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FACULTY OF AGRICULTURE

DEPARTMENT OF AGRONOMY

B.Sc. AGRICULTURE

AGR 121: AGRICULTURAL METEOROLOGY

THEORY NOTES

Chapter

No. Chapter Name Page No.

Part - I

1. Introduction to Agricultural Meteorology 2

2. Atmosphere 6

3. Weather and Climate 18

4. Solar Radiation and Light 39

5. Temperature 57

Part � II

6. Atmospheric Pressure 2

7. Wind 12

8. Atmospheric Humidity (Moisture) 27

9. Clouds and Precipitation 33

10. Evaporation and Transpiration 39

Part - III

11. Precipitation 2

12. Agroclimatic Zones 23

13. Agroclimatic normals for field crops 26

14. Weather Forecasting 30

15. Agricultural Seasons of India 35

ANNAMALAI UNIVERSITY

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Part - I

Chapter �1

Introduction to Agricultural Meteorology

Meteorology is defined as a branch of physics dealing with the lower atmosphere (Atmosphere is a deep blanket of gases surrounding the earth) with particular emphasis to the individual phenomenon. In other words it is concerned with the study of the characteristic and behavior of the atmosphere. It explains and analyses the changes of individual weather elements such as air pressure, temperature and humidity that are brought about due to the effect of insolation on the earth�s surface. (Insolation refers to radiation from the sun received by earth�s surface).

Agro meteorology is a science investigating the meteorological, climatologic and hydrologic conditions, which are significant for agriculture owing to their interaction with the objects and processes of agricultural production. In nutshell, it is a science dealing with climatic conditions, which is directly related to agriculture.

Divisions of Meteorology:

1. Dynamic Meteorology

It deals with the forces that create and maintain motion and the latest transformations associated therewith.

2. Physical Meteorology

It deals with pure physical nature such as radiation, heat, evaporation, condensation, precipitation, ice accretion (continuous coherence) and optical acoustical and electrical phenomena.

3. Climatology

Climatology refers to the study of weather patterns over time and space. It concerns with the integration of day-to-day weather over a period of time. It refers to the average

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conditions of the weather. Climatology is made up of two Greek words, kilma + logos; kilma means slope of the earth, and logos means a discourse or study. In brief, climatology is simultaneously an old and a new science. It is a statistical meteorology which determines the statistical relations, mean value normal, frequencies, variation distribution etc.

4. Synoptic meteorology

Its purpose is the analysis and forecasting of the weather phenomena. Thus synoptic meteorology comprises dynamic as well as physical meteorology and to a lesser extent climatology in order to obtain a synopsis of the state of atmosphere.

5. Aeronautical meteorology

It deals with application of meteorology to the problems of aviation.

6. Maritime meteorology

It is related to marine navigation.

7. Agricultural meteorology

It deals with application of meteorology to agriculture, soil conservation etc.

8. Hydrometeorology

It is concerned with meteorological problems relating to water supply, flood control, irrigation etc.

9. Medical meteorology

It deals with the influence of weather and climate on the human body.

10. Aerology

It is a branch of meteorology that is concerned with the conditions of the free atmosphere on the basis of direct observations.

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Meteors and its Classification:

Meteors are defined as an atmospheric phenomenon, having a luminous appearance that travels through space as aerolites, fireballs, stars etc.

a. Aerial meteors : Wind, Tornado

b. Hydro or Aqueous meteors : Rain, hail, snow and dew

c. Litho meteors : Dust and smoke

d. Luminous meteors : Rainbow and halos (circle of light and sound luminous body around the sun or moon)

e. Igneous meteors : Lightening and shooting stars.

Development and Importance of Agricultural Meteorology

Superstition served to interpret atmospheric mysteries such as rain, wind and

lightening. In the early civilization, Gods were often assigned to the climatic elements,

Indians still hold ceremonial worships/dances to Gods to produce rains at times of

drought.

The Greek philosophers showed a great interest in meteorological science. In fact the

word �Meteorology� is of Greek origin means the study on things about meteors and

optical phenomena. In fact, the word �Meteorology has been borrowed from Aristotle�s

�Meteorologica� dated about 350 BC. The period of weather tradition and superstitions

in the development of meteorology lasted until the beginning of the 17th Century when

the invention of instruments for scientific analysis of weather phenomenon was made.

In 1593, Galileo constructed a thermometer and in 1643, his student Torricelli

discovered the principles of mercurial Barometer. The climatological map was

published by British astronomer �Edmund Hally� in 1686. By 1800, dependable weather

observations were made in Europe and USA. An International Meteorological

Organization had been established in 1878. The World Meteorological Organization

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(WMO) took its present form in 1951. It serves as a specialized agency to carryout the

worldwide exchange of meteorological information with the head quarters in Geneva,

Switzerland.

The India Meteorological Department (IMD) was established in the year 1875. The

division of Agricultural Meteorology was started by the IMD in 1932 to meet the needs

of agriculture and researchers. The IMD has brought out many useful publications on

rainfall. The Rainfall Atlas of India was published based on the rainfall data from 1901

to 1950. In addition to rendering advice from time to time, the IMD began to offer

regular weather service and farmers weather bulletins from 1945. The bulletins are

broadcast daily in 20 regional languages in all the All India Radio stations on expected

weather conditions during the next 36 hrs. Weather report is also broadcasted through

television. At present 8000 rain gauge stations and 52 principal types of Agro met

observatories are available in our country.

Scope of Agricultural Meteorology

Climatic factors alone affect the yield of crops to an extent of about 40%. In India the

success of agriculture depends mainly on monsoon rains. Agricultural Meteorology is

mainly concerned with microclimatology in which the influence of the shallow layer of

atmosphere immediately above the surface is studied. Successful crop production

depends not only upon the total seasonal rainfall but also on the proper distribution.

The study of agricultural meteorology helps the farmers to know when the monsoon

rain begins, its distribution etc. Apart from this the farmer will be able to know about

the weather abnormalities and their destructive effect on crops.

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Chapter �2

Atmosphere

It is necessary to review our memory with our earlier understanding of atmosphere, which serves as a platform for studying the meteorology. The word atmosphere derives from the Greek word �Atmos� which means vapour and �Sphaira� which means sphere. It is used now to denote the gaseous sphere surrounding the earth.

Stratification and Composition of Atmosphere

The atmosphere is a mechanical mixture of many gases, not a chemical compound. In addition, it contains water vapor volume and huge number of solid particles, called aerosols. Some of the gases (N, O, Ar, CO2) may be regarded as permanent atmospheric components that remain in fixed proportions to the total gas volume. Other constituents vary in quantity from place to place and from time to time. If the suspended particles, water vapour and other variable gases were excluded from the atmospheres, we would find that the dry air is very stable all over the earth up to an altitude of about 80 kilometers.

Composition of Atmosphere

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Principal gases comprising dry air in the lower atmosphere.

Constituent Percent by volume

Nitrogen (N2) 78.08

Oxygen (O2) 20.94

*Argon (Ar) 0.93

Carbon dioxide (Co2) 0.03

*Neon (Ne) 0.0018

*Helium (He) 0.0005

Ozone (O3 ) 0.00006

Hydrogen (H2) 0.00005

*Krypton (Kr) Trace

*Xenon (Xe) Trace

Methane (Me) Trace

*Inert chemically never found in any chemical compounds

As shown in the table, two gases, nitrogen and oxygen, make up about 99 per cent of the clean, dry air. The remaining gases are mostly inert and constitute about 1 per cent of the atmosphere generally homogenous and it is called as homosphere. At higher altitudes, the chemical constituents of air changes considerably. The layer is known as the heterosphere.

Nitrogen

It is chemically inactive and an important plant nutrient, but it has to be fixed in the soil to make it available to the plant. The fixation of nitrogen in the soil is carried out by the following agencies.

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1. Nitrogen fixation by symbiotic bacteria

a. Symbiotic root nodule leguminous bacteria - Rhizobium group.

b. Symbiotic root nodule leguminous bacteria -on Casuarinas.

c. Symbiotic leaf nodule bacteria on Pavetta and Dioscorea.

d. Symbiotic root nodule - Actinomycetes in Myrstica and Almus.

2. Nitrogen fixation by free living

a. Azetobacter and clostridium group of bacteria.

b. Photosynthetic and chemo synthetic � Sulphur bacteria.

c. Free living east cells of fungi.

d. Blue green algae.

The above agencies are known as biological agencies, which fixes the atmospheric nitrogen in the soil.

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3. Lightning and powerful electrical charges

During lightning and powerful electrical charges are released and �N� and �H� in elements present in the atmosphere forms NH2 dissolved and brought down by rain and water as NH3 . About 2 to 20 lbs of nitrogen is added to the soil/acre.

4. By means of artificial methods

By electrical arc method.

Oxygen

It has got considerable importance in plant and animal life. It plays an important role in respiration, bacterial activity in soil oxidation and absorption of plant nutrients and several soil forming or weathering activities in the soil, which improve plant food availability.

3. Carbon dioxide

It plays an active part in photosynthetic activities.

4. Argon

It is used extensively in electric lamp bulbs because of its inertness. It is also used in florescent tubes. It flows with blue light.

5. Neon

Neon is used to fill florescent tubes. It flows with distinctive orange red colour.

6. Helium

It is the second highest element with a density of 0.177 gms per liter (Hydrogen 0,08988 gms /liter). It is used to inflate balloons because it will not burn.

7. Krypton

This glows with brilliant green and yellow colour.

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8. Xenon

It is chemically inert and glows with a blue green colour. Besides these the atmosphere also contains small quantities of ozone (O3 ), Methane (CH4), Nitrous oxide (N2O), Sulphur dioxide (SO2) and traces of Iodine, NaCl,NH3 Carbon monoxide etc., The amount of CO2 in the atmosphere is not quite constant. The vegetable World continuously consumes CO2, which again is produced by the animal World, through burning of fuels, volcanic action and various process of decay in the soil. But the oceans by dissolving the excess of CO2, so effectively regulate it that the amount of CO2 in t he atmosphere remains almost constant. Ozone, which is present in the lower atmosphere, has a maximum in the upper atmosphere between 10 and 25 km (30,000 and 80,000) where it amount varies considerably. Apart from this the composition of the atmosphere is remarkably constant all over the earth�s surface.

Water vapour:

The air also contains variable of water vapour. The water vapour present in the atmosphere varies up to 4% by volume as in tropical humid climate. Most of the vapour is found in the lower part pf the atmosphere. The maximum amount of water vapour that the air can absorb depends entirely on the temperature of the air, the higher the temperature of the air the more water vapour it can hold. The air is saturated with moisture when this maximum amount is reached, when air is cooled below its saturation temperature condensation takes place, water droplets formed or at low temperature ice crystals formed. Small water drop lets and ice crystals are kept afloat in the air by the ascending air currents and under special circumstances the water droplets and ice crystals coalesce and form large drops or snow flakes which are precipitated from the clouds when they become too large to be kept afloat.

Sold particles of atmosphere:

The air also contains a variable amount of impurities such as dust, soot, salts, fungal spores, bacteria and pollen (both organic and in organic) Over a city it is estimated to contain, 1,00,000 0articles per cc. A cigarette puff sends about 400 crores of dust particles. The main source of dust is the arid regions such as deserts and steppes. The minute dust particles are readily distributed throughout the lower atmosphere and

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carried for from the source. The industrial regions forest fires and volcanoes constitute the main source of soot. Through the action of winds, spray is whirled up from the oceans, and when it evaporation the salt remains in the air in the form of minute particles. The presence of dust particles in the atmosphere is important since when the air is cooled to its saturate temperature, condensation takes place on certain active nuclei. The salt particles from the oceans are most active as condensation nuclei on which the water vapour condenses to form fog or rain. They are the cause for twilight.

Layered structure of the Atmosphere:

During the international Geophysical year (1957-62), important discoveries were made about the atmosphere and many new facts came to light. The earth�s atmosphere consists of zones or layers arranged like spherical shells according to altitude and temperature variations above the earth�s surface. According to Peterson, the atmosphere is divided into the following more significant spheres.

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1. Troposphere

2. Stratosphere

3. Mesosphere (also called Ozonosphere)

4. Ionosphere

5. Exosphere

1. Troposphere:

It contains about 75 per cent of the total gaseous mass of the atmosphere. It has been derived from the Greek word �trops� meaning �mixing� or turbulence. The average height of this lowermost layer of the atmosphere is placed at about 14 km above sea level. Under normal conditions, the height of the troposphere at the poles is about 8 kilometers, while at the equator it is about 16 kilometers.

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Troposphere is marked by turbulence and eddies. It is also called connective region. Various types of clouds, thunderstorms as well as cyclones and anticyclones occur in this sphere because of the concentration of almost all the water vapour and aerosols in it. Wind velocities increase with height and attain maximum at the top. The most important feature is decrease in temperature with increasing elevation up to 14km.

Tropopause is a shallow layer separating troposphere from the next thermal layer of the atmosphere i.e., stratosphere. Tropopause (Greek word) means where the mixing stops. The temperature remains constant throughout the tropopause. The height of the tropopause is about 1 to 2 km.

2. Stratosphere: The stratosphere begins at the tropospause, which forms its lower boundary. The lower stratosphere is isothermal in character (16-30 kilometers). There is a gradual temperature increase with height beyond 20 km i.e., upper stratosphere (temperature inversion). No visible weather phenomena occur above tropopause.

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3. Mesosphere or Ozonosphere: There is maximum concentration of Ozone between 30

to 60 kilometers above the surface of the earth. Because of the concentration of ozone in

this layer it is called the ozonosphere. It is a warm layer because of selective absorption

of ultra violet radiation by ozone. In fact, it acts as a filter for ultra violet radiation from

the sun. In this layer the temperature increases with height @ 50 C/km. The maximum

temperature recorded in the ozonosphere is higher than that at the earth�s surface.

Because of the preponderance if chemical processes, this sphere is sometimes called as

chemosphere.

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4. Ionosphere: Ionosphere, according to Peterson, lies beyond the ozonosphere at a height of about 60 km above the earth�s surface. At this level the ionization atmosphere begins to occur. Above ozonosphere, the temperature falls again reaching a minimum of about 1000C at a height 80 km. above earth�s surface. Beyond this level the temperature increases again due to the absorption of short wave solar radiator by the atoms of O & N in this ionosphere.

Layers of Ionosphere

D Layer : 60-89km.

E Layer : 90-130 km.

E1 Sporadic Layer : 110 km.

E2 Layer : 150 km.

F1 Layer :

F2 Layer : 150- 380 kms.

G Layer : 400km and above.

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5. Exosphere:

The outer most layer of earth�s atmosphere is known as the exosphere, which lies between 400 and 1000 kilometers. At such great height density of atoms in the atmosphere is extremely low. Hydrogen and helium gases predominate in the outer most regions. Kinetic temperature may reach 55680 Celsius.

Modern Views Regarding the Structure of Atmosphere

On the basis of composition, the atmosphere is divided into two broad spheres.

1. Homosphere and

2. Heterosphere

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Homosphere means zone of homogenous composition height - up to 88 kilometers. The proportions of the component gases of the sphere are uniform at different levels.

It is sub-divided into

a. Troposphere - very shallow transition layer tropopause

b. Stratosphere - Stratopause

c. Mesosphere - Mesopause

Heterosphere:

The atmosphere above the homosphere is not uniform in composition. Different layers of the atmosphere in this part differ from one another in their chemical and physical properties. In this sphere gases are said to be arranged into the following four roughly spherical shells, each of which has its own distinctive composition.

1. Nitrogen layer - 200 km above earths surface molecular N.

2. Oxygen layer - Average ht. 1120km (atomic oxygen)

3. Helium layer - Average ht. 3520km.

4. Hydrogen layer - these layer are arranged according to the weight of the gases.

Lapse rate:

The rate of decrease of temperature with increase in height at a given place and time is called Lapse rate. The normal lapse rate is 6.50 C per km increase in height.

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Chapter �3

Weather and Climate

Phenology:

It indicates the coming season. It is a science, which deals with the reoccurrence of important phases of animals and vegetable life in relation to climate during the year. Events such as leafing, flowering, fruiting, leaf shedding, migration of birds, occurrence of insects etc provide indications of the coming season.

According to Indian Meteorological Department the flowering in mango tree takes place by 15th December in Chennai and Andhra Pradesh while in northern state it is as late as 15th of March.

Seasons:

1. Spring : January to March - Fresh leaves form in trees.

2. Summer : April to June � Flowering and fruiting takes place.

3. Autumn : July � September.

4. Winter : October � December.

The sequence of flowering obeys Hopkins Bioclimatic law, to which the time of flowering develops upon the latitude, longitude and altitude. According to the law,

1. For every degree of latitude north or south of equator, flowering is retarded by 4 calendar days.

2. For every 50 of longitude for East or West on land areas flowering is advanced by 4 calendar days.

3. For each 400� increase in altitude flowering is retarded by 4 calendar days.

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Boyle�s Law

The Volume of a given quantity of air varies inversely as the pressure upon it, provided the temperature remains constant.

Charle�s Law

The volume of a given quantity of air varies directly as the absolute temperature, provided the pressure remains constant.

Weather

The condition of atmosphere at a given time defined as weather and it is highly variable. I t is a heat or moisture exchange for a shorter period of time over a smaller area particularly respect to wind, temperature, cloudiness, relative humidity, and pressure etc. Weather and climate are the important factors determining the success or

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failure of agriculture. Weather influences agricultural operations from sowing of a crop to the harvest and depends on the mercy of the weather particularly rain fed agriculture. In India every year there is a considerable damage by floods in one part of the country and a severe drought causing famines in another part. The total annual pre harvest losses for the various crops are estimated from 10 to 100 per cent; while, the post harvest losses are estimated to range between 5 and 15 per cent. Hence, study of weather element is essential. Weather is the condition of atmosphere at a given time. It is the day to day interplay of temperature, humidity, pressure, rainfall etc. The weather conditions of Coimbatore on a particular day at a particular time may not be the same as that of Annamalainagar weather.

Eg: Weather data for Annamalainagar during 2005

Maximum temperature was observed on ---------------

Maximum rainfall was received on -----------------------

Maximum relative humidity was recorded on ---------

Climate

The state of atmosphere over the period of time is known as climate. It is the synthesis of these various elements of the weather. The word climate refers to the mean or normal conditions over a long period such as 20-30 years or more; where as the cold weather refers to the mean or normal conditions over a long period such as 20-30 years or more. Where as the word weather refers to more or less instantaneous conditions in the atmosphere or the trend of there conditions over a relatively short period of time.

Weather Climate

1. Occurrence of the weather elements at a time

Expressions of many such occurrences as the flow together in time. Average values of huge elements for a seasonal time is long period (for 30 years)

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2. Differentiation of climate Integration of weather

3. Concerned with how all the weather elements act as a given time

Concerned with how they affect the environment which is turn affects all the organisms.

Longitude

It is the distance east or west on the earth�s surface measured as an arc of the equator (in degrees up to 1800 or by the difference by time ) between the meridian passing through a particular place and a standard of prime meridian, usually the one passing through Greenwich, England.

Meridian

Meridian is a great circle of the earth passing through the geographical poles at any given point on the earth�s surface.

Equator

It is an imaginary circle of around the earth, equally distance at all points from both the North Pole and the South Pole. It divides the earth�s surface into the northern hemisphere and the southern hemisphere like dividing the coconut fruit at the centre horizontally.

Factors influencing the climate

The most important climate elements are temperature precipitation, humidity, wind velocity, duration of monsoons and cloudiness air pressure etc., which in different combinations decided the climate of a place. Because of the intimate relation between climate and vegetation climates are classified according to the type of plants grown or cultivated soil such as tropical climate, forest climate, desert climate, pine forest climate, tundra climate etc. The climatic elements are the results of interaction of number of factors such as

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1. Latitude- distance from the north or south of equator.

2. Altitude � the elevation of a place above mean sea level.

3. Precipitation

4. Distance from the sea

5. Topography

1. Latitude:

Latitude is the angular distance, measured in degrees, north or south from the equator. It is the main factor in determining the climatic zones such as torrid, temperate, tropical, subtropical and polar zones. It is found that the quality of grains is better in higher latitudes than that of lower latitudes.

For Example:

1. Canadian wheat is of better quality than Egyptian wheat.

2. Italian rice is superior to Indian rice.

The latitude of a place in question for its depends on the angle of incidence of the incoming radiations from the sun, the length of the day and night, the length of the seasons, the amount of incoming radiations etc.,

Distance from East to West : 2933 km

Distance from North to South : 32144 km

Land frontiers : 15200 km

Coast line : 6083 km

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Solstice

It is the time when the sun reaches its maximum distance from the equator (summer solstice when it touches tropic of cancer on 21st June and winter solstice when it touches Tropic of Capricorn on 21st December).

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0� to 23 ½ 0 L = Tropical region

23 to 66 ½ 0 L = Temperate region

Above 66 ½ 0 = Polar region.

i. Tropical Region (Climate)

The tropical climate is characterized by high temperature throughout the year because it is the region of sun�s movement.

The Equatorial Belt

The equatorial belt comes under the tropical region and the sun is north of the equator during the northern summer and south of the equator during the southern summer. At the equator the sun is in Zenith at both equinoxes. About 230 N and S the sun reaches Zenith only at the time of the solstices. Thus near the equator the sun is in zenith twice a year and there will be maximum of incoming radiation in spring and autumn. The length of the day varies but little throughout the year and the sun is high in the sky every day. The annual variation in the temperature is therefore very small. But the diurnal variation in temperature will be relatively large because the length of the day varies but little.

Zenith

Zenith is defined as the time at which part of the sun is directly overhead.

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Horizon

Horizon refers to an imaginary line at which earth of sea or sky appears to meet.

Equinoxes

Equinoxes refer to the time of the year at which the sun crosses the equator and day and night are equal.

ii. Subtropical climate

This is also characterized by high temperature alternating with low temperature in winter. Subtropical is also characterized by high temperature alternating with low temperature in winter.

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iii. Temperate Region

The temperate climate is distinguished by low temperature all through the year. It has got moderate temperature with well-distributed rainfall, humidity etc. This is the ideal climate region for successful crop production. The temperate climate has low temperature throughout the year. Here the sun does not reach the Zenith in mid summer. The days are long and the sun is high in the sky in summer and in winter the days are short and the sun is low in the sky with the result that the incoming radiation varies considerably through out the year. As a result the annual variation in temperature tends to increase from the equator towards the poles.

iv. Polar Region

Since it is far away from the suns influence this region will be extremely chill or cold through out the year. The polar climate is noted for its very low temperature throughout the year. Here the sun is below the horizon day and night in mid winter and above the horizon day and night in mid summer. At the poles there is no diurnal variation in the incoming radiation and the daily variation in temperature vanishes. On the other hand the difference between the incoming radiation in winter and summer has increased to a maximum, with the result that the annual variation in temperature increases.

2. Altitude

It is the elevation of a place, the metrological elements vary rapidly with height above the sea level and it has a profound influence on a climate. Even in the tropical climate, the high mountains have temperate climate. The temperature decreases by 0.60 for every 100 m from the sea level. Generally there is a decrease in pressure and increase in precipitation and wind velocity. The important effects of altitudes are

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1. As the height increases the pressure is decreased the barometer reading in difference heights are as follows:

a. 30� at sea level

b. 29� at 830 feet.

c. 15� at 18,500 feet.

2. As the height increases the mean temperature is decreased and the decrease is usually 10 F fro every 300� ascent (0.6 C for every 100 m increase in height from sea level)

3. As the height increases the precipitation also increases.

3. Precipitation

As the height increases the precipitation also increases and hence rainfall is more in mountainous regions. The quantity and distribution of rainfall decides the nature of vegetation and the nature of the cultivated crops. The crop region is classified on the basis of average rainfall, which is as follows:

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Rainfall (mm) Name of the climatic region

Less than 500 Arid

500-700 Semi arid

750-1000 Sub humid

More than 1000 Humid

4. Distance from the Sea

The presence of large water bodies like lakes and sea effect the climate of the surrounding areas. E.g., islands and coastal areas. The movement of air from earth surface and from water bodies to earth modifies the climate. The extreme variation in temperature during summer and winter is minimized in coastal areas and islands.

The difference between marine and continental climate can be classified as follow:

5. Topography (Relief)

The surface of landscape (levelled or uneven surface areas) produces marked changes in the climate. This involves the altitude of the place, steepness of the slope and

Marine Continental

1. Rainfall More and well distributed Less and ill distributed

2. Temperature Variation is less Variation is more

3. Land and Sea Breeze

Sea breeze is regular No sea breeze

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exposure of the slope to light and wind. The frost occurrence will be mostly in the valleys rather than the hills. Besides these, soils and vegetation as physical factors also affect climate to a smaller extent.

i) Soil type

Soil is product of climatic action on rocks as modified by landscape and vegetation over a long period of time. The colour of soil surface affects the absorption, storage and re-radiation of heat. White colour reflects while the black absorbs more radiation. Due to the differential absorption of the heat energy, variations in temperature are created at different places. In black soil areas the climate is hot while in red soil areas it is comparatively cooler due to lesser heat absorption.

E.g. Tirunelveli and Ramnad District.

ii) Vegetation

Kind of vegetation characterizes the nature of climate. Thick vegetation is found in tropical regions where temperature and precipitation are high. General types of vegetations present in a region indicates the nature of climate of that region. Thick forest areas with more vegetation will be cooler than the desert because the forest trees and by the surrounding environment becomes cooler. The black soil type regions generally a hot climate exists because of more absorption of by the black soil.

Other factors that influence the weather and climate marginally are

i. Semi permanent high and low pressure systems.

ii. Winds and air masses.

iii. Atmospheric disturbances or storms.

iv. Oceans currents.

v. Mountain barriers

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The temperature distribution near the earth surface would be as follows

a. Mean annual temperature is highest at equator and lowest at poles.

b. Annul variation in temperature is small at the equator (with maximum temperature in spring and autumn)

c. Diurnal variation in temperature is greatest at the equator and decreasing with increasing latitude.)

Classification of Climate:

Mr. Koppen has classified the climate into eleven principal types as follows:

1. Tropical rainforest climate:

It occupies the major portions of the equatorial belt. Along the west coast the belt is relatively narrow and along the east coast it spreads out 260 N and S because of the monsoons and on the land trade winds give warm weather and rainfall most of the year.

This climate is characterized by

a. High temperature coldest weather above 180C (64.40 F) annual variation in temperature less than 60C (110F).

b. Sufficient rainfall to maintain tropical forest, either rain at all seasons, two rain maxima or one long rain period and one short and dry season with at least 6 cm rainfall.

c. Vegetation of the megatherm type, which require high constant temperature, abundant precipitation and high relative humidity.

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2. Tropical �Savanna Climate:

This zone surrounds the tropical rain forest. They have a dry period caused by the migration of the doldrums and the climate is characterized by

a. High temperature, coldest much above 180C annual variations in temperature less than 120 C.

b. Relatively abundant rainfall in summer and dry winter, with at least one month with less than 6cm rainfall.

c. Vegetation related to the tropical rain forest, but because of the winter dryness the forests are replaced by open land with trees.

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3. Steppes

The steppes continue for into the interior continent where the dryness is in part due to the large distance from the coast and lack of moisture bearing winds. The equatorial part and eastern part of the steppe region has light summer rainfall chiefly because of summer showers, and the portion indicated by WR (winter rainfall) has dry summer and slight winter rainfall. The steppe climate is characterized by

a. Temperature varying within wide limits.

b. Lack of rainfall, evaporation-exceeding precipitation most of the rain at rare intervals and the amount varying considerably.

c. Vegetation adapted to high temperature large temperature variation and long day periods.

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4. Deserts

Here the descending air in the subtropical anticyclones causes extreme dryness. The deserts are characterized by:

a. High summer temperature, large diurnal variation and moderate annual variation in temperature.

b. Cloudy sky, extreme dryness, dust and sand storms, rains squalls at rare intervals.

c. Very sparse vegetation of steppe type.

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5. Warm climate with dry winter:

Adjacent to savannas and winds are mainly monsoon type, dry winter and wet summer.

a. Mean temperature of the coldest month below 180C but above �30C mean temperature of warmest month over 100C.

b. Dry winter and wet summer at least 10 times as much as rainfall in the wettest month of summer as in the driest month of winter. Warm climate with dry winter.

2. Warm Climate with dry summer:

Under the poleward part of the subtropical anticyclones where because of the annual migration of these anticyclones, the prevailing westerlies give rain in winter.

a. This zone is characterized by temperature as in climatic zone � 5.

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b. Dry summer and moist winter with at least 3 times as much rainfall in the wettest month of winter as in the driest month of summer having less than 3 cm of rainfall.

c. Vegetation of the mesothermal type adapted to dry and warm summers and moderately cold and wet winters.

The summer is frequently too dry and whether is too cold for the vegetation. As a result most plants blossom in spring and autumn where there are sufficient heat or moisture.

3. Humid temperate climate

They are under the influence of moisture throughout the year with a high temperature in winter and sufficient rainfall in all seasons.

a. Temperature as in climate zone (No. 5 and 6).

b. No appreciable annual variation in rainfall.

c. Vegetation of mesothermal type adopted for high moisture throughout the year (Evergreens).

36

8. Cold climate with moist winter

This coincides with sub polar belts of pine forests.

a. Mean temperature of coldest month less than warmest month above 100C.

b. Rains all through the year on the coast mostly in winter in land mostly in summer.

c. Vegetation � isothermal type, which required short summer and long winter and needs snow cover for protection during the long and cold winter (E.g. Pine and Fir).

9. Cold Climate with Dry Winter

In high latitudes because of the low winter temperature and the great distance from moisture bearing winds, the rain during winter is very small other characteristics are similar to zone. (8).

10. Tundra Climate

In the northern permanent most part of the continent. The mean temperature of the warmest winter is below 100C. Subsoil is frozen throughout the year and there are no forests.

37

11. Ice climate

The polar cap of /snow and ice with mean temperature of the warmest month is below 00C (32.50F)

Thornthwaite establishes five climatic provinces that correspond closely to natural plant covers.

Climatic province Type of vegetation T.E. Index

Wet Rainforest >=128

Humid Forest 64 - 127

Sub Humid Grass land 32 � 63

Semi arid Steppe 16 � 31

Arid Desert <16

If we travel along the west coast from the equator towards North Pole we pass the climatic zone in the following order.

1. Tropical rain forest

2. Savanna

3. Steppe

4. Desert

5. Warm summer rain

6. Warm winter rain

7. Temperate rain all seasons

8. Cold moist climate

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9. Cold winter dry climate

10. Tundra

11. Ice.

The following are some of the most important elements of weather, which in different combinations make up the climate of particular place or areas.

Weather parameters/Weather elements:

1. Solar radiation

2. Temperature

3. Air pressure

4. Wind velocity and wind direction

5. Moisture (humidity)

6. Cloudiness (Sunshine hours)

7. Precipitation (Rainfall)

All these are highly variable and constitute the weather / climate. A change in one of the elements generally brings about changes in the others. All these weather elements are discussed n the following chapters.

39

Chapter - 4

Solar Radiation and Light

The sun is primary source of heat to the earth and its atmosphere. The heat received

from other celestial bodies as well as the interior of the earth from the sun is about 1,

49,000,000 (1.49 * 108) kilometers. The diameter of the sun measures roughly about

13,82,400 (1.38*106) kilometers. The surface temperature of the sun is estimated between

55000C and 61000C. The interior temperature ranges from 8*106 to 40*1006 0K. Solar

radiation provides more than 99.9 percent of the energy that heats the earth and does

not change appreciably from year to year and varies only with latitude and season.

Undoubtedly, the radiant energy from the sun is the most important control of our

weather and climate. The most astonishing fact about the incoming solar radiation

(insolation) that strikes the earth�s surface is that it is equal to about 23-billon

horsepower. Actually it is this amount of energy received from the sun that acts as the

driving force for all the atmospheric as well as biological processes on the earth.

40

Besides, all other sources of energy found on earth such as coal, oil and wood etc., are

nothing but converted from of solar energy.

All matter (not at the absolute zero temperature) what ever their temperature sends out

energy into the surrounding space in the form of electromagnetic waves and the

propagation of this energy as well as the energy it self is called �Radiation�. (If we

assume that the sun is perfectly black, the temperature it should have in order for the

flux at the outer limits of the earths atmosphere to equal the solar constant and this is

know as the �Effective temperature of the sun� and is equal to 5760 0K.) A black body at

the temperature of the sun will radiate upward 99% of its energy between the

wavelengths 0.15 and 4. About ½ of the radiation will be in the region of the spectrum

between 0.38 to 0.77 and the reminder in the invisible ultraviolet and infrared regions.

The word �insolation� is abbreviated form of �incoming solar radiation�. Radiant energy

from the sun that strikes the earth is called insolation.

41

Deflection of solar radiation:

The incoming solar radiation suffers deflection as follows:

1. Absorption by ozone layer in the upper atmosphere (about 5%).

2. Scattering by dry air.

3. Absorption, scattering and diffuse reflection by suspended solid particles and

4. Absorption and scattering by water vapour.

The surface of the earth is a poor reflector of solar radiation.

i. Fresh snow reflects 80 - 85% of incoming radiation.

ii. Old snow � 40%

iii. Grass - 20 to 44%

iv. Rock � 12 to 15%

v. Dry earth � 14%

vi. Wet earth � 8 to 9%.

vii. Cloud reflects 78%

No radiation is reflected be a smooth water surface when the sun is with 400 of the

Zenith.

Reflection of solar radiation by earth�s surface and by clouds ( Albedo of earth):

The �Albedo of the earth� is a quantity used to measure the total reflecting power of the

earth and atmosphere. It is defined as the fraction of the incoming solar radiation

returned to space by scattering and reflection in the atmosphere and by reflection at

clouds and at the earth surface. It represents the unused fraction of the incoming solar

energy; the part that is absorbed neither in the atmosphere nor in the earth. The average

albedo value of the earth is 34%

Transfer of heat:

The atmosphere is a poor absorber and the earth�s surface is good absorber of incoming

radiation, and the atmosphere receives most of the heat energy via the earth surface.

42

The heat received in one place may be transported to other places by the following

mechanism

1. Conduction

2. Radiation

3. Turbulence

4. Advection

5. Convection

1. Conduction

Conduction is the process of heat transfer through matter by molecular activity. In this

process heat is transferred from one part of a body to another or between two objects

touching each other. Conduction occurs through molecular movement.

2. Radiation

Radiation is the process of transmission of energy by electromagnetic waves and is the

means by which energy emitted by the sun reaches the earth.

3. Turbulence

The wind is never a steady current. It consists of a succession of gusts and lulls of short

period (Gust refers to sudden blast of wind and Lull refers to become calm). This

irregular motion is called Turbulence is made up of number of small eddies that travel

with general air current, super imposed on it. These eddies carry heat, moisture, dust

etc, with them as they travel from one place to other. The turbulence transfer of heat is

most effective in the vicinity of earths surface is distributed through air column,

through mixing of neighboring air masses.

4. Advection or large-scale air currents

These are mainly horizontal currents and so heat is transported from one place to

another mainly through horizontal currents and hence only in horizontal direction

where as turbulence and convective currents transport heat along the vertical.

43

a. Vertical mixing

In vertical mixing of heat the air is subject to pressure changes as it moves up or down

through the atmosphere (in turbulence). In the atmosphere the ascending air will be

cooled adiabatic ally and the descending air will be heated adiabatic lapse rate. The

result of turbulence mixing along the vertical is to create dry adiabatic lapse rate if the

air is unsaturated and a moist adiabatic lapse rate if the air saturated. Vertical mixing

will tent to decrease the temperature and increase moisture content in the upper portion

of the mixed layer and increase the temperature and decrease the moisture content in

the lower portion. This will decrease the relative humidity near the earth�s surface and

increase in the upper surface.

b. Horizontal mixing

The horizontal mixing takes place at constant pressure and no adiabatic change

involved. Two different air masses of different temperature either of which is saturated

might become saturated after complete horizontal mixing.

44

5. Convection

Convection is the process of transfer of heat, through movement of a mass or substance

from one place to another. Convection is possible only in gases or fluids, for they alone

have internal mass motions. In solid substance this type of heat transfer is impossible.

The instability is created in the lower layer of the atmosphere either through the diurnal

heating of the earth�s surface by the sun or through heating of the air when it travel

towards warmer regions. Gustiness, cumulus clouds, showers and thunderstorms are

directly caused by instability. As soon as the temperature lapse rate near the earth

exceeds the dry adiabatic slightest disturbance will upset the stratification. Air from

earth�s surface rises and air from higher levels sinks to replace the ascending masses.

This process of overturning of unstable air is called �Convection�. If the rising currents

reach the condensation level, clouds will form. The descending air surrounding the

rising masses will be heated adiabatically the R.H. Will be lower and the sky will be

broken clouds of the cumulus type. The weather phenomena that convection will

produce depend on the depth of the unstable layer, the height of the condensation level

and the distribution of temperature aloft.

Heat Budget

Of the total solar radiation reaching the outer limit of the atmosphere, about 25 per cent

is reflected by clouds and 7 percent is scattered back to space by suspended particles

and it is not used to heat air. The earth surface reflects 2 percent pf radiation to the

space. About 19 per cent of solar radiation is absorbed by gases and water vapour in the

atmosphere. About 47 per cent is absorbed by the earth. Out of which 23 percent is

absorbed by the earth from scattering of clouds and atmosphere. And 24 percent is

received directly from the sun. Thus approximately two-thirds of the total radiation is

effective in heating the earth. The total energy coming to the earth over a considerable

period of time is equal to the total outward losses. In order to maintain the terrestrial

heat balance, the 66 percent of solar radiation gained must be balanced by the same

45

amount of energy radiated back to space in the form of long-wave terrestrial radiation

(transferred by conduction and convection). In this way the overall heat budget of the

earth is balanced. If this were not so, the earth would soon become either very hot or

very cold. Actually there is a deficit of heat at higher latitudes and surplus in low

latitudes.

Latent heat

Normally, when heat is given to a substance, its temperature rises. However, the heat

which changes the physical state of a substance but not raise its temperature is called

latent heat of that substance. The latent heat of a substance is thus the amount of heat

absorbed (or given out) by a unit mass of the substance to change its state without

change of temperature. The latent heat is used up in overcoming the force of attraction

between the molecules of the substances.

Sensible heat flux

It is same as enthalpy and the product of heat capacity times the Kelvin temperature, at

constant pressure for a perfect gas. This is used in meteorology in contrast to latent heat.

In crop canopies the heat energy utilized in raising the temperature is referred to as

sensible heat.

Sensible heat advection

The process in which warm dry air passing over a field supplies energy for transpiration.

Solar Constant

It is the amount of solar energy incident on a unit area at right angle to the sun�s rays at

the earth�s mean distance per unit time in the absence of atmosphere. Solar constant is 2

cal /cm2/ minute. The sun is the source of more than 99 per cent of the thermal energy

required for the physical processes taking place in the earth atmosphere system. Every

minute, the sun radiates approximately 56x1026calories of energy. In terms of the energy

46

per unit area incident on a spherical shell with a radius of 1.5x1013 cm (the mean

distance of the earth from the sun) and concentric with the sun, this energy is equal to

56 x 1026 cal. Min-1

S = ------------------------- = 2.0 langely min-1.

4 (1.5x1013 cm) 2

(Langley =gram calories cm-2).

Solar constant = 2.0 gram calories cm-1min-1

The solar constant (S) is a true constant, but fluctuates by as much as 3.5 percent about

its mean value, depending upon the distance of earth from the sun.

Solar constant is defined as the rate at which solar radiation is received outside

the earth�s atmosphere on a surface perpendicular to the sun�s rays when the earth is at

an average distance from the sun. The Smithsonian Institute, USA has come to the

conclusion that the standard value of solar constant is 1.94g cal. cm-2 min-1.

Since there is fluctuation in the amount of radiant energy emitted by the sun due

to periodic disturbances on the solar surface, the amount of solar constant, therefore,

registers a slight increase or decrease. However, this hardly exceeds 2-3 per cent.

The amount of insolation received on any date at a place on the earth is governed by

1. The solar constant which depends on (a) energy output of the sun and

(b) distance from the earth to sun.

2. Energy out put of the sun.

3. Distance from the earth to sun.

4. Transparency of the atmosphere.

5. Duration of the daily sunlight period.

6. Angle at which the sun�s rays strike the earth.

47

The distance between the earth and the sun varies between 94.5 million miles

(157.5m km) at aphelion (July 1st) and 91.5 million miles at perihelion (January1st). The

amount of radiation received is seven percent greater at perihelion than at aphelion.

This is a consequence of the inverse square law, which states, in effect, that the radiation

received on any unit area decreases in proportion to the square to the distance of the

sources.

1

Intensity = ------

d 2

(Aphelion � The point farthest from the sun in the orbit of a planet.

Perihelion - The point nearest from the sun in the orbit of a planet)

Transparency of the atmosphere has a more important bearing upon the amount

of insolation, which reaches the earth�s surface. The areas having dust, clouds, water

vapour and cloudiness or polluted air will receive less direct insolation. The

transparency of atmosphere depends on the latitude of a place. At middle and high

latitudes the sun�s rays must pass through thicker layers of reflecting/scattering

material and it is not so at tropical latitudes.

Effect of Light on Plants

Solar radiation consists of a bundle of rays of radiant energy of different wavelengths.

The sun emits radiant energy in the form of electromagnetic waves. The visible portion

of the solar spectrum appears as light. Light travels with a speed of 2,97,600 km/sec. It

takes 8 minutes and 20 second to reach the earth. Light is the total effect of the

combination of the seven different colours, Viz., Violet, Indigo, Blue, Green, Yellow,

Orange and Red (VIBGYOR). The waves that produce the effect of red colour are the

longest and those producing the violet are the shortest. Waves shorter than the violet

are called Ultraviolet rays, while those longer than the red are known as infra Red rays.

The ultra violet waves form only 6 per cent of the insolation, but have strong

48

photochemical effects on some substances. The infrared rays, even though invisible,

form 43 per cent of the isolation. They are largely absorbed by water vapour that is

concentrated in the lower atmosphere.

Solar radiation is the primary source of electromagnetic spectrum having different

wavelength. Different type of radiation is shown below. (Wavelength in micron)

1. Cosmic rays 10-7 to 10-4 micron

2. Gamma rays 10-4 to 10-3 micron

3. X rays 10-3 to 10-1 micron

4. U.V.1 to 390 micron

5. Visible 390-760 micron

6. Infrared 760-106 micron

7. Radio wave 106-1013micron

Visible solar radiation is called as light. The shorter wavelength in the solar spectrum is

harmful to the plants when exposed to excessive amounts. The atmosphere, however,

absorbs almost all the shorter wavelengths. The infra radiation has thermal effect on

plants by supplying necessary energy for evaporation of water from the plants.

The visible portion of the solar spectrum is the light with wavelength ranging from 0.4

to 0.7. Light is one of the important climatic factors for many vital functions of the

plant. It is essential for the synthesis of the most important pigment i.e., chlorophyll.

The chlorophyll absorbs the radiant energy and converts into potential energy of

carbohydrates (photosynthesis). The carbohydrate thus format is the connecting link

between solar energy and living World. In addition, it regulates the important

physiological functions like transpiration.

49

Effect of light on plant can be studied under four headings

1. Light intensity

2. Quality of light

3. Duration of light and

4. Direction of light.

1. Light Intensity:

A standard unit called candle measures the intensity of light. The amount of light

received at a distance of one meter from a standard candle is known as �Metre Candle

or Lux�. The light intensity at one foot from a standard candle is called �Foot candle� or

10.764 luxes and the instrument used is called as �Lux meter�. About one per cent of

the light energy is converted into biochemical energy. Very low light intensity reduces

the rate of photosynthesis and may even results in result in the closing of the stomata

detrimental to plants in many ways. This results in reduced plant growth. Very high

light intensity increases the rate of respiration. It causes rapid loss of water, i.e., it

increases the transpiration rate of water from the plants resulting in closure of stomata.

The most harmful effect of high intensity light is that it oxidizes the cell contents, which

is termed as �Solarisation�. This oxidation is different from respiration and is called as

�Photo oxidation�. Under natural conditions light intensity varies greatly and plants

shoe marked response to changes of light intensities. Based on the response to light

intensities the plants are classified as follows:

i) Sciophytes - (Shade loving plants)

The plants that grow better under partially shaded (low light) conditions e.g.,

Betel vines, Buckwheat, Turmeric etc.,

ii) Heliophytes - (sun loving Plants)

Many species of plants produce maximum dry matter under high light

intensities when the moisture is available at the optimum level, e.g. maize,

50

sorghum, rice etc. Except under glass house or shaded conditions, intensity of light

cannot be controlled.

2. Quality of Light

When a beam of white light is passed through a prism, it is dispersed into different

colours with their wavelengths partitude. This is called the visible part of the solar

spectrum.

The different colours and their wavelength are as follows:

Violet & Indigo 400-435nm

Blue 435-490nm

Green 490-574nm

Yellow 574-594nm

Orange 594-626nm

Red 626-750nm

Visible rays 390-760 mill micron /m/nm

51

1

Micron = ------------- meter or 10-6m

10,00,000

1

= ---------- mm = 10-3mm

1000

Milli micron: 10-9 m = nanometer

The Principal wavelengths absorbed and used in photosynthesis are in the violet-blue

and the orange-red regions. Among this, red light is the most favorable light for growth

followed b violet-blue. Ultra violet and shorter wavelengths kill bacteria and many

fungi.

52

3. Duration of light

The duration of light has greater influence than the intensity. It has a considerable

importance in the selection of crop varieties. The response of plants to the relative

length of the day and night is known as photoperiodism. The plants are classified

based on the extent of response of day length as follows.

i) Long day Plants

The plants which develop and produce normally when the photoperiod is

greater than the critical minimum (greater than 12 hours) e.g. Cereals, Potato,

Sugar beet, Wheat, Barley etc.

ii) Short day plants

The plants which develop normally when the photoperiod is less than the critical

maximum (less than 12 hours) e.g. Tobacco, Soybean, Millets, Maize, Sugarcane, etc.

53

iii) Indeterminate or day neutral plants

Those plants which are not affected by photo period, e.g., Tomato, Cotton, Sweet

potato, pineapple etc.,

The photoperiodism influences the plant characters such as floral initiation and

development, bulb and rhizome production etc. If a long day plant is grown during

periods of short days the growth of internodes are shortened and flowering is delayed

till the long days come in the season. Similarly when short day plants are subjected to

long day periods, there will be abnormal vegetative growth and there may not be any

floral initiation in Rice Cv. CO 38. But now a days many crops do have photo-

insensitive varieties.

4. Direction of light

The direction of sunlight has a greater effect on the orientation of roots shoots and

leaves. In temperate regions, the southern slopes show better growth of plants than the

northern slopes due to higher contribution of sunlight in the southern side.

54

Orientation of leaves

The changes of position or orientation of organs of plants caused by light is usually

called as �Phototropism�. For example, the leaves are oriented at right angles to

incidence of light to receive maximum radiation.

Photo morphogenesis

It is defined, as changes in the morphology of plants due to light. This is due to ultra

violet and violet rays of the sun.

Duration of sunlight period (Length of day)

55

The vertical rays of the sun at noonday fall directly overhead at the equator on March

21st and this is called �Vernal equinox�. The vertical rays continue to move northern to

the tropic of cancer and are overhead there on June 21st and this date is known as

�Summer solstice� (In northern hemisphere) and the rays return to the equator on

September 21st and this date is known as� Autumnal equinox�. Then it reaches the

tropic of Capricorn on December 21st and this date id known as �Winter solstice� (in

northern hemisphere). The summer and winter solstices will be reverse in the southern

hemisphere. At equinox days and nights are of equal length throughout the world. In

summer solstice the day will be longer whereas in winter solstice the day will be shorter

than night. The northern pole will be in daylight for the full 24 hours on summer

solstice and will be dark for full 24 hours on winter solstice of northern hemisphere.

Angle of the sunrays

The effect of varying angle at which the sun�s ray strike the earth can be seen daily by

the march of the sun across the sky. At solar noon the intensity of insolation is the

greatest but in the morning and evening hours when the sun is at low angle, the

amount of insolation is also small.

At equator the angle of incidence varies from 231/20 North of the zenith to 231/20 South

of the zenith. The intensity of solar radiation ranges from 92% on June 21st and

December 21st to 100% on March 21st and September 23rd. The range is only 8%. At 450N

latitude the angle of incidence varies from 211/20 south of zenith to 681/20 south of

only 211/20 above the horizon. The variation in intensity is due to the change in the

angle of incidence from 93% of maximum on June 21st to 98% on December 21st. The

inclination of the earth is 66033� against the plane of the orbit and this angle is the main

reason for the seasons. Only at the time of Equinox (March 21, September 23) does the

dividing line of the lighted and dark half of the earth parallel and pass through the

poles. Between March 21 and September 23 to the north pole is tilted towards the sun

and from September 23 to March 21 the south Pole is tilted towards the sun. The sun is

56

fixed in its place but rotates on its axis once in 251/3 days. The path taken by the earth

round the sun is called the �Eliptic�. The orbit of earth round the sun is roughly

circular, with only a slight eccentricity (in a conic section). The sun�s ray strikes the

surface of the earth perpendicularly near the equator and with greater obliquity as the

place moves from the equator to the poles. As the obliquity increases, the surface over

which the rays spread out is increased and the insolation received by unit surface

decreases.

Facts about earth

a. Superficial area: 19, 69, 50, 000 (1.97x108) sq. miles

b. Land surface: 5, 75, 10,000 (5.7x 107) sq. miles.

c. Water surface: 13, 94, 40, 4000 (1.39x108) sq. miles.

d. Earth makes one complete revolution on it axis in 23 hours and 56 minutes.

e. Earth rotates round the sun in 365 1/4 days.

f. Earth revolves in its orbit round the sun at a speed of 6,66,000 m.p.h.

g. Earth rotates on its axis at an equatorial speed of 1000 m.p.h.

h. The earth is closest to the sun on January 1 at about 91, 342,000 miles and

farthest away on July 2nd 94,454,000 miles.

The sun is a star with a surface temperature of about 60000C radiates into space. On a

surface exposed normal to the Sun�s rays at the mean distance from the sun, energy of

1.94 gm cal/cm2 per minute is received on an average. This energy amount of 1.94 gm

cal/minute is called the Solar constant. The mean intensity of the solar radiation

received on January 1st and July 2nd at the boundary of the atmosphere is 2.007 and

1.877 g cal/cm2/minute respectively.

57

Chapter - 5

Temperature

Temperature refers to the degree of hotness or coldness of a substance or a thing and it

provides a measure of the intensity of heat energy.

Air temperature and its Importance

Temperature is necessary for the weathering of soils, promoting bacterial activity,

sterilization of soils, killing of weeds, pests and disease, for drying grains and maturity

of crops. Every living organism, plant, or animals or insects requires optimum

temperature for carrying out the basic biochemical activities for survival. Excessive

temperature is harmful for germination growth, flowering and maturity of fruits. It

increases the transpiration from plants and evaporation from the soil and necessitates

frequent irrigation. It is the most important phenomenon of solar energy. In climatology

the word temperature denotes �Shade� temperature to avoid the influence of direct

rays of sun.

It is measured by means of thermometers. Day temperature at any given time is meant

the temperature of the air measured under standardized condition and with certain

recognized precautions against errors introduced by radiation from the sun or other

heated body.

Mean daily temperature

It is the mean of 24 readings taken at hourly intervals as in the self-recording

instruments, like thermograph. But in other thermometers these are usually taken in the

morning (8 a.m.), afternoon (2 p.m.) and evening (6 p.m.).

58

Mean monthly temperature

It is the average of the total daily mean temperature for the month divided by the

number of days in a month.

Mean annual temperature

It is the average of the 12 months temperature for the total of monthly means divided

by 12.

Mean annual range

The difference between the warmest and coldest months is the mean annual range.

Only mean temperature is usually quoted in describing climate.

The centigrade scale labels the temperature of boiling point of water under 1 atm. of

pressure 1000C and the freezing point of water as 00C. The Fahrenheit scale labels the

same temperature as 2120F and 320F respectively. The numerical relation between the

two scales is then

Sensible temperature

The temperature recorded by the thermometer does not always agree with the

sensations of heat felt by the human body. The sensation of the heat depends upon air

movement and humidity. 800F in the equatorial zone is more uncomfortable than 1000F

in the desert because of humidity.

Seasonal (temperature) variations

Temperature (Diurnal, mean and range) vary according to the season. The main factors

contributing to seasonal variations are: -

0C 100 = 0F-32 180

59

1. The angle of inclination of solar rays, which decides the intensity of radiation.

2. Distance between earth and sun

3. The movement of seasonal winds which contributes to rain and precipitation.

Diurnal variation

The difference between the maximum and minimum temperature on a day is called

diurnal range. It is smaller in the wet season than in the dry season and smaller in

coastal areas in the interior place.

Annual Variation

The difference between the temperatures in a year is annual variation. The temperature

is more in May and June and lesser in November, December in Tamilnadu.

The amount of the daily range of variation varies widely with many factors like

cloudiness and humidity of the air, nature of earth�s surface, the vertical lapse rate of

temperature, wind, elevation and latitude and is discussed below.

Cloudiness:

Cloudiness influences the penetration of insolation to earth�s surface by day and

retardation of net loss of heat by terrestrial radiation at night.

Humidity of air

There is only very small diurnal variation of temperature over the ocean; on land, after

heavy rains where soil is moist and water stands on the surface, temperature ranges are

less than during dry weather, because of the humidity of the air. The average range of

temperature increases with distance from water sources.

Air with Steep Lapse-Rate

Heating during the day is accompanied by deep convection where by energy absorbed

by air near the earths surface is distributed through a thick layer of air. Similarly at

night steep lapse rates are often accompanied winds and turbulent mixing that keep the

60

lower layers warmer than in conditions of still stable air with steep lapse-rate. Diurnal

ranges of temperature are usually smaller at in the station than nearly valleys. Daily

range of temperature increase with latitude up to subtropical latitude. Maximum daily

ranges have been recorded in subtropical deserts where clear air & dry land surface

prevails. But in the same latitude along foggy coasts parallel by cool ocean current, sea,

breeze chop off maximum temperature, fog interferes with terrestrial radiation at night

and hence daily range are the lowest in the world. In middle latitudes, daily ranges very

less with latitude than with distance from the sea. In high altitudes diurnal range

decrease again, owing to the lessened effectiveness of the daily successive of the

sunlight and darkness.

Vertical distribution of temperature (Altitude)

As a general rule throughout the troposphere, the temperature decreases with

elevation. The rate of decrease with altitude this condition is reversed at certain levels

so that temperature temporarily increases with altitude is not uniform; it varies with

time of the day, season and location. The average decrease is approximately

0.650C/100m. (6.50C/km). This is known as normal lapse rate or vertical temperature

gradient.

Temperature Inversion

Although normally, the lower several miles of atmosphere show a decrease in

temperature with increasing altitude when the colder air lies below warmer air and

closer to earth�s surface the normal lapse rate is reversed and this is called temperature

inversion.

Horizontal distribution of temperature (Latitude)

The lines connecting places, which have same air temperature, are called isotherms.

Thus, all the points on a map through which any one isotherm passes have identical

average temperature for the period indicated. There is general decrease from equator to

poles (increase in latitude).

61

Factors affecting temperature

Elevation of a place

Soil type

Nearness to water body

Presence of hill or mountain

Location of the earth (co-ordinate)

Anthrophic factors

Effect of temperature on Plant growth / Crop Productivity

Air temperature is the most important weather parameter, which affects the plant life.

The growth of higher plants is restricted to a temperature between 0 to 600C and the

optimum For example 100C to 400C. Beyond these limits, plants are damaged severely

and even get killed. The maximum production of dry matter occurs when the

temperature ranges from 20 and 300C. As already seen the temperature of a place is

largely determined by latitude and altitude. Based on the above the vegetations are

classified as tropical (rain forest, desert, grassland), temperature (Grassland, deciduous

forest), taiga (coniferous forest), tundra (lowshrubly growth, lichen) and polar. Some

investigators have classified the vegetation of the world into four classes based on the

prevailing temperature conditions. The four classes are

1. Megatherms - Equatorial and tropical rain forests

2. Mesotherms - tropical and sub tropical, tropical deciduous forests

3. Microtherms - temperate and high altitude, alpine vegetation and mixed

coniferous forests and

4. Hekistotherms - artic and alpine regions

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High night temperature favors growth of shoots and leaves and it also affects plant

metabolism. On the other hand low night temperature injures the plants. Tender leaves

and flowers are very sensitive to low temperature and frost.

Temperature is of paramount importance for organic life because of the following

factors: -

a. Temperature governs the physical and chemical processes within the

plants, which in turn control biological reactions which take place within

the plants.

b. The diffusion rate of gases and liquids change with temperature.

c. Solubility of different substances is depending upon temperature.

d. The rate of reactions varies with variations in temperature.

e. Equilibrium of various systems and compounds is a function of

temperature and

f. Temperature affects the stability of the enzyme system.

Every plant has its own minimum, optimum and maximum temperature limits for its

normal growth and reproduction. The vital physiological activities of a plant stop both

at below the minimum level ad at above the maximum level, whereas physiological

activities will be at its maximum at optimum temperature levels. These levels of

temperature are known as cardinal temperature points.

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Cardinal temperature for the germination of some important crops (Bierhyzen, 1973)

Cardinal Temperature 0C S. No Plant

Minimum Optimum Maximum

1 Rice 10-12 30-32 36-38

2 Sorghum 8-10 32-35 40-42

3 Maize 8-10 32-35 40-44

4 Wheat 3-4.5 25 30-32

5 Barley 3-4.5 20 38-40

6 Sugar beat 4-5 25 28-30

7 Tobacco 13-14 28 35

8 Carrot 4-5 8 25

9 Peas 12 32-34 40

10 Oats 4-5 25 28-30

11 Lentil 4-5 30 36

In General

Cool season crops 0-15 25-31 31-37

Hot season crops 15-18 31-37 44-50

Apart from yield reductions, many visible injuries on the plants are seen due to very

high temperature.

Cold injury: (Low Air Temperature and Plant Injury)

1. Chilling injury

Plants, which are adapted to hot climate, exposed to low temperature for sometime, are

found to be severely injured. Some effects of chilling are development of chlorotic

condition (Yellowing)

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Example: Chlorotic bands in the leaves of Sugarcane, Sorghum and Maize in winter

months when the night temperature is below 200C.

Based on the reaction to chillness, plants can be divided into five categories.

a. Plants killed by exposure to temperature in the range of 0.5 to 5.00C for 60

hours. Rice, Cotton, Cowpea.

b. Plants injured by the above condition but recovered after being placed in

favorable conditions. For example Sudan grass, Spanish and Valencia

peanut

c. Plant not likely to suffer serious injury. For example Corn, Sorghum and

Pumpkin.

d. Plants injured by prolonged chillness. For example Buck wheat and

Soybean.

e. Plants not injured by prolonged chillness. For example Sunflower, Tomato

and Potato.

In temperate climate two types of injury viz., delayed growth and sterility occur

because of low temperature for example, In Japan, rice yields decreases due to

insufficient grain maturation caused by low temperatures during the ripening period.

Low temperatures delay flowering at a certain stage before heading. Rice yield

decreases due to sterility of spike let caused by low temperatures at the booting stage or

anthesis. The observed injuries may be stoppage of anther development, pollen

unrippeness, partial or no dehiscence, pollen grains remaining in anther loculi, little or

no shedding of pollen grains on stigma and failure of germination of pollen on stigma.

2. Freezing injury

Plant parts or entire plant may be killed or damaged beyond repair as a result of actual

freezing of tissues. Ice crystals are formed first in the intercellular spaces and then

within the cells. Ice, within the cells, causes more injury by mechanical damage on the

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structure of the protoplasm and plasma membrane. Freezing of water in intercellular

spaces results in withdrawal of water from the cell sap due to dehydration and causes

death of cells. E.g., Frost damage in Potato, Tea, etc.,

3. Suffocation

In temperate regions, usually during the winter season, the ice or snow forms a thick

cover on the soil surface. As a result the entry of O2 is prevented and plants suffer for

want of O2. Ice coming in contact with the roots prevents the diffusion of CO2 outside

the root zone. This prevents the respiratory activities of roots leading to accumulation of

harmful substances.

4. Heaving

This is a kind of injury caused by lifting up of the plants along with soil from its normal

position. This type of injury is common in temperate regions. The presence of ice

crystals increases the volume of soil. This causes mechanical lifting of the soil.

Effect of high Temperatures

Cells of most plant species get killed when the temperature ranges from 50 to 60 C.

This point of temperature is called Thermal death point and varies with the following

factors.

1. The species

2. The age of tissue and

3. The time of exposure to high temperature

It is reported that most plant cells are killed at a temperature of 45 to 55 C. Some plants

tissues withstand a temperature of up to 105C. The aquatic plants and shade loving

plants are killed at comparatively, lower temperature (40C); where as, for xerophytes it

is 50C. High temperature results in desiccation of the plants and disturbs the balance

between photosynthesis and respiration. Higher temperature increases the respiration

leading to rapid depletion of reserve food in plants resulting in growth stunted due to

incipient or starvation

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Heat injuries

1. Sun clad

Injury caused by high temperature on the sides of bark is known as sun clad, this is

nothing but exposure of barks of the stems to high temperature during daytime and low

temperature during night time.

2. Stem girdle

It is another injury associated with high temperature. High temperature at the soil

surface scorches the stems at ground level. This type of injury is very common in young

seedlings of cotton in sandy soil where the after noon soil temperature exceeds 60C to

65C. The stem girdle injury is first noticed through a discolored band a few millimeters

wide. This is followed by shrinkage of the tissues, which have been discoloured. The

stem girdle causes the death of the plant by destroying the conductive and cambial

tissues or by the establishment of pathogens in the injury. As direct effects on crop

plants high temperature causes sterility in flowers. The general effects of excessive heat

are defoliation, pre-mature dropping of fruits. In extreme cases, death of the plants may

also occur.

Temperature aberrations

Heat Wave

A region is considered to be in the grip of moderate heat wave when it recorded

maximum temperature exceeds the normal by 5 to 8C. Heat wave is common in the

state of Uttar Pradesh (54%Probablity) in the month of June. Incidences are maximum

in western UP. Persistence is 5-6 days particularly more in June.

Effect of Heat wave

As already discussed under the effect of temperature crop growth the thermal death

point affects photosynthesis and respiration. Increased respiration results in depletion

of reserve food, sun clad, stem girdle.

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Cold Waves:

A region is said to be in the grip of a moderate cold wave when it recorded minimum

temperature falls short of the normal by 6C to 8 C and severe cold wave is prevailed

when the minimum temperature short falls up to 8C. Generally experienced from Nov.

to March. Severe clod wave generally prevail form January to March that is common in

Uttar Pradesh. In Western U.P it is for 1day where as in Eastern U.P it is 2-7days.

Degree-days:

At a given location, the period between planting and harvesting is not a specific number

of calendar days but rather a summation of energy units, which may be represented as

degree-days. A degree-day for a given crop is defined as a day on which the mean daily

temp is one degree above the zero temps. (that is the minimum temperature for

growth) of the plant.

Zero temperature:

Spring wheat: 32-400F (depending on variety)

Oat: 430F; Field Corn: 54-570F Sweet Corn: 500F,

Potatoes: 450F, peas: 400F cotton 620F-640F

The period required for achieving maturity is also a function of the length of day is

photoperiod. Crop, planted in early in the spring requires more calendar days to

mature than the same crop planted later.

Growing Degree days (GDD):

GDD or accumulated day degrees is also called as Effective Heat Unit. This is an

arithmetic accumulation of daily mean temperature above certain threshold

temperature. This is computed by using the formula suggested by IWATA (1984).

Maximum temperature + Minimum temperature

GDD = ------------------------------------------------------------ � base temperature

2

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Soil Temperature

In many cases soil temperature is more important to plant life than air temperature. It

influences the germination of seeds and root activities. It influences the soil-borne

diseases like seeding blight, root rot etc. The decomposition of organic matter will be

higher in higher soil temperature with necessary moisture. It controls the nutrient

availability. It tropics high temperature of soil causes regeneration of potato tubers. It

also affects nodulation on legumes. The surface of the soil is exposed to the direct

radiation and movement. It gains heat during the daytime and losses some parts during

the night to the atmosphere.

Diurnal variation of soil temperature

- As depth of soil increases temperature increases up to 20 cm remains unchanged

beyond 30 cm.

- Surface temperature is doubled in the afternoon compared to morning due to

insolation.

- Variation due to morning and afternoon temperature beyond 30cm depth is

negligible.

- A variation beyond 30 cm is only seasonal changes.

Effect on crop growth

In many instances soil temperatures is of greater importance to plant life than air

temperature. For example, Peach and Oak trees can withstand air temperature of �250C

but roots of these trees cannot tolerate even up to �160C. It influences the soil borne

disease like seedling blight, root rot etc and decomposition of organic matter. Input =

storage + output. Storage causes changes below surface. Conduction of heat, to lower

layer depends on thermal properties of the soil like, specific heat, thermal conductivity

and thermal diffusivity of the soil. It influences the germination of seeds and root

activities. It also influences the soil borne, diseases like seedling blight, root rot etc., and

decomposition of organic matter. Greater the soil temperature higher will be the

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decomposition of organic matter. It controls the nutrient availability. In the tropics high

temperature of soil causes degeneration of potato tubers. It affects nodulation in

legumes.

Cardinal Temperature � Temperature of vital activities

Cool season crops Hot season crops Minimum 0-50C 15-180C Optimum 25-310C 31-370C Maximum 31-370C 44-500C