S.O., London.

Mitchell, J. M., 1953: On the causes of instrumentally observed secular température trends. Jour. Met. 10, 244-61.

Mitchell, J. M., 1961 : The température of cities. Weatherwise, 14, 224-9.

Mitchell, J. M., 1962: The thermal climate of cities. In Symposium: Air Over Cities. U.S. Public Health Service, Taft Sanitary Eng. Center, Cincinnati, Ohio, Technical Rept. A 62-5, 131-43.

Monteith, J. L., 1962: Atténuation of solar radiation: a Climatological Study. Quart. J. Roy. Met. Soc, 92, 254-62.

Moore, D. J., 1975: A simple boundary-layer model for predicting time mean ground Ievel concentrations of material emitted from tall chimneys. Proc. Instn. Mech. Engrs. 189, 33-43.

Morgan, D. L. and Rogers, D. L., 1972: Texture of a city: the foundation of urban meteorology. In Conférence on Urban Environment and Second Conférence on Biometeorology, Amer. Meteorol. Soc, 295-302.

Motto, H. L., 1970: Lead in soils and plants: its relationship to traffic volume and proximity to highways. Env. Science Tech., 4(3), 231-51.

Muller, R. D., 1968: Some effects of urbanization on run-off as evaluated by Thornthwaite water balance models. Proc. 3rd Ann. Conf. Amer. Water Res. Assoc, 15 pp.

Munn, R. E., 1970: Airflow in Urban Areas. In Urban Climates, Technical Note No. 108, World Meteorological Organization, Geneva, 15-39.

Munn, R. E., 1973: Urban meteorology: some selected topics. Bull. Am. Met. Soc, 54, 90-3.

Munn, R. E. and Stewart, I. M., 1967: The use of meteorological towers in urban air pollution programs. Jour. Air Poil. Control Assoc, 17, 98-101.

Myrup, L. O., 1969: A numerical model of the urban heat island. / . Appl. Meteorol., 8, 908-18.

Myrup, L. O., 1970: Corrigendum. / . Appl. Meteorol., 9, 541.

REFERENCES 57

Myrup, L. O. and Morgan, D. L., 1972: Numerical model of the urban atmosphère. Vol. 1 : The city-surface interface. Contributions in Atmos. Science. No. 4, Univ. of California, Davis, 237 pp.

Nader, J. S., 1967: Pilot Study of Ultraviolet Radiation in Los Angeles, October 1965. Public Health Service Publication, 999-AP-38. Nat. Center for Air Poil. Control, Cincinnati, Ohio.

Nakamura, K., 1966: City température of Nairobi. / . Geog. Japan, 75, 316-7.

Nappo, C. J., 1972: A Numerical Study of the Urban Heat Island. In Preprints, Conférence on Urban Environment and Second Conférence on Biometeorology, Amer. Meteorol. Soc, Philadelphia, 1-4.

Neuberger, H., 1967: Végétation as aérosol filter. Biometeorology 2, Proc. 3rdlnt. Congress, Pergamon Press, New York, 693-702.

Neuberger, H., Hosler, C. L. and Kochmond, W. C , 1967: Végétation as an aérosol filter. Biometeorology, Proc. 3rd Int. Congr. Biometeorology, 2, 693-702.

Nieuwolt, S., 1966: The urban microclimate of Singapore. / . Trop. Geog., 22, 30-7.

Norwine, J. R., 1972: Heat-island properties of an enclosed multilevel shopping center. In Preprints, Conférence on Urban Environment and Second Conférence on Biometeorology, Amer. Meteorol. Soc, Philadelphia, 139-43.

O.E.C.D., 1972: Air managementproblems andrelated technical studies. Paris.

Ogden, T. L., 1969: The effect on rainfall of a large steel works. / . Appl. Meteorol., 8, 585-91.

Oguntoyinbo, J. S., 1970: Reflection coefficient of natural végétation crops and urban surfaces in Nigeria. Quart. J. R. Met. Soc, 96, 430-41.

Oke, T. R., 1972(a) : City size and the urban heat island. In Preprints, Conférence on Urban Environment and Second Conférence on Biometeorology, Amer. Meteorol. Soc, Philadelphia, 144-6.

Oke, T. R., 1972(6): Evapotranspiration in urban areas and its implications for urban climate planning. In Preprints, C.I.B. Colloquium, Teaching the Teqchers on Building Climatology, Stockholm, Sept. 4-6, 1972, 9 pp.

Oke, T. R., 1974: Review of Urban Climatology, 1968-1973. Technical Note No. 134, World Meteorological Organization, Geneva, 132 pp.

Oke, T. R. and East, C , 1971 : The urban boundary layer in Montréal. Boundary Layer Meteorol., 1, 411-37.

Oke, T. R. and Fuggle, R. F., 1972: Comparison of urban/rural counter and net radiation at night. Boundary Layer Met., 2, 290-307.

Oke, T. R. and Hannell, F. G., 1970: The form of the urban heat island in Hamilton, Canada. In Urban Climates, Technical Note No. 108, World Meteorological Organization, Geneva, 113-26.

Oke, T. R., Maxwell, G. B. and Yap, D., 1972(a): Comparison of urban/rural cooling rates at night. In Preprints, Conférence on Urban Climates and Second Conférence on Biometeorology. Amer. Meteorol. Soc, Philadelphia, 17-21.

Oke, T. R., Yap, D. and Fuggle, R. F., 1972(6): Détermination of urban sensible heat fluxes. In International Geography, 1972 (Ed. Adams, W. P. and Helleiner, F. M.), Univ. of Toronto Press, 176-8.

Okita, T., 1960: Estimation of the direction of air flow from observations of rime ice. Jour. Met. Soc. Japon, 38, 207-9.

Oujezdsky, T. M., 1973: Diurnal and Seasonal Change of the Urban Heat Island in Austin, Texas. Report No. 34, Atmospheric Science Group, Collège of Engineering, University of Texas.

Page, J. K., 1964: Air pollution and town planning. In Proceedings of the Clean Air Conférence, Harrogate, National Society for Clean Air, London, 71-81.

Page, J. K., 1973(a): Selecting an air pollution control policy. In T. McManus (Ed.), Report of the Symposium on Air Pollution, Dublin, 1972, pp. 39-65, Technical Inf. Div., Institute for Industrial Research and Standards, Dublin.

Page, J. K., 1973(6): Energy, environment and planning. In Fuel and the Environment, Proceedings, Conférence, The Institute of Fuel, Vol. 1, 157-65.

Panofsky, H. A., 1971 : Comments on "The urban wind velocity profile". Atmos. Environ., 5, 733.

Panofsky, H.A.andTownsend, A. A., 1964: Change in terrain roughness and the wind profile. Quart. J. Roy. Met. Soc, 90,147-55.


Parry, M., 1956: Local température variations in the Reading area. Quart. J. Roy. Met. Soc, 82, 45-57.

Parry, M., 1970: Sources of Reading's air pollution. In Urban Climates, Technical Note No. 108, World Meteorological Organiza-tion, Geneva, 295-305.

Pasquill, F., 1970(a): Wind structure in the atmospheric boundary layer. Phil. Traits. Roy. Soc, Ser. A. 269, No. 1199.

Pasquill, F., 1970(6): Prédiction of diffusion over an urban area — Current practice and future prospects. In Proc. Symp. on Multiple Source Urban Diffusion Models. U.S. Env. Protection Agency.

Pelle, W. J., 1964: Bibliography on the Planning Aspects of Air Pollution Control, Summary and Evaluation. Public Health Service, USGPO, Washington.

Peschier, J., 1973: Wind and Température Profiles in an Urban Area. Atmos. Sci. Group. Report No. 33, Univ. of Texas, 32 pp.

Peterson, J. T., 1969: The Climate ofCities: A Survey of Récent Literature. U.S. Dept. of Health, Education and Welfare, National Air Polln. Control Admin., Rayleigh, N.C., 48 pp.

Peterson, J.T., 1970: Distribution of sulphur dioxide over Metropolitan St. Louis as descnbed by empirical eigenvectors and its relation to meteorological parameters. Atmos. Environ., 4, 501-18.

Peterson, J.T., 1972: Calculation of sulphur dioxide concentrations over Metropolitan St. Louis. Atmos. Environ., 6, 433-42.

Pool, T. R., 1970: Some aspects of the urban climate of Chichester. The South Hants Geog., 3, 25-32.

Pooler, F., 1963: Airflow over a city in terrain of moderate relief. Jour. ofApp. Met., 2, 446-56.

Potter, J. G., 1961 : Changes in seasonal snow fall in cities. Canadian Geography, 5, 37.

Probald, F., 1972: Déviations in the heat balance: the basis of Budapest's urban climate. In International Geography, 1972 (Ed. Adams, W. P. and Helliner, I. M.), Univ. of Toronto Press, 184-6.

Programmes Analysis Unit, 1972: An Economie and Technical Assessment of Air Pollution in the United Kingdom. Report M 20, P.A.U., M 20, H.M.S.O., London.

Quitt, E., 1972: Mesoklima im Umweltkomplex (Mesoclimate in the complex of environment). Studia Geographica, 26, Brno, C.S.S.R.

Raman, C. R. V. and Kelkar, L. L., 1972: Urban air pollution potential over Bombay, India, derived from mixing depths and mean layer wind. In Preprints, Conférence on Urban Environment and Second Conférence on Biometeorology, Amer. Meteorol. Soc, Philadelphia, 303-9.

Rao, P. K., 1972: Remote sensing of urban heat islands from an environmental satellite. Bull. Amer. Meteorol. Soc, 53, 647-8.

Rathbun, J. C , 1940: Wind forces on tall buildings. Trans. Am. Soc Civ. Engng., 105, 1.

Reagan, R., Livermore, N. B. and Steans, J. C , 1971: Environmental Impact of Urbanization on the Foothills and Mountainous Lands of California. The State of California Resources Agency and Dept. of Conservation.

Reed, L. E., 1966: Vehicle exhausts in relation to public health. The Royal Society of Health Journal, 86, 231.

Reiquam, H., 1970: Preliminary trial of a model in the Oslo airshed. Proc of Second Int. Clean Air Congress, IUPPA, Washington, D.C., 6-11.

Roach, W. T., 1961 : Some aircraft observations of fluxes of solar radiation in the atmosphère. Quart. J. R. Met. Soc, 87, 346-63.

Roberts, J. J., Croke, E. J. and Kennedy, A. S., 1970: An urban atmospheric dispersion model. In Proc Symp. Multi-Source Urban Diff. Models, U.S. Environ. Protect. Agency, Research Triangle Park, 6.1-6.72.

Robins, A. G., 1975: Expérimental model techniques for the investigation of the dispersion of chimney plumes. Proc Instn. Mech. Engrs., 189, 44-54.

Rouse, W. R. and McCutcheon, J. G., 1972: The diurnal behaviour of incoming solar and infra-red radiation in Hamilton, Canada. International Geography 1972, Toronto University Press, 191-6.

Ryd, H., 1973 : Use of climatological data in building planning with respect to comfort. In Teaching the Teachers on Building Climat-ology, CIB Proceedings No. 25, Document D20, National Swedish Institute for Building Research, 49-89.

Rydell, C. P. and Schwarz, G., 1968: Air pollution and urban form: a review of current literature. Journal American Institute of Planning, 34. 115-20.

REFERENCES 59

Sargent, F. and Tromp, S. W., 1964: A Survey ofHuman Biometeorology. Technical Note No. 65, World Meteorological Organiza-tion, Geneva, 113 pp.

Sasakura, J., 1965: On the distribution of relative humidity in Tokyo and its secular change on the heat of Tokyo. Tokyo Jour. Climatol., 2, 45-6.

Saule, P. L., 1971 : Studies of Urban Runoffand Floods — Virginia. U.S. Geological Survey Prof. Paper, 750-A.

Saunders, P. J. W. and Wood, C.-M., 1974: Plants and air pollution. Landscape Design, 105, 28-30.

Sawyer, R. M., 1963: Effect of Urbanizatlon on Storm Discharge and Ground-Water Recharge in Nassau County, New York. U.S. Geological Survey Prof. Paper, 475-C, pp. 185-187.

Schaefer, V. J., 1968: Ice nuclei from auto exhaust and organic vapours. Jour. App. Meteor. 7, 113.

Schmidt, F. H. and Velds, C. A., 1969: On the relation between changing meteorological circumstances and the decrease of the sulphur dioxide concentration around Rotterdam. Atmos. Environ., 3, 455-60.

Schmauss, A., 1927: Grosstâdte und Niederschlag. Met. Zeit. 64, 339-41.

Schueneman, E., 1968 : Planning and zoning in air pollution control. In Stcrn, A.C. (Ed.), Air Pollution, 2, Académie Press, London.

Schuman, S. H., Anderson, C. P. and Oliver, J. T., 1964: Epidemiology of successive heat waves in Michigan, 1962 and 1963. / . Amer. Med. Assoc, 189, 733-8.

Scorer, R. S., 1972: Air Pollution. Pergamon Press, London, 148 pp.

Sekiguti, T., 1970: Thermal situations of urban areas, horizontally and vertically. In Urban Climates, Technical Note No. 108, World Meteorological Organization, Geneva, 137-40.

Sekiguti, T., 1972: Thermal environment of residential new towns. In Preprints WMO-CIB CoUoquium Teaching the Teachers on Building Climatology, Vol. 3, Statens Instit. fdr Byggnadsforskning, Stockholm, 65.1-65.8.

Sekiguti, T., Shinha, T., Muto, S., Tamiya, H., Kobayshi. M. and Kawakami, K, 1972: Local température observations for a basin and city climate complex. Tokyo Geogr. Paper s, 16, 79-93.

Sharon, D. and Koplowitz, R., 1972: Observations of the heat island of a small town. Meteorol. Rundschau, 25, 143-6.

Shellard, H. C , 1965: The estimation of design wind speeds. In Proc. Int. Conf. on WindEffects on Buildings and Structures., Nat. Phys. Lab., Teddington, England, London, H.M.S.O., 29-51.

Shellard, H. C , 1967: Wind records and their application to structural design. Met. Mag., 96, 235-43.

Shellard, H. C , 1968: Results of some récent spécial wind measurements in the United Kingdom relevant to wind loading problems. In Proc. Int. Seminar. WindEffects on Buildings and Structures, Nat. Res. Cncl., Ottawa, Vol. I, 515-35.

Sheppard, P. A., 1958 : The effect of pollution on radiation in the atmosphère. Int. Jour. Air Poil., 1, 31-43.

Sherry, J. P., Achinger, W. C. and Simon, R. A., 1969: Handbook ofAir Pollution. U.S. Government Printing Office, Washington.

Shiotani, M. and Yamamoto, G., 1950: Atmospheric turbulence over the large city turbulence in the free atmosphère. Geophys. Mag., 21, Tokyo, 134-47.

Shitara, H., 1959: The minute diurnal fluctuation of air température and its vertical profile in a built-up area. Geogr. Rev. Japon, 32, 313-9.

Singer, T. A. and Smith, M. E., 1970: A summary of the Recommended Guide for the Prédiction of the Dispersion of Air-Borne Effluents. In Urban Climates, Technical Note No. 108, World Meteorological Organization, Geneva, 306-24.

Skelton, J., 1972: Studies of Urban Runoffand Floods in Missouri. U.S. Geological Survey. Prof. Paper 800A.

Smith, M., 1968: Recommended Guide for the Prédiction ofthe Dispersion of Airborne Effluents. ASME, New York, 85 pp.

Soma, S., 1964: The properties of atmospheric turbulence in high winds. / . Met. Soc. Japon, 42, 372.

Spangler, T. C. and Dirks, R. A., 1972: Mesoscale variations ofthe urban mixing layer depth. In Preprints, Conférence on Urban Environment and Second Conférence on Biometeorology, Am. Met. Soc, Philadelphia, 37-42.

Squires, P., 1966: An estimate ofthe anthropogenic production of cloud nuclei. Jour. Rech. Atmos., 2-3, 297-308.

Stair, R., 1966: The measurement of solar radiation, with principal emphasis on the ultraviolet component. Int. J. Air Water Poil., 10, 665-88.

60 URBAN CLIMÀTOLOGY AND ITS RELEVANCE TO URBAN DESIGN

Steinhauser, J., Eckel, O. and Sauberer, F., 1955: Klima undBioklima von Wien. Pr I, Wetter undLeben, 120 pp.

Steinhauser, J., Eckel, O. and Sauberer, F., 1959: Ibid., Pt. 3, 136 pp.

Stern, A. C. (Ed.), 1968: Air Pollution (3 vols). Académie Press, London (2nd édition).

Summers, P. W., 1965: An Urban Heat-Island Model — Us Soie in Air Pollution Problems, with Application to Montréal. Paper presented to the First Canadian Conférence on Micrometeorology, Toronto, 22 pp.

Sundborg, A., 1950: Local clunatological studies of the température conditions in an urban area. Tellus, 2,222-32.

Sutton, O. G., 1947: The problem of diffusion in the lower atmosphère, Quart. Jour. Roy. Met. Soc, 73,257-81.

Taesler, R., 1973(a): The relevance of urban climate to building and planning. In Preprints, WHO-CIB Colloquium on Teaching the Teachers on Building Climatology, Vol. 3, Statens Inst. fôr Byggnadsforskning, Stockholm.

Taesler, R., 1973(6): General clunatological data for planning and building — Summary and discussion. In Teaching the Teachers on Building Climatology, Proceedings No. 25, Document D20, National Swedish Institute for Building Research, 188-203.

Taylor, J. A. (Ed.), 1972: Weather Forecasting for Agriculture and Industry. David and Charles, Newton Abbot, 250 pp.

Taylor, J. A. (Ed.), 1974: Climatic Resources and Economie Activity. David and Charles, Newton Abbot, 264 pp.

Taylor, J. R., Hasegawa, A. and Chambers, L. A., 1961 : Control of air pollution by site sélection and zoning. In Air Pollution, World Health Organization, Geneva, 46.

Taylor, R., 1962: Small-scale advection and the neutral wind profile. / . Fluîd Mech., 13, 529.

Terjung, W. H., 1967: The geographical application of some selected physio-climatic indices to Africa. Int. J. Biomet., 2, 5-19.

Terjung, W. H., 1969: The city-man system in urban energy balance climatology. Froc. Assoc. Amer. Geogr., 1, 145-8.

Terjung, W. H., 1970(a): The energy balance climatology of a city-man system. Annals Assoc. Am. Geogr., 60, 466-92.

Terjung, W. H., 1970(6): Urban energy balance climatology: A preliminary investigation of the city-man system in down-town Los Angeles. The Geogr. Rev., 60(1), 31-53.

Terjung, W. H., 1971: The effect of a cyclonic storm on the energy fluxes at the urban interface — a preliminary experiment. Arch. Met. Geoph. Biokl., Ser. B, 19, 367-416.

Terjung, W. H., 1974: A climatic model of urban energy budgets. In Geogr. Anal., 341-67.

Terjung, W. H. and Louie, S. S .-F., 1973: Solar radiation and urban heat islands. Annals. Assoc. Amer. Geogr., 63(2), 181-207.

Thorpe, G. R., 1973: Forecasting runoff from storms moving over partially urbanised catchments. Paper 8 in CIRIA Symposium on Rainfall, Runoff and Surface Water Drainage of Urban Catchments, Bristol, April 1973.

Tyson, P. D., Garstang, M. and Emmitt, G. D., 1973: The Structure of Heat Islands. Dept. of Geography and Environmental Studies, University of the Witwatersrand, Johannesburg, Occasional Paper No. 12, 71 pp.

Unesco Working Group on the Influence of Man on the Hydrological Cycle, 1972: Influence of man on the hydrological cycle: Guidelines to Policies for the Safe Development of Land and Water Resources. In Status and Trends of Research in Hydrology, 1965-74, pp. 31-70.

Unesco/FAO Working Group on the International Hydrological Décade, 1973: Maris Influence on the Hydrological Cycle. FAO Irrigation and Drainage Paper Spécial Issue, 17, 71 pp.

United States Council on Environmental Quality, 1971 : Second Annual Report.

United States Council on Environmental Quality, 1973: Environmental Quality: Fourth Annual Report. U.S. Government Printing Office, Washington.

United States Environmental Protection Agency, 1970: Proceedings of Symposium on Multiple-Source Urban Diffusion Models. APCO Pub. No. AP-86.

Waananen, A. O., 1969: Urban effects on water yield. In Effects of Watershed Changes on Streamflow. (Ed. Moore, W. L. and Morgan, C. W.), University of Texas Press, 169-82.

Watkins, L. H., 1962: The Design of Urban Sewer Systems. Road Research Tech. Paper No. 55, H.M.S.O., 96 pp.

Watkins, L. H., 1963: Research on surface-water drainage. Proc. Instit. Civil Engin., 24, 305-30.

REFERENCES 61

Wiitala, S. W., 1961 : Some Aspects of the Effect of Urban and Suburban Development on Runoff. U.S. Geological Survey open file report.

Williams, F. P., 1960: Pollution levels in cities. In Proc. Harrogate Conférence Nat. Soc. Clean Air, London, 83-8.

Wilson, K. V., 1966: Flood Frequency of Streams in Jackson, Mississippi, U.S. Geological Survey open file report.

Wise, A. F. E., 1970: Wind Effects Due to Groups of Buildings. Building Research Station (U.K.), Current Paper, 23/70, H.M.S.O., London.

World Meteorological Organization, 1970(a): Meteorological Aspects of Air Pollution. Technical Note No. 106, Geneva, 69 pp.

World Meteorological Organization, 1970(*): Urban Climates. Technical Note No. 108, Geneva, 390 pp.

World Meteorological Organization, 1972: Dispersion and Forecasting of Air Pollution. Technical Note No. 121, Geneva, 116 pp.

Yamamoto, G. and Shimanuki, A., 1964: Profiles of wind and température in the lowest 250 m in Tokyo. Tohoku University Science Reports, Séries 5, Geophysics, 19, 111-4.

Yamashita, S., 1970: An urban-climatological study of the atténuation of solar radiation by air pollution in Tokyo. Geogr. Rev. Japon 43, 285-96.

Yap, D., Gunn, K. L. and East, C , 1969: Vertical température distribution over Montréal. Naturalists Con. 96, 561-80.

Récent WMO TECHNICAL NOTES

No. 106 Mcteorological aspects of air pollution. No. 107 Mcteorological observations in animal experiments. Edited by C. V. Smith. No. 108 Urban climates (Proceedings of the Symposium on Urban Climates and Building Climatology, jointly organized

by the World Health Organization and WMO, Brussels, October 1968) (Volume I). No. 109 Building climatology (Proceedings of the Symposium on Urban Climates and Building Climatology, jointly

organized by the World Health Organization and WMO, Brussels, October 1968) (Volume II). No. 110 Use of weather radar for aviation (Report by a working group of the Commission for Instruments and Methods

of Observation, prepared by H. Treussart, c h a i r m a n - W . B. Beckwith - S. G. Bigler - K. O t a n i - V . V. Kostarev - R. Schwarz).

No. 111 The planning of meteorological station networks. By Professor L. S. Gandin. No. 112 Performance requirements of aerological instruments. An asscssment based on atmospheric variability. By

C. L. Hawson. No. 113 Weather and animal diseases. By L. P . Smith. No. 114 Meteorological factors in air pollution. By A. G. Forsdyke. No. 115 Machine processing of hydrometcorological data. Prepared by a working group of the Commission for Hydro-

metcorology. No. 116 Investigations on the climatic conditions of the advaiiceinent of the Tunisian Sahara. By Professor H. Flohn

(with the collaboration of M. Kctata). No. 117 Use of weirs and flumes in stream gauging (Report of a working group of the Commission for Hydrology). No. 118 Protection of plants against adverse weather. By G. W. Hurst and R. P. Rumney. No. 119 The application of micrometeorology to agricultural problems (Report of the Working Group on Agromcteoro-

logical Aspects of Micrometeorology of the Commission for Agricultural Meteorology). No. 120 Review of forecast vérification techniques. By E. M. Dobryshman. No. 121 Dispersion and forecasting of air pollution (Report by a working group of the Commission for Atmospheric

Sciences, prepared by R. G. Munn, chairman - A. E. J . Eggleton - L. Facy - D. H. Pack - F . H. Schmidt). No. 122 Some cnvironmental problems of livestock housing. By C. V. Smith. No. 123 The asscssment of human bioclimate — A limited review of physical parameters. By Professor H. E. Landsbcrg. No. 124 The use of satellite pictures in weather analysis and forecasting (Reviscd version of Tcchnical Note No. 75). By

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3-5 August 1971). No. 130 Lectures presented at the IMO/WMO Centenary Conférences, Vienna and Geneva, September 1973. No. 131 Climate under glass. By Professor Dr. J . Seeman. No. 132 Applications of mcteorology to économie and social development. By R. Schneider, J . D. McOuigg, L. L. Mcans

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