'Season': Towards a Workable Climatological ConceptAuthor(s): Ian WatsonSource: Area, Vol. 6, No. 4 (1974), pp. 283-287Published by: The Royal Geographical Society (with the Institute of British Geographers)Stable URL: http://www.jstor.org/stable/20000900 .

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'Season': towards a workable climatological concept

lan Watson, University of Western Australia

Summary. ' Seasons' can be constructed to provide operational temporal frameworks for climatological analyses.

The purpose of climatology has long been regarded as the synthesis of vast numbers of meteorological observations into meaningful statistical assemblages. Thus means, frequencies, periodicities and intensities have been used in an attempt to elicit explanatory relationships from otherwise sterile data. Much has been written to illustrate the need for discriminatory selection of these statistical techniques. Just as important, but less documented, has been the need for an equally discriminatory selection of the temporal framework of a study. This is vital to any climatological investigation which purports to explain a sequence of atmospheric events, for the temporal extent of the framework largely determines the process resolution, or the level of understanding upon

which an explanatory relationship is based. Barrett's (1970) timely call for more effort in climatology to be spent in developing an understanding of ' process',

may be in part fulfilled by directing analytical techniques to data at more appropriate temporal resolutions. It is suggested here that ' season', a rarely used concept, has potential as a temporal framework for studies of atmospheric phenomena where detailed analysis is required.

The concept' season ' has not figured prominently in climatological literature, as most analyses of atmospheric events over relatively short time scales (relative that is, to thirty- or hundred-year averages) use the ' year' as a temporal frame

work. An explanation is not difficult to discover. 'Season ' unlike ' year' has no universally accepted limits. To those living in lands adjacent to the equator, the notion of ' season' is an often vague differentiation between wet and dry periods, whereas in higher lattitudes weather pattern changes throughout the year are more notable and demand the full ' summer', ' autumn ', ' winter', ' spring' designation. ' Season' is perceived differently by people and as such is a relative rather than an absolute concept. Despite this, it is argued that if seasonal differentiation exists in a study area, adoption of the ' year ' rather than the ' season ' as a temporal framework may tend to mask statistical regularities by which possibly important aspects of phenomena can be identified and isolated for explanatory analysis.

An advantage of the concept 'season' is that because it has no absolute temporal extent, it can be constructed to relate to both the location and nature of the phenomenon being studied. However, the 'season' may be constructed from the same data by which considered aspects of the phenomena are to be analysed. The potential danger of ' circularity ' is immediately apparent, and to overcome this it is suggested that, first, temporal dimensions of the 'season' be broadly constructed, and second, construction should be based on a multi variate approach.

Most raw data are collected or grouped in absolute temporal form: hourly, daily or monthly. A problem thus arises in attempting to construct a ' season'

283

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284 'Season': towards a workable climatological concept

that is relative from data in an absolute temporal form. Absolute temporal terms must inevitably be used, and to avoid marshalling statistical singularities, it is expedient to use the widest possible temporal division. Within a ' season', the ' month' appears most viable as a time-unit into which data can be grouped. It is suggested then that ' seasons ' be constructed from monthly groupings of data. Variables used to construct the temporal dimensions of the ' season' should also be selected with care. A ' season' constructed on the basis of one variable may be an invalid temporal framework in that the point of constructing a' season 'is not to depict the seasonal character of one aspect of an atmospheric phenomenon, but to create a general framework into which study of any aspect of that phenomenon can be couched. Thus a multi-variate approach is advocated. Combined use of wide temporal dimensions and more than one variable should enable construction of a ' season' that is general enough to permit valid analysis of data within that temporal framework.

The potential value of the ' season' concept can be best illustrated by the construction of a ' season' as a temporal framework for, as an example, investi gation of the causal characteristics of temperature inversions in the lower troposphere near Perth, Western Australia. Available data comprise temperature-height diagrams from morning and evening radiosondes released during 1972. These data yield three types of information. First, lapse rate con ditions indicate whether or not the temperature gradient is inverted; second, inversion height can be estimated in graduations of approximately 100 m; and third, the inversion intensity can be found by calculating the difference in temperature between the level of commencement and the level of cessation of the negative lapse rate. Frequency, height and intensity of temperature inver sions are the three variables to be used in determining the duration of the 'temperature inversion summer'.

40 35 30

B ~-0

U0 0

10

5

JAN FEB t MAR APR MA-J-N l

- L AUG I SP OT NION DEC

Figure 1. Monthly frequency of lower tropospheric temperature inversions during 1972.

Visual interpretation of Figure 1 showing monthly frequency of inversions reveals quite a definite trend. The peak of inversion occurrence appears in May and then, with some fluctuations, a marked downward trend occurs until December. The trend indicates that frequency of inversions is not purely a

function of mean temperature changes during the year (November and Decem ber are hot months) but must be linked also with rate, direction and source of

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'Season': towards a workable climatological concept 285

airflow. Fluctuations in frequency show also a bi-modal pattern. Months exhibit

ing notably few inversions are October, November, December and January and this seems to point to an obvious summer pattern, but a distinction between

this and a winter pattern is somewhat less precise due to frequency peaks occur ring in months marginal to the cooler season, namely August and May. A

summer inversion season, on the basis of inversion frequency, existed in 1972

between October and the end of January. While this does not correlate neatly

with the usual sequence of warmer months, it is nevertheless a clear enough

trend to indicate that the 'inversion summer' commences slightly earlier than the generally accepted ' calendar summer' at Perth.

Distributional patterns of inversion intensity during 1972 confirm this summer difference, although less markedly. As mentioned above, inversion intensity is taken as the temperature difference between the levels of negative lapse rate

cessation and commencement, and for the purpose of obtaining a yearly fre

quency of inversion intensity, these can conveniently be expressed as terciles.

The lowest terciles (T1) includes inversions of 1-2?C; tercile two (T2) includes those of 3-5?C and the highest group (T3), consists of intensities of 6-1 1C.

] ~~~~~~JAN4UARY

O B DECEME FEBRUARY

SEPTEM1EE

J L................ .......-----t.1-2?. -- -- - - - t2,. 3-5 C

t 3 6-11,C

Figure2. Monthlyfrequency of temperatureinversion intensitytercilesduring 1972

Figure 2 shows the circular sequence in monthly frequency of intensity

terciles. For T1 a seasonal division occurs when the months October to February

are isolated, as the frequency during these five months is lower than for the rest

of the year (except for April). Inversions of T2 intensity however, appear to be

more consistent if the ' season ' is extended from September to January. Apart

from October which seems to be anomalous, the range of T2 frequency is rela

tively small. An alternative alignment could be between March and September as the T2 pattern appears to be roughly bi-symmetrical either side of the March

September radii. Inversions of T3 intensity do not exhibit any clear pattern of

seasonal variation comparable with the major climatic division between standard

cool and warm seasons, although their frequency is greater between February

and May than between June and January. In general, frequency distribution

of intensity terciles does not offer any clear basis for strict delineation of a sum

mer inversion season but it may be suggested from the behaviour of T2 distri

bution, that the season be extended to commence from September.

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286 'Season': towards a workable climatological concept

00~~~~~0 - - ~ ~ ~ 00

0000

~~~~~~~~~~~~~~~~3 Z_

Figure 3. Frequency distribution of temperature inversion heights from Septem ber to February 1972.

Three-dimensional construction of inversion height percentage frequencies for September to February (Figure 3) and March to August (Figure 4) illustrates clearly a seasonal pattern of reversal of height frequency which occurs between the two six-month periods. These two groups represent the most distinct of the

height frequency changes. From September until February the greatest percent

age frequency of inversions reach heights of 300-800 m with very few below

the 100 m level (Figure 3), however the reverse exists during March to August where the greatest frequency is between 100 and 300 m with a small percentage

occurring over the 300 m level (Figure 4). As the alignment of monthly height frequency in these two diagrams changes

so noticeably between the two six-month periods, this criterion constitutes

60

8

400

Figure 4. Frequency distribution of temperature inversion heights from March to August, 1972.

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'Season': towards a workable climatological concept 287

the clearest seasonal demarcation of the three criteria considered here, but although this may be the most obvious a similar seasonal trend is also discern ible from frequency data of intensity and number of inversions. The pattern of these inversion characteristics seems to indicate that this ' summer season' of inversions extends from September to February. This then, once established,

will be the temporal framework from which a study of the causal characteristics of these temperature inversions will proceed (Watson and Gentilli, 1974).

This is merely one instance of a study that could well use a workable' season'. Studies based on this temporal framework, could produce interesting insights into atmospheric 'processes ' hitherto obscured by data aggregated within inappropriate temporal frameworks. However ' seasons ' should be constructed

with discrimination, otherwise they may be inappropriate to an analysis where a higher or lower temporal resolution is required. Care should be exercised too, that selection and implementation of the delineating variables does not render ensuing anaLlysis meaningless.

Acknowledgements

The author would like to acknowledge the comments of Dr J. Gentilli, Dr L. J. Wood, Dr J. A. Dawson and Dr M. A. Hirst on an earlier draft of this paper.

References

Barrett, E. C., 1970. Rethinking climatology, an introduction to the uses of weather satellite photographs in climatological studies, in Board, C. et al. (eds), Progress in Geography, Vol. 2, Arnold, London.

Watson, I. and Gentilli, J., 1974. Thermal inversion by advection in a subtropical summer, Aust. Geog. Studies, 12, 119-25.

The SSRC Human Geography Committee's symposium on resource management

William Birch (University of Leeds) introduces four reports:

The initiative for this symposium came from an ad hoc Steering Group on research in resource management convened for the Human Geography Committee by Prof.

William Birch of the University of Leeds. At a joint meeting with the Institute of British Geographers in Norwich in January 1974 (see Area 6 (1974), 1, 66-7) working parties were established to explore researchable topics within four problem areas:

1. Rural land planning-convenor Dr R. Munton, University College London; 2. Environmental information for planning-convenor Prof. J. T. Coppock,

University of Edinburgh; 3. Environmental hazards-convenor Dr Anne Kirkby, University College, London; 4. Water resource management-convenor Judith Rees, London School of Econo

mics.

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