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  • AGRICULTURAL METEOROLOGY Dr. S.M. Sureshkumar Dr. G. Baradhan Dr. S. Manimaran Dr. N. Ramesh Department of Agronomy Faculty of Agriculture Annamalai University 1
  • Imprint 2
  • Chapter -1 Introduction to Agricultural Meteorology Agricultural Meteorology – definition ofmeteorology, agro – climatology- weather and climate,factors affecting weather and climate – climate types –scope of Agricultural meteorology, co-ordinates of Indiaand Tamil Nadu – Atmosphere – stratification andcomposition.Chapter – 2Solar Radiation and Temperature Solar radiation – light and heat energy – Intensity,Quality, day length and direction of light, effect on cropproduction, measurement – Temperature effect on cropgrowth, cardinal temperature, diurnal and seasonvariations, isotherm – soil temperature and effect on cropgrowth – measurement of temperatureChapter - 3Atmospheric pressure and wind systems of the world Atmosphere pressure – pressure system – causesfor variation, importance, isobar, measurement –Atmospheric humidity – Absolute and specific humidity,vapour pressure, effect on crop growth , measurement. 7
  • Wind systems of world, tropical convergence zones, speedand direction of wind measurement – effect on crops.Chapter – 4Rainfall and Evaporation Clods, classification and characteristics –precipitation – forms – Hydrological cycle – rainfall –monsoons of India, onset and withdrawal, effect on cropproduction – measurement of rainfall. Isohyet – artificialrain making – evaporation and transpiration, factorsaffecting evaporation.Chapter – 5Weather Aberrations and forecasting Weather aberrations – deficient rainfall (drought),cyclone, anticyclones, hurricane, tornado, storms – effecton agriculture – phenology and bioclimatic law,Agricultural seasons of India – Agroclimatic zones ofworld, India and Tamil Nadu – weather forecasting –importance. Types – Agrimet observatory – synopticchart, crop weather calendar – remote sensing and cropweather modelling – agroclimatic normals for field crops –effect of climate and weather on crop production and pestand diseases. 8
  • Appendix IAppendix II 9
  • 10
  • Chapter 1INTRODUCTION TO AGRICULTURAL METEOROLOGY AND ATMOSPHEREMeteorology The word meteorology was derived from a Greekword “meteoros” means ‘above the earth’s surface’ whichis nothing but atmosphere. The meaning of “logy” is astudy of science. Therefore, meteorology is defined as abranch of science dealing with that of the atmosphere.Meteorology is concerned with the physical, chemical anddynamical state of the earth’s atmosphere and it’sinteraction with the underlying surface. Meteorology is essentially known as observationalscience dealing with different atmosphere variables suchas pressure, temperature, wind, humidity, clouds, andradiation at a given moment. Meteorology predicts thebehavior of the atmosphere over a period of a few hoursto a few days ahead. The main task of meteorology is toprovide weather forecasting and communicate the samein time to the users. Another important duty of themeteorology is to increase our knowledge of the 1
  • atmosphere through research and aiming at the accuracyof weather forecast. Meteorology is a combination of bothphysics and geography. Meteorology is the science of weather. It isessentially an inter-disciplinary science because theatmosphere, land and ocean constitute an integratedsystem. The three basic aspects of meteorology areobservation, understanding and prediction of weather.There are many kinds of routine meteorologicalobservations. Some of them are made with simpleinstruments like the thermometer for measuringtemperature or the anemometer for recording windspeed. Countries around the world exchange the weatherobservations through fast telecommunications channels.These are plotted on weather charts and analyzed byprofessional meteorologists at forecasting centers.Weather forecasts are then made with the help of moderncomputers and supercomputers. Weather informationand forecasts are of vital importance to many activitieslike agriculture, aviation, shipping, fisheries, tourism,defence, industrial projects, water management anddisaster mitigation. Recent advances in satellite and 2
  • computer technology have led to significant progress inmeteorology.Agricultural meteorology In simple terms, agricultural meteorology is theapplication of meteorological information and data for theenhancement of crop yields and reduction of crop lossesbecause of adverse weather. This has linkages withforestry, horticulture and animal husbandry. The agrometeorologist requires not only a sound knowledge ofmeteorology, but also of agronomy, plant physiology andplant and animal pathology, in addition to commonagricultural practices. This branch of meteorology is ofparticular relevance to India because of the highdependence of our agriculture on monsoon rainfall whichhas its own vagaries. So, Agricultural meteorology canalso be defined as follows:  A branch of applied meteorology which investigates the physical conditions of the environment of growing plants or animal organisms  An applied science which deals with the relationship between weather/climatic conditions and agricultural production 3
  •  A science concerned with the application of meteorology to the measurement and analysis of the physical environment in agricultural systems. The word ‘Agro meteorology’ is the abbreviated form of agricultural meteorology  To study the interaction between meteorological and hydrological factors on the one hand and agriculture in the widest sense, including horticulture, animal husbandry and forestry on the other (WMO, Geneva)Importance to crop production1. Helps in planning the cropping patterns/systems2. Selection of sowing dates for optimum crop yields3. Cost effective ploughing, harrowing, weeding etc4. Reducing losses of applied chemicals and fertilizers5. Judicious irrigation to crops6. Efficient harvesting of all crops7. Reducing or eliminating outbreak of pests and diseases8. Efficient management of soils which are formed out of weather action9. Managing weather abnormalities like cyclones, heavy rainfall, floods, drought etc. This can be achieved by 4
  • (a) Protection: When rain is forecast avoid irrigation. But, when frost is forecast apply irrigation(b) Avoidance: Avoid fertilizer and chemical sprays when rain is forecast(c) Mitigation: Use shelter belts against cold and heat waves10.Effective environmental protection11.Avoiding or minimizing losses due to forest fires. Future scope of agricultural meteorology1. To study climatic resources of a given area for effective crop planning.2. To evolve weather based effective farm operations3. To study crop weather relationships for all important crops and forecast crop yields based on agro climatic and spectral indices using remote sensing4. To study the relationship between weather factors and incidence of pests and diseases of various crops5. To delineate climatic/agro ecological/agro climatic zones for defining agro climatic analogues so as to make effective and fast transfer of technology for improving crop yields6. To prepare crop weather diagrams and crop weather calendars 5
  • 7. To develop crop growth simulation models for assessing/obtaining potential yields in different agro climatic zones8. To monitor agricultural droughts on crop-wise for effective drought management9. To develop weather based agro advisories to sustain crop production utilizing various types of weather forecast and seasonal climate forecast10.To investigate microclimatic aspects of crop canopy in order to modify them for increased crop growth11.To study the influence of weather on soil environment on which the crop is grown12.To investigate the influence of weather in protected environment (eg. Glass houses) for improving their design aiming at increasing crop production.Divisions of Meteorology Sl. Division Application No. name 1. Dynamic Deals with the forces that meteorology create and maintain motion and the latest transformations associated therewith 2. Physical Deals with physical nature meteorology such as radiation, heat, evaporation, condensation and precipitation 6
  • 3. Climatology Study of weather patterns over time and space 4. Synoptic Deals with analysis and meteorology forecasting of the weather phenomena 5. Aeronautical Application of meteorology to meteorology the problem of aviation 6. Maritime Deals with marine navigation meteorology 7. Agricultural Application of meteorology in meteorology relation to crop production 8. Hydrometeor Deals with meteorological ology problems related to water supply, flood control, irrigation etc. 9. Medical Influence of weather and meteorology climate on the human body 10. Aerology Deals with the conditions of the free atmosphere on the basis of direct observationsCoordinates of India and Tamil naduCoordinates of IndiaLies between 8º N and 37º N latitude 68ºE and 97º E longitudeCoordinates of Tamil NaduLies between 08º 05’ N and 13º 35’ N latitude 76º 15’ E and 80º 20’ E longitudeThe Earth is elliptical in shape and has three spheres Hydrosphere - the water portion 7
  • Lithosphere - the solid portion Atmosphere - the gaseous portionAtmosphere The word atmosphere derives from the Greek word“Atmos” which means vapour and “Sphaira” whichmeans sphere (layers).The atmosphere is defined as thecolourless, odourless and tasteless physical mixture ofgases which surrounds the earth on all sides. It ismobile, compressible and expandable. It contains hugenumber of solid and liquid particles called aerosol.Atmosphere is a huge envelope of mixture of gasesextending several hundreds of kilometers from earthsurface. Some gases are permanent atmosphericconstituents in fixed proportions to the total gas volumeand others vary from place to place and time to time.Uses of atmosphere1. Provides oxygen which is useful for respiration in crops2. Provides carbon-dioxide to build biomass in photosynthesis3. Provides nitrogen which is essential for plant growth4. Acts as a medium for transportation of pollen5. Protects crops plants on earth from harmful UV rays 8
  • 6. Maintains warmth to plant life and7. Provides rain to field crops as it is a source of water vapour, cloud, etc.Composition of atmosphere (Fig.1)The following all the different gases that are present inpercentage by volume approximately.Nitrogen (N2) = 78.08 Oxygen (O2) = 20.95Argon (Ar) = 0.93 Carbon di oxideCO2 = 0.03Neon (Ne) = 0.0018 Helium(He) = 0.0005Ozone(O3) = 0.00004 Hydrogen(H2) = 0.00006Methane (CH4) = 0.00017 Fig.1 9
  • Fig.2 Vertical Layers of atmosphere based on temperature/Stratification (Fig.2) On the basis of vertical temperature variation theatmosphere is divided into different spheres or layers. A) Troposphere1. The troposphere is the atmospheric layer closest to the planet and contains the largest percentage (around 80%) of the mass of the total atmosphere.2. The word “Tropo” means mixing or turbulence and “sphere” means region.3. The troposphere is approximately 12 kilometers thick, but there are slight variations. The average height of this lower most layer of the atmosphere is about 14 km above the mean sea level; at the equator it is 16 km and 7-8 km at the poles.4. All weather phenomenon that we are primarily interested like various types of clouds, 10
  • thunderstorms, cyclone and anticyclones occurs in the troposphere because of the concentration of almost all the water vapour and aerosols in it. So, this layer is called as “seat of weather phenomena”.5. The wind velocities increase with height and attain the maximum at the top of this layer.7. Another important feature of the troposphere is that as height increases, temperature decreases. The temperature drops about 6.5 ºC for every kilometer above the earths surface (Lapse rate).8. Most of the radiation received from the sun is absorbed by the earth’s surface. So, the troposphere is heated from below.9. In this layer, about 75 per cent of total gases and most of the moisture and dust particles present.10. The top of the troposphere there is a shallow layer separating it from stratosphere which is known as the “Tropopause “.11. The upper boundary of the layer Tropopause, ranges in height from 5 miles (8 km) near the poles up to 11 miles (18 km) above the equator. Its height also 11
  • varies with the seasons; highest in the summer and lowest in the winter.B). Stratosphere1. The stratosphere is the second major strata of air in the atmosphere. It extends above the tropopause (around 20 km onwards) and extends to an altitude of about 30 miles (50 km) above the planets surface.2. The air temperature in the stratosphere remains relatively constant up to an altitude of 15 miles (25 km). Then it increases gradually with height until it reaches about 10°C at an altitude of 50 km.3. Solar energy is converted to kinetic energy when ozone molecules absorb ultraviolet radiation, resulting in heating of the stratosphere.4. This layer is called as “Seat of photochemical reactions”.5. Since the stratosphere is not turbulent this is where most planes like to fly.6. Less convection takes place in the stratosphere because it is warm at the top and cold at the bottom.7. There is also persistence of circulation patterns and high wind speeds. 12
  • 8. The boundary between the stratosphere and the next layer mesosphere is called the stratopause.C). Mesosphere/Ozonosphere1. The layer between 50 and 80 km is called as “Mesosphere”. In this layer the temperature decreases with height.2. There is a maximum concentration of ozone between 30 and 60 km above the surface of the earth and this layer is known as the ozonosphere.3. A property of the ozone is that it absorbs UV rays. Had there been no layer of the ozone in the atmosphere, the ultraviolet rays might have reached the surface of the earth and no life can exist.4. The temperature drops in this layer to about -100 ºC. This is the coldest region of the atmosphere.5. The stratosphere and mesosphere together are sometimes referred to as the middle atmosphere. Because of the preponderance of chemical process this sphere is called as the “chemosphere”6. The upper boundary of this layer is called the “Mesopause”. 13
  • D). Thermosphere (Ionosphere)1. The thermosphere layer lies beyond the ozonosphere (mesosphere) at a height of about 80 km above the earth’s surface and extends up to 400 km. The air is very thin.2. Thermosphere means "heat sphere". The temperature is very high in this layer because ultraviolet radiation is turned into heat. Temperatures often reach 2000°C or more. This increase in temperature is due to the absorption of intense solar radiation by the limited amount of remaining molecular oxygen.3. The atmosphere in the ionosphere is partly ionised enriched ion zones exist in the form of distinct ionised layers. So, this layer is also called as the ionosphere. Long distance radio communication is possible due to this layer.E). Exosphere1. The outer most layer of the earth’s atmosphere isnamed as the exosphere and this layer lies between 400and 1000 km. The exosphere is the most distantatmospheric region from Earths surface.2. The upper boundary can be defined theoretically bythe altitude (about 120,000 miles, half the distance to 14
  • the Moon) at which the influence of solar radiationpressure on atomic hydrogen velocities exceeds that ofthe Earths gravitational pull.3. At such a greater height the density of atoms in theatmosphere is extremely low.4. Hydrogen and Helium gases predominate in this outermost region.5. At an altitude of about 500 to 600 km the density ofthe atmosphere becomes so low that collisions betweenthe natural particles become extremely rare.Weather It is defined as a physical state of the atmosphereat a given place and given time or a state or condition ofthe atmosphere at a given place and at a The daily or short term variations of differentconditions of lower air in terms of temperature, pressure,wind, rainfall etc. Weather is highly variable and itchanges constantly sometimes from hour to hour and atother times from day to day. The different weather elements are solar radiation,temperature, pressure, wind, humidity, rainfall,evaporation etc. 15
  • Eg: The air temperature of Annamalai nagar on20.01.2011 at 10.30 a.m. is 31º C.Climate It is defined as the generalized weather or summationof weather conditions over a given region duringcomparatively longer period. The sum of all statisticalinformation of weather in a particular area duringspecified interval of time, usually a season or a year oreven a decade or long term regime of atmosphericvariables of a given place or area. The aspects involvedare larger areas like a zone/ a state/ a country and isdescribed as normals. The climatic normals aregenerally worked out for a period of 30 years. Theclimatic elements are latitude, longitude and altitude.Eg: In Tamil Nadu the summer temperature ranges from29ºC to 40º C.Factors affecting climate There are number of factors that affect the climateof a particular place in the World. They are,i) Latitude Latitude controls the amount of solar radiationthat reaches the surface of the earth. The distance fromthe equator, either south or north, largely creates 16
  • variations in the climate. Based on the latitude, theclimate has been classified as i) Tropical ii) Sub-tropical iii)Temperate iv) Polar The tropical climate is characterized by hightemperature throughout the year. Subtropical ischaracterized by high temperature alternating with lowtemperature in winter. The temperate climate has lowtemperature throughout the year. The polar climate isnoted for its very low temperature throughout the year.In general the latitude incease the temperature decrease.ii) Altitude (elevation) The height from the MSL (Mean Sea Level) createsvariation in climate. Even in the tropical regions, thehigh mountains have temperate climate. The temperaturedecreases by 6.5 ºC/km from the sea level. Generally,there is also a decrease in pressure and increase inprecipitation and wind velocity. The above factors alterthe kind of vegetation, soil types and the cropproduction. With increasing altitude the decreasingtemperature.iii) Precipitation 17
  • The quantity and distribution of rainfall decidesthe nature of vegetation and the nature of the cultivatedcrops.iv) Mountain barriers or Relief: Mountains form a natural barrier that causes airmasses to rise. As air is forced to rise it expands asgravity decreases, it becomes less dense and cools.v) Soil type Soil is a product of climatic action on rocks asmodified by landscape and vegetation over a long periodof time. The colour of the soil surface affects theabsorption, storage and re-radiation of heat. White colourreflects while black absorbs more radiation. Hence,where ever black soil type is predominant there will bemore absorption of heat and hot climate will Nearness to large water bodies The presence of large water bodies like lakes andsea including its current affect the climate of thesurrounding areas, Eg. Islands and coastal areas. Themovement of air from earth’s surface and from waterbodies to earth modifies the climate. The extremevariation in temperature during summer and winter isminimized in coastal areas and island. 18
  • vii) Topography The surface of landscape (leveled or unevensurface areas) produces marked change in the climate.This involves the altitude of the place, steepness of theslope and exposure of the slope to light and wind.viii) Vegetation Kinds of vegetation characterize the nature ofclimate. Thick vegetation is found in tropical regionswhere temperature and precipitation are high.Others factors are Wind and air masses, atmospheric disturbances orstorms and oceans currents.Scales of climate and their importancei) Microclimate Microclimate deals with the climatic featurespeculiar to small areas and with the physical processesthat take place in the layer of air very near to the groundwhich differ sharply from general climatic conditions.ii) Mesoclimate The scale of meso climate falls between micro andmacro climates. It is concerned with the study of climateover relatively smaller areas between 10 & 100 kmacross. 19
  • iii) Macroclimate Macro climate deals with the study of atmosphereover large areas of the earth and with the large scaleatmospheric motions that cause weather. The scales ofair motion in different climates are given in the Tablebelow. Type of Horizontal Vertical Time climate scale (km) scale(km) Scale (hrs) A. Macro climate 1. 2000-5000& 10 200 to Planetary more 400 scale 2. Synoptic 500-2000 10 100 scale B. Meso 1 to 100 1-10 1-10 climate C. Micro <100m 200 m 6-12 climate minutesIf any weather system develops under different types ofclimate, it persists longer periods under themacroclimate while smaller periods under microclimates. 20
  • Climates of India and Tamilnadu and theircharacterization Climate classification was tried by manyscientists from beginning of 19th century using manyparameters. Thornthwaite during 1931 and 1948classified the climate using precipitation and evaporation/Potential evaporation and was subsequently modified byMathur (1955) for the Moisture Index (Im) and is givenbelow Im = 100 [(P-PE)/PE] Where P = Precipitation, PE = Potentialevapo-transpiration Using the moisture Index (Im) the followingclassification was made Im Quantity Climateclassification 100 and above Per humid 20 to 100 Humid 0 to 20 Moist sub humid -33.3 to 0 Dry sub humid -66.7 to -33.3 Semi arid -100 to -66.7 Arid 21
  • Another classification by Troll (1965) basedon number of humid months, said to be of moreagricultural use was modified by ICRISAT for India.Humid month is one having mean rainfall exceeding themean Potential evapo transpiration. Climate Number of % geographical humid months area of India Arid <2.0 17.00 Semiarid- 2.0-4.5 57.17 dry Semiarid- 4.5-7.0 12.31 wet Humid >7.0 1.10 The ICAR under All India Coordinated ResearchProject on Dryland Agriculture adopted classificationbased Moisture Deficit Index (MDI) P - PET MDI = --------- x 100 PETWhere P is annual precipitation (cm) and PET is PotentialEvapotranspiration. Based on MDI the climate is dividedinto three regions as below. Type of climate MDI Subhumid 0.0 to 33.3 Semiarid -33.3 to -66.6 Arid > -66.6Koppen classification of climate 22
  • Koppen classified climate into eleven types. Theyare as follows:Sl.N Name Sl.N Name o. o.1. Tropical rainforest 7. Humid temperate climate climate2. Tropical –Savanna 8. Cold climate with Climate moist winter3. Steppes 9. Cold Climate with Dry Winter4. Deserts 10. Tundra Climate5. Warm climate 11. Ice climate with dry winter6. Warm Climate with dry summerRainfall based classification The crop regions are classified on the basis ofaverage rainfall which is as follow. Rainfall(mm) Name of the climatic region Less than 500 Arid 500-750 Semi-arid 750-1000 Sub-arid More than 1000 HumidTemperature based classification 23
  • The tropic ofcancer, which passesthrough the middle of thecountry, divides it into twodistinct climates. Thetropical climate in the Southwhere all the 12 months ofthe year have mean dailytemperature exceeding20°C; and in the Northwhere a sub-tropical climate prevails. In sub-tropicsduring the winter months, it is cool to cold. Frosts occursometime during the months of December and January.Some areas in the Northern India have a temperateclimate. Here it snows during the winter months andfreezing temperatures may extend to two months or moreduring the year. Three main climatic zones of India basedon temperature are shown in the map below. Chapter 2 Solar Radiation and Temperature 24
  • THE SUN Sun is an ordinary star, nearest to ourEarth. The planets and satellites are called the membersof the solar system. Sun is the star at the center of theSolar System. All the planets revolve around the Sun inelliptical orbits. Sun is the prime source of energy onEarth and life depends on.The main features of the Sun1. Diameter of the Sun is 13,92,000 km and its mass about 2 X 1030 kg (3,30,000 times that of Earth)2. It rotates on its axis about once in every four weeks(27 days near equator & 30 days –polar)3. The mean distance of the Sun from the Earth isapproximately 149.6 million km4. Sun is the brightest object in the sky5. Light travels from the Sun to Earth in about 8 minutesand 19 seconds6. The surface temperature of the Sun is around 5778°K(5505 °C)7. The interior mass of the Sun has a density of 80 to100 times that of water 25
  • 8. Every minute, the Sun radiates approximately 56 X1026 calories of energy9. Energy is due to the fusion, Hydrogen is transformedto Helium10. 99% of the energy to biosphere is only from the Sunand the rest oneper cent is from stars, lightning discharge. Sun’sradiation reflected from the Moon, re-radiation from theEarth etc.,SEASONAL VARIATIONS The primary cause of seasonal extremes on Earthis the 23.5 degree tilt of our planets spin axis. When thenorth pole is tilted toward the Sun, northern days arelong and the weather is warm. Six months later, as thesouth pole tilts toward the Sun, the southern hemispheretakes its turn at summer. 26
  • Seasons in the two hemispheres are alwaysreversed. When it is summer in New York, it is winter inSydney. On a spring day in Paris, autumn leaves arefalling in Argentina. The name "equinox" is derived from the Latinaequus (equal) and nox (night). The name “solstice” isderived from the Latin sol (sun) and sistere (to stand still) EQUINOX - when the Sun shines down directlyover a planets equator. Two times of the year when nightand day are about the same length. SOLSTICE - when one of the poles is tilted towardthe Sun to its maximum extent. The Sun stands still indeclination; that is, the apparent movement of the Sunspath north or south comes to a stop before reversingdirection. Summer Solstice: The Sun is farthest north andthe length of time between Sunrise and Sunset is thelongest of the year. Winter Solstice: The Sun is farthest south and thelength of time between Sunrise and Sunset is theshortest of the year. An equinox happens each year at two specificmoments in time, where the center of the Sun can be 27
  • observed to be vertically overhead, occurring aroundMarch 21 and September 23 each year. A solstice is an astronomical event that happenstwice each year when the Suns apparent position in thesky reaches its northernmost or southernmost extremes.From the vernal equinox (March 21), the Earth is movingnorthward around the Sun towards the summer solstice(June 21), when the Sun is farthest North. The autumnalequinox (September 23), the Earth moving southwardtowards winter solstice (December 21), the Sun reachesits lowest position in the sky (South). If you live in the Southern Hemisphere, thesummer solstice and winter solstice will be reversed.INSOLATION Insolation means Incoming solar radiation.Electromagnetic energy radiated into the space by theSun. It consists of bands of radiant energy of differentwave lengths.Factors affecting insolation1. The solar constant which depends on a. Energy output of the Sun b. Distance from earth to Sun2. Transparency of the atmosphere 28
  • 3. Duration of daily sunlight period4. Angle at which Sun’s noon rays strike the Earth.SOLAR CONSTANT Solar constant is the energy received on a unit areaat the outer most boundary of the Earth (atmosphere)surface held perpendicular to the Sun’s direction, at themean distance between the Sun and the Earth. The rate at which solar wave radiation is received atthe top of the atmosphere on a unit surface unit time. • Solar constant is not a true constant. It fluctuates by as much as ± 3% about its mean value depending upon the distance of the Earth from the Sun. • Value is about 2 cal / cm2 / min. (1.92 and 2.02) of the upper atmosphere (cal/cm2= Langley). It is equal to 1370 watts per square meter • 35% of the energy is contributed by U.V. and visible parts and 65% by Infra RedTRANSFER OF HEAT All matter, at a temperature above theabsolute zero, imparts energy to the surrounding space.Three processes viz. conduction, convection andradiation are involved in heat flow or heat transfer. 29
  • 1. Conduction Heat transfer through the matter without theactual movement of molecules of the substances ormatter. Heat flows from the warmer to cooler part of thebody so that the temperatures between them areequalized.Eg. The energy transmission through an iron rod whichis made warmer at one end.2. Convection Processes of transmission of heat throughactual movement of molecules of the medium. This ispredominant form of energy transmission on the earth asall the weather related processes involve this process.Eg. Boiling of water in a vessel.3. Radiation Transfer of energy from one body to anotherwithout the aid of the material medium (solid, liquid orgas). Radiation is not a heat, only when radiation isabsorbed by surface of a body heat is produced.Eg. The energy transmission through space from the Sunto the Earth. 30
  • SOLAR RADIATION The flux of radiant energy from the Sun is solarradiation. Heavenly bodies emit – short wave radiation Near surfaces including earth emit - longwave radiationImportance of Solar Radiation on Crops1. From germination of seeds to harvesting and even postharvest processes are affected by solar radiation2. Solar radiation provides energy for • All the phenomena related to biomass production • All photosynthetic processes • All physical processes taking place in the soil, the plant and their environment3. Solar radiation controls the distribution oftemperature thereby distribution of crops into differentregions.Radiation flux The amount of radiant energy emitted,received, transmitted across a particular area is knownas radiant flux.Radiant flux density 31
  • The radiant flux divided by the area acrosswhich the radiation is transmitted is called radiant fluxdensity.Emissive power The radiant flux density emitted by a sourceis called its emissive power.Energy measurement Units Cal cm-2 J cm-2 mi-1 W cm-2 min-1 Cal cm-2 1 4.1868 0.069 min-1 J cm-2 mi-1 0.238 1 0.00165 W cm-2 14.3 60.6 1Where, J – Joule; W – Watt.SOLAR SPECTRUM Radiation energy is transmitted in the formof electromagnetic waves by the Sun. The energy fromthe Sun is spread over a very broad band of wavelengthsknown as solar spectrum. It is also known aselectromagnetic spectrum. The spectrum is thecombination of different wavelengths.Eg. U.V. rays, visible light, I.R.rays etc. 32
  • Different Bands in Solar Spectrum Wavele % of Spectr Band ngth Importance Ener um (µ) gy 1.Ultra Cosmic Shorter wave < 0.005 violet rays lengths of Gamma spectrum & rays 0.005 – Chemically and X- 0.20 active, unless 9% rays filtered there is Ultra danger of life on 0.20 – Violet earth 0.39 rays 0.39 – Visible spectrum Violet 0.42 known as Light 0.42 – essential for all Blue 0.49 plant processes 0.49 – Green 2.Visib 0.54 41% le 0.54 – Yellow 0.59 0.59 – Orange 0.65 0.65 – Red 0.76 Essential for 3.Infra Infrared thermal energy > 0.76 50% red rays of the plant (Source of heat)Units of measurements of wavelength 33
  • Micron (µ) = 10-6 m = 10-4 cm Milli micron (mµ) = 10-9 m = 10-7 cm Angstrom (Å) = 10-10 m = 10-8 cmSolar radiation and crop plants Crop production is exploitation of solar radiation;the three broad spectrums are,1. Shorter than visible range: Chemically very active • When plants are exposed to this radiation the effects are detrimental • Atmosphere acts as regulator for this radiation and none of cosmic, Gamma and X-rays reaches to the earth • The UV rays of this segment reaching to the earth are very low and it is normally tolerated by the plants2. Visible spectrum: Light • Between UV & IR radiation and also referred as light • All plant parts are directly or indirectly influenced by the light 34
  • • Intensity, quality and duration are important for normal plant growth • Poor light leads to plant abnormalities • Light is indispensable to photosynthesis • Light affect the production of tillers, the stability, strength and length of culms • It affects the yield, total weight of plant structures, size of the leaves and root development • It affects the flowering and fruiting and dormancy of the seed3. Higher than visible wavelength: Source of heat • Referred to IR radiation • It has thermal effect on plants • In the presence of water vapour, this radiation does not harm plants, rather it supplies the necessary thermal energy to the plant environment Effect of solar radiation on crop plantsS.N Band Specific effect on planto. Wavelengt h (µ) No specific effect on plant Radiation activity such as bio-chemical1. within 1.0 and photochemical processes. and more Radiations are transformed into heat2. 1.0 - 0.72 Radiation in this band helps in 35
  • plant elongation activity This light is strongly absorbed by 0.72 – chlorophylls. It generates strong 3. 0.61 photosynthetic and photo- periodic activity Absorption in this region has low 0.61 – 4. photosynthetic effectiveness and 0.51 weak formative activity It is the strongest chlorophyll and yellow pigment absorption 0.51 – 5. region. Very strong on 0.40 photosynthetic activity and effect on formation of tissues Produces formative effects. It has 0.40 – 6. dwarfing effect on plants and 0.32 thickening effect on plant leaf 0.32 – Detrimental effect on most 7. 0.28 plants Lethal effects most of the plants Less than 8. get killed UV ranges have 0.28 germicidal actionAbsorption, Reflection and Transmission A part of the incident radiation on thesurface is absorbed, while a part is reflected and theremaining is transmitted.Absorptivity Absorptivity of a substance is defined as theratio of the amount of radiant energy absorbed to thetotal amount incident upon that substance. The 36
  • absorptivity of a blackbody is unity. Natural bodies likeSun and Earth are near perfect black bodies.Reflectivity Reflectivity is defined as the ratio of theradiant energy reflected to the total incident radiationupon that surface. If it is expressed in percentage itbecomes albedo.Transmittivity Transmittivity is defined as ratio of thetransmitted radiation to the total incident radiation uponthe surface.Emissivity Emissivity is defined as the ratio of theradiant energy emitted by a given surface to the totalheat energy emitted by a black body. The emissivity of ablack body is unity.Blackbody radiation A Blackbody is defined as a body, whichcompletely absorbs all the heat radiations falling on itwithout reflecting and transmitting any of it. It meansreflectivity and transmittivity become zero. When such ablack body is heated it emits radiation of all wavelengthsdepending upon its temperature. 37
  • ALBEDO It is the percentage of reflected radiation tothe incident radiation (varies with colour andcomposition of the Earth’s surface, season, angle of theSun rays). Value is Highest in winter and at Sunrise andSunset. Pure water 5-20%, Vegetation 10-40%, dark soils5-10%, desert soil 25-30%, snow 45-90% and clouds5-85%. High albedo indicates that much of the incidentsolar radiation is reflected rather than absorbed.Depends up on 1. Angle of incidence of radiation. Albedo increase with decreasing elevation of sun with minimum during noon 2. Physical characteristics of surface 3. Season 4. Time of the dayFor plant community albedo depends upon 1. Age of the crop 2. Percentage of ground cover 3. Colour and reflectivity of the foliageRadiation balance The difference between all incoming andoutgoing radiation at the earth’s surface and top of the 38
  • atmosphere is known as radiation balance at the earth’ssurface.Temperature It is defined as the measure of the averagespeed of atoms and moleculesKinetic energy Energy of motion.Heat It is the aggregate internal energy of motion andmolecules of a body. It is often defined as energy in theprocess of being transferred from one object to anotherbecause of the temperature between them.Sensible heat It is the heat that can be measured by athermometer and thus sensed by humans. Normallymeasured in Celsius, Fahrenheit and Kelvin.Latent heat It is the energy required to change a substance to ahigher state of matter. This same energy is released onthe reverse process. Change of state through Evaporationand condensation is known as latent heat of evaporationand latent heat of condensation. From water to water 39
  • vapour takes 600 calories and water to ice takes 80calories.Disposition of Solar radiation • 25% of solar radiation is reflected back to the space by clouds (more by middle and high latitudes and less in the sub tropics) • 6% reflected back by air, dust and water vapour. • 30% scatted downwards (more in the form of shorter wavelengths than that in longer wave length (red)). • 17% of solar radiation is absorbed by the atmosphere. (Mostly by Oxygen, O3, CO2 & H2O vapour). O2 – absorb the extreme UV wavelengths (0.12to 0.6 µ) O3 – UV (0.2 to 0.32 µ) and Visible part ofradiation (0.44 to 0.7 µ) H2O vapour – Near infra red (0.93, 1.13, 1.42µ) CO2 - IR band 2.7 µ.• About 50% of solar radiation reaches earth’s surface, after reflection, scattering and absorption.LIGHT 40
  • Light is the visible portion of the solar spectrumwith wavelength range is from 0.39 to 0.76μ. Light is oneof the important climatic factors for many vital functionsof the plant.Functions of Light • It is essential for the synthesis of the most important pigment ie. Chlorophyll, Chlorophyll absorbs the radiant energy and converts it into potential energy of carbohydrate (photosynthesis). • The carbohydrate thus formed is the connecting link between solar energy and living world. • In addition, it regulates the important physiological functions. - Effects tiller production, stability, strength and length of culms - Effects dry matter production - Effects the size of the leaves and root development - Effects the flowering, fruiting and dormancy of the seedCharacteristics of Light The characteristics of light viz. intensity, quality,duration and direction are important for crops.1. Light intensity 41
  • The intensity of light is measured by comparingwith a standard candle. The amount of light received at adistance of one meter from a standard candle is knownas “Meter candle or Lux”. The light intensity at one footfrom a standard candle is called “foot candle” or 10.764luxes. - Very low light intensity reduces the rate of photosynthesis resulting in reduced growth. - Similarly, very high intensity is detrimental to plant in many ways as below. - It increases the rate of respiration. - It also causes rapid loss of water (i.e.) increases the transpiration rate of water from the plants. - The most harmful effect of high intensity light is that it oxidises the cell contents which is termed as ‘Solarisation’. This oxidation is different from respiration and is called as photo-oxidation.• Under field conditions the light is not spread evenly over the crop canopy but commonly passed by reflection and transmission through several layers of leaves.• The intensity of light falls at exponential rate with path length through absorbing layers according to Beer’s 42
  • law. i.e. the relative radiation intensity decreases exponentially with increasing leaf area.• At ground level the light intensity is below the light compensation point (The light intensity at which the gas exchange resulting from photosynthesis is equal to that resulting from respiration) Based on the response to light intensities theplants are classified as follows.(i) Sciophytes (shade loving plants) The plants grow better under partially shaded conditions. Eg. Betel vine, buck wheat etc.(ii) Hetrophytes (Sun loving) Many species of plants produce maximum dry matter under high light intensities when the moisture is available at the optimum level. Eg. Maize, sorghum, rice etc.2. Quality of Light When a beam of white light is passed through aprism, it is dispersed into wavelengths of differentcolours. This is called the visible part of the solarspectrum (VIBGYOR 0.39 – 0.76 µ). 43
  • • The principal wavelength absorbed and used in photosynthesis are in the violet – blue and the orange - red regions. • Among this, short rays beyond violet such as X rays, gamma rays and larger rays beyond red such as infrared, are detrimental to plant growth. • Red light is the most favourable light for growth followed by violet – blue. • Red light helps mature apples to turn red, induces germination and suppresses elongation of stem. • Blue and green lights inhibit germination of seeds. • Ultra – violet and shorter wave lengths kill bacteria and many fungi (Germicidal effect).3. Duration of light:• The duration of light has greater influence than the intensity for canopy development and final yield.• 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 responseto day length which is as follows.(i) Long day plants 44
  • The plants which develop and produce normallywhen the photoperiod is greater than the criticalminimum (more than 10 hours). Eg. Potato, Sugar beet,Wheat, Barley, Oats etc.(ii) Short day plants The plants which develop normally when thephotoperiod is less than the critical maximum (less than10 hours). Eg. Rice, Sorghum, Cotton, Sunflower,Cucumber etc.(iii) Day neutral plants / Indeterminate Those plants which are not affected by photoperiod.Eg. Tomato, Maize etc.• The photoperiodism influences the plant character such as floral initiation or development, bulb and rhizome production etc.• In long day plant, 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• The varying photoperiodicity of plants is a very important factor in crop rotation which helps not only 45
  • in operational nutrient management but also in disease eradication.• The choice of varieties and cultivars in newly cultivated areas depend on duration of light.4. Direction of light• The direction of sunlight has a greater effect on the orientation of roots and leaves.• In temperate regions, the southern slopes plants produce better growth than the northern slopes due to higher contribution of sunlight in the southern side.• The change of position or orientation of organs of plants caused by light is usually called as “phototropism” i.e. the leaves are oriented at right angles to incidence of light to receive maximum light radiation.Photomorphogenesis Change in the morphology of plants due to light.This is mainly due to U.V and violet ray of the Sun.Factors Affecting the Distribution of Solar Radiationwithin the Plant Canopy1. Type of Plantsa. The leaves of cereal crops like paddy, wheat etc. have atransmissivity from 5 to 10 per cent. 46
  • b. The broad leaves of ever green plants have lower valueof 2 to 8 per cent.2. Age of the LeavesThe transmissivity of younger leaves is more as comparedto older leaves.3. Chlorophyll ContentAs the chlorophyll content increases the value oftransmissivity decreases.4. Arrangement of Leavesa. The relative light interception by horizontal and erectfoliage is 1:0.44b. When the leaf area index is one (1) the lighttransmissivity of more upright leaves is 74 % as against50 % for horizontal leaves.5. Angle of Leavesa. In full sunlight, the optimum inclination for efficientlight use is 81°. The leaf placed at the optimuminclination is 4 – 5 times as efficient in using light as ahorizontal leaf.c. The ideal arrangement of leaves is that the lowestleaves lay at angles between 0° to 30° to be horizontal (13% ), the adjoining leaves lay at 30° to 60° (37 % ) and theupper leaves lay at 60° to 90°(50 % ). 47
  • 6. Plant Density In sparse crop stands not only the per cent of lighttransmissivity is more but it is also variable with the timeof the day. In dense crop canopies the lighttransmissivity is less.7. Plant Height When the plant height increases the transmissivityof light by the canopy decreases.8. Angle of the suna. The highest radiation penetration also occurs at noon.b. Relatively high radiation penetration also occurs bothin the morning as well as before the sunset due to highproportion of diffuse light.Instruments to measure Solar Radiation1. Pyranometer: Total short wave radiation orcomponents on a horizontal plain surface fromhemispherical sky.2. Net pyranometer or solarimeter: Net short waveradiation in W m-2.3. Pyrradiometer: Global or total radiation from sky ona horizontal surface in W m-2.4. Net pyrradiometer or net radiometer: Net shortwave and long wave radiation in W m-2. 48
  • 5. Pyrheliometer: Direct solar beam on a plane surfaceat normal incidence in W m-2.6. Pyrgeometer: Net infrared radiation of theatmosphere in W m-2.7. Luxmeter or Photometric sensors: The brightness(intensity) is measured in foot candles or lux.8. Photometer or light meter: Measured the light.9. Sunshine shine recorder: The number of hours ofbright sunshine is recorded.AIR TEMPERATURE Temperature is defined as, “The measure ofspeed per molecule of all the molecules of a body”.Whereas heat is, “the energy arising from random motionof all the molecules of a body” (Degree of molecularactivity). It is the intensity aspect of heat energy. Temperature is a measure of “the average energy ofthe molecules of a body”, whereas heat is a measure of“the total amount of thermal energy in a body”. Forexample, the temperature of a cup of boiling water is thesame as that of a large pot of boiling water (212°F, or100°C), the large pot has more heat, or thermal energy,and it takes more energy to boil a pot of water than a cupof water. 49
  • In meteorology, the temperature of the atmospherewhich represents the average kinetic energy of themolecular motion in a small region and is defined interms of a standard or calibrated thermometer in thermalequilibrium with the air. • The degree of hotness or coldness of a body or environment.HEAT TRANSFERConduction Heat transfer when two bodies of unequaltemperatures come into contact. Heat passes from pointto point by means of adjacent molecules.Convection Transfer through movement of particles (part ofmass) in fluids and gasses. These are able to circulateinternally and distribute heated part of the mass.Radiation It is the process of transmission of energy byelectromagnetic waves between two bodies without thenecessary aid of an intervening material medium.Role of temperature in crop production:1. Temperature influences distribution of crop plants and vegetation. 50
  • 2. The surface air temperature is one of the important variables, which influences all stages of crop during its growth development and reproductive phase.3. Air temperature affects leaf production, expansion and flowering.4. The diffusion rates of gases and liquid changes with temperature.5. Solubility of different substances is dependent on temperature.6. Biochemical reactions in crops (double or more with each 10°C rise) are influenced by air temperature.7. Equilibrium of various systems and compounds is a function of temperature.8. Temperature affects the stability of enzymatic systems in the plants.9. Most of the higher plants grow between 0°C – 60°C and crop plants are further restricted from 10 – 40°C, however, maximum dry matter is produced between 20 and 30°C10.At high temperature and high humidity, most of the crop plants are affected by pests and diseases.11.High night temperature increases respiration and metabolism. 51
  • 12. Most of the crops have upper and lower limits of temperature below or above which, they may not come up and an optimum temperature when the crop growth is maximum. These are known as “cardinal temperatures” and different crops have different temperatures. Sl Crop Minimum Optimum Maximum No 1 Wheat and 0–5 25 – 31 31 – 37 Barley 2 Sorghum 15 – 18 31 – 36 40 – 42Factors affecting air temperature  Latitude  Altitude  Distribution of land and water  Ocean currents  Prevailing winds  Cloudiness  Mountain barriers  Nature of surface  Relief  Convection and turbulence etc.1. Latitude 52
  • The time of occurrence of maximum monthlymean temperature and minimum monthly meantemperature also depends on latitude of a place. (Eg.) Thecoldest month is January in northern regions of Indiawhile December in the south. Similarly, the warmestmonth is May in the south while June in the northacross the country.2. Altitude The surface air temperature decreases withincreasing altitude from the mean sea level as the densityof air decreases. Since the density of air is less at higheraltitudes, the absorbing capacity of air is relatively lesswith reference to earth’s long wave radiation.3. Distribution of land and water Land and water surfaces react differently tothe insolation. Because of the great contrasts betweenland and water surfaces their capacity for heating theatmosphere varies. The differential heating processbetween land and sea surfaces are due to theirproperties. It is one of the reasons for Indian monsoon.4. Ocean currents The energy received over the ocean surfacecarried away by the ocean currents from the warm areas 53
  • to cool areas. This results in temperature contrastbetween the equator and poles. The occurrence of El-Nino is due to change in sea surface temperaturebetween two oceanic regions over the globe.5. Prevailing winds Winds can moderate the surface temperatureof the continents and oceans. In the absence of winds, wefeel warm in hot climates. At the same time, the weatheris pleasant if wind blows.6. Cloudiness The amount of cloudiness affects thetemperature of the earth’s surface and the atmosphere. Athick cloud reduces the amount of insolation received ata particular place and thus the day time temperature islow. At the same time, the lower layers in the atmosphereabsorb earth’s radiation. This results in increasingatmospheric temperature during night. That is why,cloudy nights are warmer. This is common in the humidtropical climates.7. Mountain barriers Air at the top of the mountain makes littlecontact with the ground and is therefore cold while in thevalley at the foothills makes a great deal of contact and is 54
  • therefore warm. That is, the lower region of the earth’s atmosphere is relatively warmer when compared to hillocks. 8. Nature of surface 9. Relief In the northern hemisphere north facing slopes generally receive less insolation than south facing slopes and temperature are normally lower. 10. Convection and turbulence etc. Diurnal and Seasonal Variation of Air Temperature• The minimum air temperature occurs at about sunrise, after which there is a constant rise till it reaches to maximum.• The maximum air temperature is recorded between 13.00 hrs and 15.00 hrs although the maximum solar radiation is reaches at the noon.• A steady fall in temperature till sunrise is noticed after is attains maximum. Thus the daily displays one maximum and one minimum. The difference between the two is called the diurnal range of air temperature.• The diurnal range of air temperature is more on clear days while cloudy weather sharply reduces daily amplitudes. 55
  • • The diurnal range of temperature is also influenced by soils and their coverage in addition to seasons.• Addition of daily maximum and minimum temperature divided by two is nothing but daily mean / average temperature. Temperature Distribution Each day the earth receives energy in the form of incoming solar radiation from the sun. There are three temperature distributions viz., horizontal, vertical and periodic distributions that are observed on earth. 1. Horizontal air temperature distribution Sun rays make different angles at the same place at different times. Due to the variation in angle of sun’s rays, distribution of solar heat on earth decreases both ways from equator to poles. This is known as horizontal distribution of air temperature. On maps, the horizontal distribution of air temperature is shown by isotherms. The isotherms are imaginary lines drawn connecting points of equal temperature. Horizontal distribution of air temperature influenced by,  Latitude 56
  • A general decrease in temperature from equator towardspoles is one of the most fundamental factors ofclimatology.  Land and water Irregular distribution of land and water on earth’ssurface breaks the latitudinal variation in temperature.Land areas warm and cool rapidly than water bodies.  Mountain barriers Influence horizontal distribution of temperature byrestricting movement of air masses. Eg. Himalayasprotect India from polar air.  Topography On local scale topographic relief exerts aninfluence on temperature distribution. In northernhemisphere north facing slopes receive less insolationthan south facing slopes and temperatures are normallylower.2. Vertical air temperature distribution The decrease in temperature with increase inheight (altitude) is known as vertical temperaturedistribution. Eg. Permanent snow caps in highmountains.3. Periodical temperature distribution 57
  • The air temperature changes continuously duringa day or a year.  Maximum insolation is received around noon (12.00 hr) at maximum temperature is recorded from 1 p.m. to 3 p.m. and delay is known as thermal lag.  In northern hemisphere winter minimum occurs in January and summer maximum in July.Temperature and Climatic Zones The climatic zones of the earth are:  Tropical  Sub – tropical  Temperate  Alpine These zones are differentiated on the basis oftemperature. The tropical zone is a hot winterless zone;sub-tropical zone is a hot zone with a cool winter;temperate zone is with a warm summer and pronouncedwinter; and the alpine zone has a short summer and along severe winter (Himalayas, Tibetan Plateau).Temperature inversion Occasionally at some altitude the temperatureabruptly increases instead of decreasing. This condition 58
  • in which this “abrupt rise instead of fall in temperatureoccurs in the air” is known as the temperature inversion.This may occurs under the following conditions. When the air near the ground cools off faster than the overlying layer, because of heat loss during cooling nights. When an actual warm layer passing over a lower cold layer Cold air from hill tops and slopes tend to flow downward and replaced by warm airSignificance of Temperature inversion Cloud formation, precipitation and atmospheric visibility are greatly influenced by inversion phenomenon Impurities like smoke, dust, etc., are confined to lower layers Fog formation may take place near the ground which may affect the visibility to both human beings and animals. Affects air navigation. Diurnal temperature (day and night) is affected by temperature inversions. The incoming solar radiation and its conversion into heat is affected. 59
  • Heat Units It is a measure of relative warmth of growingseason of a given length. Normally it is indicated as“Growing Degree Days” (GDD) or “thermal units” or“effective heat units” or “growth units”. A heat unit is thedeparture from the mean daily temperature above theminimum threshold temperature. The growth of plants is dependent on the totalamount of heat to which it is subjected during its lifetime. The minimum threshold temperature is thetemperature below which no growth takes place. Usuallyranges from 4.5 to 12.5 ºC for different crops (tropicalcrop 10ºC).Degree Day Concept A degree day is obtained by subtracting thethreshold temperature from daily mean temperature.Summation of the daily values over the growth periodgives degree days of the crops. Tmax + Tmin GDD = Σ ------------------- - Tb 2 Where 60
  • Tmax – Maximum air temperature of the day Tmin – Minimum air temperature of the day Tb - Base temperature of the crop The base temperature is the thresholdtemperature.Advantages / Importance of growing degree DayConcept• In guiding the agricultural operations and planting land use.• To forecast crop harvest dates, yield and quality• In forecasting labour required for agricultural operations• Introduction of new crops and new varieties in new areas• In predicting the likelihood of successful growth of a crop in an area.Air Temperature and Plant Injury The injury to crop plants by air temperatureis by both low and high air temperatures.Thermal death point The temperature at which the plant cell gets killedwhen the temperature ranges from 50-60°C. This varies 61
  • with plant species. The aquatic and shade loving plantsare killed at comparatively lower temperature (40°C).I. High Temperature Injury- results in desiccation of plants- disturbs the physiological activities like photosynthesis and respiration- increases respiration leading to rapid depletion of reserve food.1. Sun clad Injury caused on the barks of stem by hightemperature during day time and low temperature duringthe night time.2. Stem griddle The stem at ground level scorches arounddue to high soil temperature. It causes death of plant bydestroying conductive tissues. Eg. This type of injury isvery common in young seedlings of cotton in sandy soilwhen soil temperature exceeds 60°C.II. Low Temperature (COLD) Injury 62
  • The primary effect of low air temperaturebelow that optimum temperature is the reduction of ratesof growth and metabolic processes.(i) Suffocation In temperature regions, usually during the winterseason, the ice or snow forms a thick cover on the soilsurface. As a result, the entry of oxygen is prevented(deficient oxygen) and crop suffers for want of oxygen. Icecoming in contact with the root prevents the diffusion ofCO2 outside the root zone. This prevents the respiratoryactivities of roots leading to accumulation of harmfulsubstances.(ii) Heaving This is a kind of injury caused by lifting up of theplants along with soil from its normal position. This typeof injury is commonly seen in temperate regions. Thepresence of ice crystals increases the volume of soil. Thiscauses mechanical lifting of the soil.(iii) Chilling injury Plants which are adapted to hot climate, if exposed tolow temperature (0 to 10°C) for sometime, are found to bekilled or severely injured or development of chloraticcondition (yellowing) (Eg.) chloratic bands on the leaves 63
  • of sugarcane, sorghum and maize in winter months whenthe night temperature is below 20°C.(iv) Freezing injury This type of injury is commonly observed in plantsof temperate regions. When the plants are exposed tovery low temperature, water freezes into ice crystals inthe intercellular spaces of plants. The protoplasm of cellis dehydrated resulting in the death of cells. (Eg.) Frost damage in potato, tea etc.Thermo periodic response Response of living organism to regularchanges in temperature either day or night or seasonal iscalled thermoperiodism.Instruments to measure the air temperature1. Maximum Thermometer: To record the maximumtemperature in a day.2. Minimum Thermometer: To record the minimumtemperature in a day3. Dry bulb Thermometer: To record the temperature at atime.4. Wet bulb Thermometer: To record the temperature at atime under wet condition. 64
  • 5. Thermograph: To record the temperature in a day(continuous recording on a chart).SOIL TEMPERATURE Soil thermal properties are strongly influenced bythe soil volumetric water content, volume fraction ofsolids and volume fraction of air. Air is a poor thermalconductor and reduces the effectiveness of the solid andliquid phases to conduct heat. While the solid phase hasthe highest conductivity it is the variability of soilmoisture that largely determines thermal conductivity. Temperature variations are most extreme at thesurface of the soil and these variations are transferred tosub surface layers but at reduced rates as depthincreases. Additionally there is a time delay as to whenmaximum and minimum temperatures are achieved atincreasing soil depth (sometimes referred to as thermallag). The temperature of the soil is an indicator of theenergy needed to sustain the normal activity of plantsand soil organisms. The soil temperature is one of themost important factors that influence the crop growth.Temperature can have a profound impact on the successof seed germination, root and shoot growth, nutrient 65
  • uptake, crop growth and micro organisms live in the soil.The physio-chemical as well as life processes are directlyaffected by the temperature of the soil. Under the low soiltemperature conditions nitrification is inhibited and theintake of water by root is reduced. In a similar wayextreme soil temperatures injures plant and its growth isaffected.Eg. On the sunny side, plants are likely to develop fasternear a wall that stores and radiates heat. If shaded bythe wall, however, the same variety may mature later. Insuch cases soil temperature is an important factor.Importance of Soil Temperature on Crop PlantsThe soil temperature influences many process.1. Governs uptake of water, nutrients etc needed for photosynthesis.2. Controls soil microbial activities and the optimum range is 18-30°C.3. Influences the germination of seeds and development of roots.4. Plays a vital role in mineralization of organic forms of nitrogen.(increases with increases in temperature)5. Influences the presence of organic matter in the soil (more under low soil temperature) 66
  • 6. Affects the speed of reactions and consequently weathering of minerals.7. Influences the soil structure (types of clay formed, the exchangeable ions present, etc.)Factors Affecting Soil Temperature Heat at ground surface is propagated downward inthe form of waves. The amplitude deceases with depth.Both meteorological and soil factors contribute inbringing about changes of soil temperature.I) Meteorological Factors1. Solar Radiationa) The amount of solar radiation available at any given location and point of time is directly proportional to soil temperature.b) Even though a part of total net radiation available is utilised in evapotranspiration and heating the air by radiation (latent and sensible heat fluxes) a relatively substantial amount of solar radiation is utilized in heating up of soil (ground heat flux) depending up on the nature of surface.c) Radiation from the sky contributes a large amount of heat to the soil in areas where the sun’s rays have to penetrate the earth’s atmosphere very obliquely. 67
  • 2. Wind Air convection or wind is necessary to heatup the soil by conduction from the atmosphere. (Eg.) Themountain and valley winds influence the soiltemperature.3. Evaporation and Condensationa) The greater the rate of evaporation the more the soil is cooled. This is the reason for coolness of moist soil in windy conditions.b) On the other hand whenever water vapour from the atmosphere or from other soil depths condenses in the soil it heats up noticeably. Freezing of water generates heat.4. Rainfall (Precipitation)Depending on its temperature, precipitation can eithercool or warm the soil.II. Soil Factors1. Aspect and Slopea) In the middle and high latitudes of the northern hemisphere, the southern slopes receive more insolation per unit area than the northern exposure.b) The south west slopes, are usually warmer than the south east slopes. The reason is that the direct beam 68
  • of sunshine on the south east slope occur shortly after prolonged cooling at night, but the evaporation of dew in the morning also requires energy.2. Soil Texturea) Because of lower heat capacity and poor thermal conductivity, sandy soils warm up more rapidly than clay soils. The energy received by it is concentrated mainly in a thin layer resulting in extraordinary rise in temperature.b) Radiational cooling at night is greater in light soils than in heavy soils. In the top layer, sand has the greatest temperature range, followed by loam and clay.c) The decrease of range with depth is more rapid in light soils than heavy soils when they are dry but slower when they are wet.d) A soil with rough surface absorbs more solar radiation than one with a smooth surface.3. Tillage and Tiltha) By loosening the top soil and creating a mulch, tillage reduces the heat flow between the surface and the sub soil. 69
  • b) Since, the soil mulch has a greater exposure surface than the undisturbed soil and no capillary connection with moist layers below, the cultivated soil dries up quickly by evaporation, but the moisture in the sub- soil underneath the dry mulch is conserved.c) In general soil warms up faster than air. The diurnal temperature wave of the cultivated soil has a much larger amplitude than that of the uncultivated soil.d) The air 2-3 cm above the tilled soil is often hotter (10°C or above) than that over an untilled soil.e) At night loosened ground is colder and more liable to frost than the uncultivated soil. 4. Organic mattera) The addition of organic matter to a soil reduces the heat capacity and thermal conductivity. But, the water holding capacity increases.b) The absorbtivity of the soil increases because of the dark colour of the organic matter.c) At night, the rapid flow of heat from sub-soil by radiation is reduced with the addition of organic matter because of its low thermal conductivity.d) The darker the colour, the smaller the fraction of reflected radiation. 70
  • e) The dark soils and moist soils reflect less than the light coloured and dry soils.5. Soil moisturea) Moisture has an effect on heat capacity and heat conductivity.b) Moisture at the soil surface cools the soil through evaporation.c) Therefore, a moist soil will not heat up as much as a dry one.d) Moist soil is more uniform in temperature throughout its depth as it is a better conductor of heat than the dry soil.Variations in soil temperature There are two types of soil temperature variations;daily and seasonal variation of soil temperature1. Daily variations of soil temperaturea) These variations occur at the surface of the soil.b) At 5 cm depth the change exceeds 10°C. At 20 cm the change is less and at 80 cm diurnal changes are practically nil.c) On cooler days the changes are smaller due to increased heat capacity as the soils become wetter on these days. 71
  • d) On a clear sunny day a bare soil surface is hotter than the air temperature.e) The time of the peak temperature of the soil reaches earlier than the air temperature due to the lag of the air temperature.f) At around 20 cm in the soil the temperature in the ground reaches peak after the surface reaches its maximum due to more time the heat takes to penetrate the soil. The rate of penetration of heat wave within the soil takes around 3 hours to reach 10 cm depth.g) The cooling period of the daily cycle of the soil surface temperature is almost double than the warming period.h) Undesirable daily temperature variations can be minimised by scheduling irrigation.2. Seasonal variations of soil temperature:a) Seasonal variations occur much deeper into the soil.b) When the plant canopy is fully developed the seasonal variations are smaller.c) In winter, the depth to which the soil freezes depends on the duration and severeness of the winter. 72
  • d) In summer the soil temperature variations are much more than winter in tropics and sub tropics.Daily and monthly variation of soil temperature 73
  • Measurement of soil temperatureSoil thermometer: To measure the earth temperature atshallow depth.Units of MeasurementsIn Agro meteorology the two basic scales of temperatureused are:a. Centigrade °C b. Fahrenheit °FRelationship between °C and °F is 1. °F = °C x 9/5 + 32 2. °C = (F- 32) x 5/9In meteorology for upper air temperature measurementsthe absolute scale °A. °A = 273 + °C. 74
  • Chapter - 3 ATMOSPERIC PRESSURE AND WIND SYSTEMS OF THE WORLD Pressure is defined as the force per unit area. Theatmospheric pressure is the weight of the air column perunit area, above the earth surface extending to the limitsof the atmosphere. The earth’s atmosphere weighs about60,000 billion tones. At sea level, it exerts a pressure ofabout 1 kg/cm2. It is expressed in millibar (mb) (1000millibar = 1 bar = 1 million dynes per square centimeter).The average pressure over the earth’s surface at sea levelis 1013.25 millibar, equivalent to the weight of a columnof 76.0 cm (30 inches) (1cm of Hg =13.33mb) of mercuryat 0º Celsius or to a weight of air of 1033.3 gram persquare cm. Unequal heating of the earth and itsatmosphere by the sun and rotation of the earth bringabout differences in atmospheric pressure. Air pressurecan tell us about what kind of weather to expect as well.If a high pressure system is on its way, often you canexpect cooler temperatures and clear skies. If a lowpressure system is coming, then look for warmerweather, storms and rain. 75
  • Isobars (Greek word “isos” – equal and “baros” –weight) The distribution of pressure is represented onmaps by ‘isobars’. Isobars are defined as the imaginarylines drawn on a map to join places having the sameatmospheric pressure.Measuring Atmospheric Pressure Any instrument that measures air pressure iscalled a barometer. The first measurement ofatmospheric pressure began with a simple experimentperformed by Evangelista Torricelli in 1643. The most common type barometer used areaneroid barometer and mercury barometer.Pressure systems of the world The shape of the earth is not uniform and subjectedto uneven distribution of solar radiation, when it revolvesaround the sun. The uneven distribution of solarradiation over different regions of the globe leads tocontrast in surface air temperature. This results invariations of surface atmospheric pressure systems,which are known as standard atmospheric pressuresystems / belts. There are altogether seven alternating 76
  • low and high pressure belts on the earth’s surface. Theyare as follows:  Equatorial trough of low pressure (between 5°N and 5°S)  Subtropical high pressure belt (Northern hemisphere) (25° and 35°N)  Subtropical high pressure belt (Southern hemisphere) (25° and 35°S)  Subpolar low pressure belt (Northern hemisphere) (60° and 70°N)  Subpolar low pressure belt (Southern hemisphere) (60° and 70°S)  Polar high (Northern hemisphere)  Polar high (Southern hemisphere) The equatorial region receives more solar radiationand thus the surface air temperature is high, whichcreates lighter air near the ground compared to higherlatitudes. The above condition leads to low atmosphericpressure over the equatorial region while sub-tropicalhigh pressure belts develop in both the hemispheresbetween 25 and 35 degree latitudes due to relatively lowsurface air temperature. It is due to low solar radiationreceived due to inclined sun’s rays over the subtropical 77
  • region when compared to the equatorial belt. Likewisealternate low and high atmospheric pressure beltsystems are developed across the globe from the equatorto the poles. Polar high 70° Subpolar low 60° 35° Subtropical high 25° 0° Equatorial low trough 25° 35° Subtropical high 60° Subpolar low 70° Polar highVariation in Atmospheric Pressure(a)Variation with height (Vertical pressure variation)a. Pressure decreases with height. 78
  • b. This is natural since the height of the atmosphere column above decreases with height.(b) Horizontal pressure variationa. Temperature and pressure are inverse to each other. Along equator low pressure exists and in cold polar latitudes polar high exists.b. Horizontal variation of air pressure on the surface of the earth are indicated by Isobars.c. The horizontal variation of pressure depends on Temperature, extent of water vapour, land water realationship etc.,(c )Diurnal pressure variation a) There is a definite rhythm in the rise and fall of the pressure in a day. b) Radiational heating (air expansion) and radiational cooling (air contraction) are the main reasons for diurnal variation in the air pressure. c) Diurnal variation is more prominent near the equator than at the mid latitudes. d) The areas closer to sea level record relatively larger amount of variation than in land areas. 79
  • e) Equatorial regions absorbs more heat than it loses while the polar region gives up more heat than they receive.(d) Seasonal pressure variation a) Due to the effect caused by annual variation in the amount of insolation, distinct seasonal pressure variations occur. b) These variations are larger in the tropical region than the mid latitude and polar regions. c) Usually, high pressures are recorded over the continents during the cold season and over the oceans during the warm season.Causes of variation The atmospheric pressure changes continuouslydue to several factors. The most important factors arechanges with temperature, altitude, water vapourcontent and rotation of earth.a) Temperature Hot air expands and exerts low pressure. Cold aircontracts and exerts high pressure. So the equator has a 80
  • low pressure due to prevalence of high temperature butpoles have a high pressure.b) Altitude At sea level, the air column exerts its full pressure,but when we stand on a hill or when we go in the upperlayers of atmosphere, we leave a portion of air whichcannot exert its full pressure. At sea level, a coastal townenjoys high pressure but on high altitude one willregister a low pressure. For every 10 m of ascend, thepressure get reduced by 1 hPa.c) Water vapour The water vapour content is lighter in cold areathan in air which is dry with the result that moist air of ahigh temperature exerts a less pressure when comparedto cold air.d) Rotation of the earth On account of rotation of the earth, the pressureat 60-70°N and S becomes low. The rotation of the earthnear sub-polar belts, makes the air to escape from thesebelts which move towards the horse latitude (30° - 35°Nand 30 – 35°S). These latitudes absorb the air from subpolar belts making the pressure high. G.D.Coriolis (1844)a French Mathematician indicated that air is deflected 81
  • towards right in the Northern Hemisphere and Left in theSouthern hemisphere due to rotation of earth and thiswas termed after him as Coriolis force. Coriolis force isnot actually a force but it is effect created by rotation ofearth.Importance of pressure on crop plantsPressure patternsLow / Depression or cyclone When the isobars are circular or elliptical in shape,and the pressure is lowest at the centre, such a pressuresystem is called ‘Low’ or ‘Depression’ or ‘Cyclone’. Themovement will be anti-clockwise in the Northernhemisphere while it is clockwise in the southernhemisphere. Wind speed hardly exceeds 40 km per hour.High pressure or Anticyclone When isobars are circular, elliptical in shape andthe pressure is highest at the centre such a pressuresystem is called ‘High’ or ‘Anticyclone’. When the isobarsare elliptical rather than circular the system is called as‘Ridge’ or ‘Wedge’. The movement will be clockwise in theNorthern hemisphere while it is anti-clockwise in thesouthern hemisphere.Atmospheric Humidity 82
  • The amount of water vapour that is present inatmosphere is known as atmospheric moisture orhumidity.Absolute humidity The actual mass of water vapour present in agiven volume of moist air. It is expressed as grams ofwater vapour per cubic meter or cubic feet.Specific humidity Weight of water vapour per unit weight of moist air.It is expressed as grams of water vapour per kilogram ofair (g/kg).Mixing ratio The ratio of the mass of water vapour contained ina sample of moist air to the mass of dry air. It isexpressed as gram of water vapour per kilogram dry air.Relative Humidity The ratio between the amount of water vapourpresent in a given volume of air and the amount of watervapour required for saturation under fixed temperatureand pressure. There are no units and this is expressed aspercentage. In other terms it is the ratio of the air’s watervapour content to its maximum water vapour capacity ata given temperature expressed in percentage. The relative 83
  • humidity gives only the degree of saturation of air. Therelative humidity of saturated air is 100 per cent.Dew Point temperature The temperature to which a given parcel of airmust be cooled in order to become saturation at constantpressure and water vapour content. In this case, theinvisible water vapour begins to condense into visibleform like water droplets.Vapour Pressure deficit The difference between the saturated vapourpressure (SVP) and actual vapour pressure (AVP) at agiven temperature. This is an another measure ofmoisture in the atmosphere which is useful in cropgrowth studies. When air contains all the moisture that itcan hold to its maximum limit, it is called as saturatedair, otherwise it is unsaturated air, at that temperature.The vapour pressure created at this temperature undersaturated conditions is vapour pressure or saturatedvapour pressure (SVP).Importance of Humidity on crop plants The humidity is not an independent factor. It isclosely related to rainfall, wind and temperature. It playsa significant role in crop production 84
  • 1. The humidity determines the crops grown in a given region2. It affects the internal water potential of plants3. It influences certain physiological phenomena in crop plants including transpiration4. The humidity is a major determinant of potential evapotranspiration. So, it determines the water requirement of crops5. High humidity reduces irrigation water requirement of crops as the evapotranspiration losses from crops depends on atmospheric humidity6. High humidity can prolong the survival of crops under moisture stress. However, very high or very low relative humidity is not conducive to higher yields of crops7. There are harmful effects of high humidity. It enhances the growth of some saprophytic and parasitic fungi, bacteria and pests, the growth of which causes extensive damage to crop plants.Eg: a. Blight disease on potato. b. The damage caused by thrips and jassids on several crops8. High humidity at grain filling reduces the crop yields 85
  • 9. A very high relative humidity is beneficial to maize, sorghum, sugarcane etc, while it is harmful to crops like sunflower and tobacco10.For almost all the crops, it is always safe to have a moderate relative humidity of above 40%.Variation in Humidity1. Absolute humidity is highest at the equator and minimum at the poles.2. Absolute humidity is minimum at sunrise and maximum in afternoon from 2 to 3 p.m. The diurnal variations are small in desert regions.3. The relative humidity is maximum at about the sunrise and minimum between 2 to 3 p.m.4. The behaviour of relative humidity differs a lot from absolute humidity. At the equator it is at a maximum of 80 per cent and around 85 per cent at the poles. But, near horse latitudes it is around 70 per cent. Measurement of Humidity 86
  • Wind Air in horizontal motion is known as wind. Verticalmovement is noticed but negligibly small compared tohorizontal movement as the height of the atmosphere isonly for few kilometers. However vertical movement oruplift of air only causes significant weather changes incloud formation and rain.Wind systems of the worldThe wind belts found on earth’s surface in eachhemisphere are: a) Doldrums b) Trade wind belt c) Prevailing westerlies d) Polar easterlies1. DoldrumsOwing to continuous heating of the earth by insolation,pressures are low and winds converge and rise near theequator. This intertropical convergent zone is known as‘Doldrums’. a) These are the equatorial belts of calms and variable winds. b) The location is 5°S and 5°N latitudes. c) Wind is light due to negligible pressure gradient. 87
  • d) Mostly, there are vertical movements in the atmosphere. e) The atmosphere is hot and sticky.2. Trade winds (Tropical Easterlies) a) The regular high temperature at the equator results in a high pressure forming in the upper levels of the equator. b) Then, the air is transferred to the northward and southward directions until 35° North and South in both the hemisphere. 88
  • c) Due to this reduction in surface pressure on the equator (doldrums) there is an increase in pressure at 35°N and 35°S which are known as horse latitude (sub-tropical high). d) As a result, the winds flow from the horse latitude to the equatorial region. e) While moving, these winds are deflected by Coriolis force to the right in northern hemisphere and to the left in southern hemisphere. f) These winds flow from 35°N to the equator in NE direction in the northern hemisphere and from 35°S to the equator in SE direction in the southern hemisphere. These are known as ‘Trade winds’. These are known as ‘Tropical easterlies’. g) These are most constant winds in force and direction and flow over nearly half the globe.3. Anti-trade winds a) This is a supplementary wind system of the earth which is effective at higher levels. b) This system works in opposite direction to the surface winds. c) The anti-trade winds mostly flow from land to ocean and brings no rain. 89
  • 4. Prevailing Westerlies a) The winds that flow from sub-tropical high to the low-pressure area about 60-70° latitudes in both the hemispheres are known as ‘Prevailing westerlies’. b) In the northern hemisphere the direction of Prevailing westerlies is SW and in southern hemisphere NW. c) These winds are forceful and are irregular as compare to the trade winds in the tropical regions. This is high precipitation zone5. Polar Easterlies / Polar winds a) A permanent high pressure exists on the poles. b) From these high pressure polar regions, cold winds flow to areas at about 60-65° latitudes in both the hemispheres. c) The winds flow in NE direction in the northern hemisphere and in SE direction in the southern hemisphere.Mountain winds a) Blows from mountain up slope to base b) Occurs during night time c) Cooling of air close to slope takes place 90
  • d) Adiabatic heating decreases this phenomenon e) Also known as ‘Katabalic winds’Valley winds a) Blow from valley base to up slope. b) Occurs during day time c) Over heating of air adjacent to slope takes place d) Adiabatic cooling decreases this phenomenon e) Also known as ‘Anabatic winds’Sea breeze During the daytime, more in summer, land isheated more than the adjacent body of water. As a resultwarmed air over the land expands producing an area oflow pressure. The isobaric surfaces bend upward as aresult of which the cooler air starts moving across thecoast line from sea to land. This is the ‘Sea breeze; or ‘Onshore breeze’.Land breeze At night because of nocturnal radiation land iscolder than adjacent sea and the pressure gradient isdirected from land to sea. There is a gentle flow of windfrom land to sea. This ‘off-share’ wind is called ‘Landbreeze’.Effect of wind on crop plants 91
  • 1. Transports heat in either sensible or latent heat form from lower to higher altitudes 2. Wind affects the plant directly by increasing transpiration and the intake of CO2 and also causes several types of mechanical damage 3. Wind helps in pollination and dispersal of seeds 4. Light and gentle winds are helpful for cleaning the agricultural produce 5. Hot dry winds frequently do much damage to vegetation in the growing crops by promoting excessive water loss 6. Wind has powerful effect on humidity 7. Long, continued warm, dry winds injured blossoms by evaporating the secretion of the stigma 8. Provides moisture which is necessary for precipitation 9. Wind prevents frost by disrupting atmospheric inversion 10.Causes soil erosionSpeed and Direction of wind Measurement of wind 92
  • Chapter 4 Rainfall and EvaporationClouds Clouds are condensed moisture consisting ofdroplets of water and ice crystals. Clouds are defined as“An aggregation of minute drops of water suspended inthe air at higher altitudes”. The rising air currents tendto keep the clouds from falling to the ground.Cloud formation When air rises due to increase in temperature, thepressure begins less it expands and cools until thetemperature is equalized. If the cooling proceeds furthertill the saturation point, the water vapour condenses andthe cloud is formed.Basic types of cloudsCirrus (Ci) - means “curl” which is recognized by its veillike fibrous or feathery form.Cumulus (Cu) – means heaps or globular mass.Stratus (St) - look like a sheet.Nimbus (Nb) – Looks dark and ragged. 93
  • WMO cloud classification The World Meteorological Organisation(WMO) classified the clouds according to their height andappearance into 10 categories. From the height, cloudsare grouped into 4 categories (viz., High clouds, Middleclouds, low clouds and vertical clouds) as stated belowand there are sub- categories in each of these maincategories.Family “A” or High clouds The clouds in this category are high. Themean lower level is 7 kilometers and the mean upperlevel is 12 kilometers in tropics and sub-tropics. In thisfamily there are three sub-categories.1. Cirrus (Ci)  In these clouds ice crystals are present.  Looks like wispy and feathery. Delicate, desist, white fibrous, and silky appearance.  Sun rays pass through these clouds and sunshine without shadow.  Does not produce precipitation.2. Cirrocumulus (Cc)  Like cirrus clouds ice crystals are present in these clouds also. 94
  •  Looks like rippled sand or waves of the sea shore.  White globular masses, transparent with no shading effect.  Mackerel sky.3. Cirrostraturs (Cs) Like the above two clouds ice crystals are present in these clouds also. Looks like whitish veil and covers the entire sky with milky white appearance. Produces “Halo”.Family “B” or Middle clouds The clouds in this category are middle clouds. Themean lower level is 2.5 kilometers and the mean upperlevel is 7 kilometers in tropics and sub-tropics. In thisfamily there are 2 sub-categories as details below:1. Altocumulus (Ac) In these clouds ice water is present. Greyish or bluish globular masses. Looks like sheep back and also known as flock clouds or wool packed clouds.2. Alto-stratus (As) In these clouds water and ice are present separately. 95
  •  Looks like fibrous veil or sheet and grey or bluish in colour. Produces coronos and cast shadow. Rain occurs in middle and high latitudes.Family “C” or Low clouds The clouds in this category are lower clouds. Theheight of these clouds extends from ground to upper levelof 2.5 kilometers in tropics and sub-tropics. In thisfamily, like high clouds, there are 3 sub-categorises.1. Strato cumulus (Sc) These clouds are composed of water. Looks soft and grey, large globular masses and darker than altocumuls. Long parallel rolls pushed together or broken masses. The air is smooth above these clouds but strong updrafts occur below.2. Stratus (St) These clouds are also composed of water. Looks like for as these clouds resemble grayish white sheet covering the entire portion of the sky (cloud near the ground). Mainly seen in winter season and occasional drizzle occurs. 96
  • 3. Nimbostratus (Ns) These clouds are composed of water or ice crystals. Looks thick dark, grey and uniform layer which reduces the day light effectively. Gives steady precipitation. Sometimes looks like irregular, broken and shapeless sheet like.Family “D” or Vertical clouds These clouds form due to vertical development i.e.,due to convection. The mean low level is 0.5 and meansupper level goes up to 16 kilometers.In this family two sub-categories are present.1. Cumulus (Cu) These clouds are composed of water with white majestic appearance with flat base. Irregular dome shaped and looks like cauliflower with wool pack and dark appearance below due to shadow. These clouds usually develop into cumuolo-nimbus clouds with flat base.2. Cumulonimbus (Cb)  The upper levels of these clouds possess ice and water is present at the lower levels. 97
  •  These clouds have thunder head with towering evil top and develop vertically.  These clouds produces violent winds, thunder storms, hails and lightening, during summer.PrecipitationPrecipitation is defined as water in liquid or solid formfalling on the earth’s surface.Isohyets: Isohyets are the lines connecting variouslocations, having an equal amount of precipitation.Types of precipitation 1. Cyclonic precipitation 2. Conventional precipitation 3. Orographic precipitation 4. Precipitation due to turbulent ascent1. Cyclonic precipitationCyclonic precipitation results from lifting of air massesconverging into low pressure area of cyclones.2.Conventional precipitation This type of precipitation occurs due to convectionover turning of moist air. Conventional precipitationresults in heavy showery rainfall. The major forms ofprecipitation associated with this type are rains or snowshowers, hail or snow pellets. 98
  • 3. Orographic precipitation Precipitation resulting from raising and cooling ofair masses when they are blocked by a topographicalbarrier (mountains). The barriers are important factor inincreasing the rainfall on windward slopes. 4. Precipitation due to turbulent ascent It is produced when airs current converge and rise.Most precipitation results from condensation andsublimation. This type occurs mainly in middle latitude.a) Rainfall intensityb) Aerial extent of rainstormc) Frequency of rainstormsForms of PrecipitationLiquid formRain It is precipitation of liquid water particles either inthe form of drops having diameter greater than 0.5 mmor in the form of smaller widely scattered drops. Whenthe precipitation process is very active, the lower air ismoist and the clouds are very deep, rainfall is in the formof heavy downpour. On occasions, falling raindropscompletely evaporate before reaching the ground.Drizzle 99
  • It is fairly uniform precipitation composed of finedrops of water having diameter less than 0.5 mm smalland uniform size and seems to be floated in the air, it isreferred as drizzle. If the drops in a drizzle completelyevaporates before reaching the ground, the condition isreferred to as ‘mist’.ShowerSolid formSnow It is the precipitation of white and opaque grains ofice. Snow is the precipitation of solid water mainly in theform of branched hexagonal crystals of stars. In winter,when temperatures are below freezing in the wholeatmosphere, the ice crystals falling from the Altostratusdo not melt and reach the ground as snow.Hail Precipitation of small pieces of ice with diameter rangingfrom 5 to 50 mm or something more is known as hail.Hailstorms are frequent in tropics. In India, the periodfrom March to May offers the ideal condition forhailstorms. It is the most dreaded and destructive form of 100
  • precipitation produced in thunderstorms orcumulonimbus clouds.Mixed formSleet It refers to precipitation in the form of a mixture of rainand snow. It consists of small pellets of transparent ice, 5mm or less in diameter. It refers to a frozen rain thatforms when rain falling to the earth passing through alayer of cold air and freezes. This happens whentemperature is very low. It is not commonly seen in Indiaexpect high ranges, that too in winter, in extreme northand northeast India.GlazeFreezing rain is known as glaze. This is formed at sub-freezing temperature, when rain falls on objects or onground. It looks like a sheet or coat.Condensation 101
  • Hydrological cycle The water cycle also known as the hydrologicalcycle, describes the continuous movement of water onabove and below the surface of the earth. There is nobeginning or end. Water can change states among liquid,vapour and ice at various places in the water cycle.Although the balance of water on earth remains fairlyconstant over time, individual water molecules can comeand go. It consists of four major steps viz., Evaporation,transportation, condensation and precipitation.Monsoons of India The term monsoon is derived from an Arabic word“Mausim” means “Season”. 102
  • Indian continent receives its annual rainfall by thepeculiar phenomenon known as monsoon. It consists ofseries of cyclones that arise in India Ocean. These travelin northeast direction and enter the Peninsular Indiaalong its west coast. The most important of thesecyclones usually occur from June to September resultingin southwest monsoon. Southwest monsoon is the mostimportant as it contribute 80 – 95% of the total rainfall ofthe country. Two types of monsoon systems are a) South WestMonsoon, b) North East Monsoon.(a) South West Monsoon Beginning of the year temperature of theIndian Peninsular rapidly rises under the increasing heatof the sun. A minimum barometric pressure isestablished in the interior parts of the Peninsular by themonth of March. Westerly winds prevail on the westKerala and south winds on the west of northern Circars,Orissa and Bengal. During April and May the region ofhigh temperature is shifted to north viz., upper Sind,lower Punjab and Western Rajasthan. This area becomesthe minimum barometric pressure area to whichmonsoon winds are directed. 103
  • The western branch of South West monsoontouches North Karnataka, Southern Maharastra andthen it make its way to Gujarat. When the South WestMonsoon is fully operating on the Western India, anotherbranch of the same is acting in the Bay of Bengal. Itcarries rains to Burma, Northern portions of the eastcoast of India, Bengal, Assam and the whole of NorthIndia in general.b) North east Monsoon During September end, the South WestMonsoon penetrates to North Western India but stays onfor a full month in Bengal. On account of the increase inbarometric pressure in Northern India, there is a shift ofthe barometric pressure to the South East and NorthEasterly winds begin to flow on the eastern coast. Thesechanges bring on heavy and continuous rainfall to theSouthern and South Eastern India.c) Winter Rainfall It is restricted more to Northern India and isreceived in the form of snow on the hills and as rains inthe plains of Punjab, Rajasthan and central India.Western disturbance is a dominant factor for rainfallduring these months in northwestern India. 104
  • d) Summer Rainfall: The summer Rainfall is received from March toMay as local storms. It is mostly received in the SouthEast of Peninsular and in Bengal. Western India does notgenerally receive these rains.Rainfall distribution in Tamilnadu in differentseasons Rainfall Quantity (mm) Percentage season shareSouth West 311.7 32monsoonNorth East 457.8 47MonsoonCold weather 48.7 5periodHot weather 155.9 16periodEffect of monsoon on crop production1. Nearly 54 per cent of population of the world depends on monsoon for their income.2. Monsoon rains are considered as life giving rains.3. Heavy rain during harvesting causes lodging of crop and seed germination. 105
  • 4. As in the case of other weather elements the amount and distribution of rainfall influence the crop yield considerably.5. Many farm operations such as seed bed preparation, sowing, intercultivation etc., depend on rainfall.Harmful effect of rainfall1. When heavy rainfall occurs sowing, weeding, harvesting and drying becomes very difficult.2. Rainfall received immediately after harvesting causes germination of seeds and growth of fungal population3. If low rainfall occurs, it results in non- supply of sufficient water to the plant which finally results in drying up.Artificial rain making Clouds are classified into warm and cold cloudsbased on cloud top temperature. If the cloud temperatureis positive these clouds are called warm clouds and if it isnegative they are called as cold clouds. The nucleusneeded for precipitation differs with type of clouds.Hygroscopic materials are necessary as nucleus for warmcloudsCloud seeding 106
  • Cloud seeding is one of the tools to mitigate the effects of drought. It is defined as a process in which the precipitation is encouraged by injecting artificial condensation nuclei through aircrafts or suitable mechanism to induce rain from rain bearing cloud. The rain drops are several times heavier than cloud droplets. These mechanisms are different for cold and warm clouds.Seeding of cold clouds This can be achieved by two ways 1. Dry ice seeding and 2. Silver Iodide seeding 1. Dry ice seeding Dry ice (solid carbon-dioxide) has certain specific features. It remains as it is at –80°C and evaporates, but does not melt. Dry ice is heavy and falls rapidly from top of cloud and has no persistent effects due to cloud seeding. Aircrafts are commonly used for cloud seeding with dry ice. Aircraft flies across the top of a cloud and 0.5 – 1.0 cm dry ice pellets are released in a steady stream. While falling through the cloud a sheet of ice crystals is formed. 107
  •  From these ice crystals rain occurs. This method is not economical as 250 kg of dry ice is required for seeding one cloud. To carry the heavy dry ice over the top of clouds special aircrafts are required, which is an expensive process. 2. Silver Iodide seeding Minute crystals of silver iodide produced in the form of smoke acts as efficient ice-farming nuclei at temperatures below –5°C. When these nuclei are produced from the ground generators, these particles are fine enough to diffuse with air currents. Silver iodide is the most effective nucleating substance because; its atomic arrangement is similar to that of ice. The time for silver iodide smoke released from ground generator to reach the super cooled clouds was offer some hours, during which it would draft a long way and decay under the sun light. The appropriate procedure for seeding cold clouds would be to release silver iodide smoke into super cooled cloud from an aircraft. In seeding cold clouds silver iodide technique is more useful than dry ice techniques, because, very much less of silver iodide is required per cloud. There is no necessity to fly to the top of the cloud, if area to be covered is large. 108
  • Seeding of warm clouds1) Water drop Technique2) Common salt techniqueEvaporation A physical process in which liquid water isconverted into its vapour. Evaporation is the mostimportant water loss term in water balance equation.Importance of Evaporation in crop plants1) Evaporation is an important process of hydrologic cycle.2) The evaporation from the soil is an important factor deciding the irrigation water requirements of a crop3) In modifying the microclimate of a crop the evaporation from the soils is an important factor for consideration.4) Evaporation is the most important of all the factors in the heat budget, after radiation.5) The evaporation is also one of the most important factors in the water economy.6) Since, a certain amount of evaporation also demands a definite amount of heat, it provides a link between water budget and heat budget.Factors affecting evaporation 109
  • The evaporation from a fully exposed water surface isthe function of several environmental factors1. Environmental factorsa. Water temperature With an increase of temperature the kinetic energy of water molecules increases and surface tension decreases which increases evaporation.b. Wind The evaporation from fully exposed surface is directly proportional to the velocity of wind and vice-versa, because dry wind replaces the moist air near water. The process of evaporation takes place continuously when there is a supply of energy to provide latent heat of evaporation (540 calories / gram of water).c. Relative humidity The evaporation is greater at low RH than at high RH.d. Pressure The evaporation is more at low pressure and less at high pressure.2. Water factorsa. Composition of water 110
  • The dissolved salts and other impurities decreases the rate of evaporation. The evaporation is inversely proportional to the salinity of water. b. Area of evaporation The larger the area of exposure, greater will be the evaporation. Transpiration This is a physiological phenomenon, which takes place only in living plants. The loss of water from living parts of the plant is known as Transpiration. The loss of water through stomatal openings of the leaves is termed as stomatal transpiration. The loss of water through cuticle is known as cuticular transpiration and from lenticels is known as lenticular transpiration. Importance of transpiration on crop plants 1. Dissipation of radiant energy by plant parts 2. Translocation of water in the plants 3. Translocation of minerals in the plant Factors affecting transpirationI. Environmental factors1. Light: By directly opening and closing the of stomata there is periodicity in the transpiration 111
  • rate. Indirectly by increasing the temperature of leaf cells the transpiration is increased.2. Atmospheric humidity: The rate of transpiration is almost inversely proportional to atmospheric humidity.3. Air Temperature: Increase in Temperature results in opening of stomata which in turn increases transpiration.4. Wind velocity: The higher the wind speed higher the transpirationII. Plant factors1. Plant height: Water need of the crop varies with height.2. Leaf characteristics: Reduction in leaf area brings reduction in transpiration.3. Availability of water to the plant: If there is little water in the soil the tendency for dehydration of leaf causes stomatal closure and a consequent fall in transpiration. Evapotranspiration Evapotranspiration denotes the quantity of water transpired by plants or retained in the plant tissue plus the moisture evaporated from the surface of the soil. As 112
  • long as the rate of root uptake of soil moisture balancesthe water losses from the canopy, evapotranspirationcontinues to occur at its potential rate. When the rate ofroot water uptake falls below the transpiration demand,actual transpiration begins to fall below the potentialrate. This is either because the soil cannot supply waterto roots quickly or the plant can no longer extract waterto meet the evaporational demand.Reference Evapotranspiration (ET0) This represents the maximum rate ofevapotranspiration from an extended surface of 8 to 10centimeters tall green grass cover, actually growing andcompletely shading the ground under limited supply ofwater.Potential Evapotranspiration (PET) is the upper limit ofevapotranspiration. PET is the water transpired fromuniform, short, green, actively growing vegetation whenwater supply is unlimited. ET data are used as a basisfor estimating the amount of water required to irrigate. Lysimeter is a device for measuring ET. There aretwo types of Lysimeter, they are namely volumetric,which is used for paddy, and the other is gravimetric 113
  • which is used in dry land and irrigated crop like maize,cotton and groundnut.Importance of Evapotranspiration and PotentialEvapotranspiration for crop plants 1. Estimation of the soil moisture there by planning irrigation schedule of crops. 2. Understanding relationship between the crop yield and irrigation water. 3. Guiding for the production of a crop with a fully developed canopy. 4. The evapotranspiration can also help to demarcate soil climatic zones including the drought prone areas. 5. These will form the base for developing suitable soil and crop management practices, crop varieties, water conservation techniques, cropping pattern and ways to improve productivity rainfed crops. 114
  • Chapter 5 Weather Aberrations and Forecasting Weather Aberrations Weather is not always normal. Some abrupt or sudden changes take place in weather which is called weather abnormalities. They cause considerable damage to the crops. Some of them are droughts, floods, untimely rains, thunder storms, hail storms, dust storms and cold waves. Drought or deficient rainfall The term drought can be defined in several ways.1. The condition under which crops fail to mature because of insufficient supply of water through rain.2. The situation in which the amount of water required for transpiration and evaporation by crop plants in a defined area exceeds the amount of available moisture in the soil.3. A situation of no precipitation in a rainy season for more than 15 days continuously. Such length of non-rainy days can also be called as dry spells. Classification of Drought 115
  • Droughts are broadly divided into 3 categories based on the nature of impact and spatial extent.1. Meteorological Drought If annual rainfall is significantly short of certain level (75 per cent) of the climatologically expected normal rainfall over a wide area, then the situation is called meteorological drought. In every state each region receives certain amount of normal rainfall. This is the basis for planning the cropping pattern of that region or area.2. Hydrological drought This is a situation in which the hydrological resources like streams, rivers, reservoirs, lakes, wells etc., dry up because of marked depletion of surface water. The ground water table also depletes. The industry, power generation and other income generating major sources are affected. If Meteorological drought is significantly prolonged, the hydrological drought sets in.3. Agricultural Drought This is a situation, which is a result of inadequate rainfall and followed by soil moisture deficit. As a result, the soil moisture falls short to meet the demands of the crops during its growth. Since, the soil moisture available 116
  • to a crop is insufficient, it affects growth and finallyresults in the reduction of yield. This is further classifiedas early season drought, mid-season drought and lateseason drought.Flood Years in which actual rainfall is ‘above’ the normalby twice the mean deviation or more is defined as yearsof flood or excessive rainfall. Like droughts, the definitionof floods also varies one situation to another and formsone region to other. 2005 is the heavy flood year in Tamilnadu.Cyclone Cyclones are most common in India and causeheavy damage to crops, livestock and loss of human life.Tropical cyclones as the name implies, are cyclonicwhirlpools that are formed over the oceans in thelatitudes between 5 and 20 degree from the equator.These storms are called by different local names, themost common being hurricane (North America), typhoon(Eastern Asia, Philippines, Cyclone (India, Australia).Tornado 117
  • Tornado is the most violent storms of lower troposphere. They mostly occur during spring and early summer. Crop losses are heavy due to this event. Thunder storm Thunderstorms occur regularly in the tropics. Thunder and lightning accompany the typical mature thunderstorm. Raindrops are large and it is literally a “cloud burst”. Thunderstorms are most common in summer due to surface heating of land mass. Dust storms Dust storms take place during summer in the months of April, May and June. During this period, high temperature and low humidity prevail. Dust and particles may deposit on the crops. Effect of weather aberrations on agriculture:1. The crops fail to grow and mature because of insufficient availability of moisture.2. Due to submergence due to muddy water, the fields are silted.3. Due to higher rainfall tank bunds are breached. 118
  • 4. Floods cause damage to crops, livestock and human life.5. Cyclones cause damage either by high wind or by flooding. Phenology Phenology is the study of the annual cycles of plants and how they respond to seasonal changes in their environment. For example, in botany phenology refers to the timing of flower emergence, sequence of bloom, fruiting, and leaf drop in autumn. Uses of Phenology  Correlation with insect emergence and pest control  Correlation with crop planting dates  Farmscaping with insect refugia (cover crops, hedge rows, strip crops) to attract beneficial insects and enhance natural biological control  Designing orchards for pollination and ripening sequence  Designing bee forage plantings  Designing perennial flower beds and wildflower plantings  Prediction of global warming trends Bioclimatic law 119
  • The law which states that phenological events are altered by about 4 days for each 5° change of latitude northward or longitude eastward; events are accelerated in spring and retreat in autumn. Different season of India Based on the rainfall pattern and temperature distribution, the India Meteorological Department, Government of India has divided the whole year into four seasons viz. 1. South west monsoon (June – September) 2. Post Monsoon (October – November) 3. Winter (December – February) 4. Summer or Pre monsoon (March / April – May / June) The above seasons coincide with the agricultural seasons viz.1) Kharif2) Rabi and3) Summer The Kharif season is nothing but the Southwest monsoon or autumn. The Rabi coincides with post monsoon and winter seasons. In summer, the cultivable land under seasonal crops is kept in many 120
  • regions of the country. Wherever water is plenty, vegetables are grown in some parts of the country during summer. Spring falls between January and March. Based on temperatures, ranges, there are three distinct crops seasons in India. They are1) Hot weather (Mid February – Mid June)2) Kharif or rainy season (Mid June – Mid October)3) Rabi (Mid October – Mid February) In Tamil Nadu there is a slight variation in the seasons based on rainfall duration as2) Winter – January and February3) Summer – March to May4) Rainy seasonsa) South west monsoon (June – September)b) North east monsoon (October – December) The criteria for division of growing seasons are broadly based on monthly precipitation and temperature. The pattern followed asa) Hot month – if the average temperature is above 20°Cb) Cold month – if the mean temperature is between 0-10°Cc) Warm month – if the mean temperature is 15-20°C 121
  • MAPPING OF AGROCLIMATIC ZONES OF INDIAANDTAMIL NADU CLIMATE Climate in general is the totality of weather duringa large period and wider areas.AGRO CLIMATE Agro climate can be defined as the conditions and theeffect of varying parameters like solar radiation, rainfall,relative humidity on crop growth and production. Theclimate of one place is not similar to other area. However identical climatic conditions are prevalent in regions of the same latitude.ADVANTAGES OF AGRO CLIMATIC ZONES1. This would enable in exploring agricultural potentiality of the area.2. Location of homo climatic zones will enable in identifying the soil and climate problems related to agriculture to each region.3. This will help in introduction of new crops from other similar area. Eg. Introduction of oil palm in Tamil Nadu.4. Development of crop production technologies specific for the region.5. Take a research to solve the regional problems. 122
  • 6. To transfer the technology easily. 123
  • A. AGRO CLIMATIC ZONES OF INDIA Based on the criteria of homogeneity in agro-characteristics such as rainfall, temperature, soil,topography, cropping, farming systems and waterresources, the country has been divided into 15 agro-climatic zones. This was mainly done to identify aproduction constraints and to plan future strategies forpurpose of cropping with the involvement of academists,scientists, planners, administrators, voluntary agencies,bank and others for balanced regional growth.1. WESTERN HIMALAYAN ZONE This zone consists of three distinct sub-zones of Jammu and Kashmir, Himachal Pradesh andUttar Pradesh Hills. The region consists of skeletal soilsof cold region, podsolic soils, mountain meadow soils,hilly brown soils. Lands of the region have steep slopesin undulating terrain. Soils are generally silty loams andthese are prone to erosion hazards.2. EASTERN HIMALAYAN ZONE 124
  • Sikkim and Darjeeling Hills, Arunachal Pradesh,Meghalaya, Nagaland, Manipur, Tripura, Mizoram,Assam and Jalpaiguri and Coochbihar districts of WestBengal fall under this region, with high rainfall and highforest cover. Shifting cultivation is practiced in nearlyone-third of the cultivated area and this has causeddenudation and degradation of soils with the resultantheavy runoff, massive soil erosion and floods in the lowerreaches and basins.3. LOWER GANGETIC PLAINS ZONE This zone consists of West Bengal-lower Gangeticplains region. The soils are mostly alluvial and are proneto floods.4. MIDDLE GANGETIC PLAINS This zone consists of 12 districts of eastern UttarPradesh and 27 districts of Bihar plains. This zone has ageographical area of 16 million hectares and rainfall ishigh. About 39 per cent of gross cropped area isirrigated and the cropping intensity is 142 per cent.5. UPPER GANGETIC PLAINS ZONE This zone consists of 32 districts of UttarPradesh. Irrigation is through canals and tube wells. Agood potential for exploitation of ground water exists. 125
  • 6. TRANS GENETIC PLAINS ZONE This zone consists of Punjab, Haryana,Union Territories of Delhi and Chandigarh andSriganganagar district of Rajasthan. The majorcharacteristics of this area are highest net sown area,highest irrigated area, high cropping intensity and highgroundwater utilization.7. EASTERN PLATEAU AND HILLS ZONE This zone consists of eastern part of MadhyaPradesh, southern part of West Bengal and most ofinland Orissa. The soils are shallow and medium indepth and the topography is undulating with a slope ofone to ten per cent. Irrigation is through tanks and tubewells.8. CENTRAL PLATEAU AND HILLS This zone comprises 46 districts of UttarPradesh, Madhya Pradesh and Rajasthan. Nearly ⅓rd ofland is not available for cultivation.9. WESTERN PLATEAU AND HILLS ZONE This zone comprises the major part ofMaharashtra, parts of Madhya Pradesh and one districtof Rajasthan. The average rainfall of the zone is 904 mm.The net sown area is 65 per cent and forests occupy 11 126
  • per cent. The irrigated area is only 12.4 per cent withcanals being the main source.10. SOUTHERN PLATEAU AND HILLS This zone comprises 35 districts of AndhraPradesh, Karnataka and Tamil Nadu which are typicallysemi-arid zones. Dryland farming is adopted in 81 percent of the area and the cropping intensity is 111 percent.11. EAST COAST PLAINS AND HILLS This zone comprises west coast of TamilNadu, Andhra Pradesh and Orissa. Soils are mainlyalluvial and coastal sands. Irrigation is through canalsand tanks.12. WEST COAST PLAINS AND GHATS ZONE This zone comprises west coast of TamilNadu, Kerala, Karnataka, Maharashtra and Goa with avariety of crop patterns, rainfall and soil types.13. GUJARAT PLAINS AND HILLS ZONE This zone consists of 19 districts of Gujarat.This zone is arid with low rainfall in most parts and only32.5 per cent of the area is irrigated largely through wellsand tubewells.14. WESTERN DRY ZONE 127
  • This zone comprises nine districts ofRajastan and is charecterised by hot sandy desert,erratic rainfall, high evaporation, scanty vegetation. Theground water is deep and often brackish. Famine anddrought are common features of the region.15. ISLAND ZONE This zone converts the island territories ofAndaman and Nicobar and Lakshadeep which aretypically equatorial with rainfall of 3000 mm spread overeight to nine months. It is largely a forest zone withundulated lands.B. AGRO CLIMATIC ZONES OF TAMIL NADU Tamil Nadu recorded the mean annual rainfall of974 mm. The highest contribution from north-east (NE)monsoon (47 per cent of total rainfall), followed by south-west (SE) monsoon (32 per cent), summer (16 per cent)and winter(5 per cent). Based on the rainfall, altitude and irrigationsource, the state has been marked into sevenagroclimatic zones.1. North – Eastern Zone Thiruvallur, Vellore, Kanchipuram,Thiruvannamalai, Villupuram and Cuddalore district. 128
  • The mean annual rainfall is 1100 mm out of which 566mm is received during the NE monsoon period withfrequent occurrence of cyclones. The major soil types arered sandy loam, clayey loam and saline coastal alluvium.The major irrigation sources are tanks and wells. Sincethe rainfall is uniformly distributed from July toDecember, two crop sequences are practiced. In thecoastal areas rainfed rice is cultivated. The crops likesugarcane and millets are raised with the help of wellirrigation. 129
  • Agro Climatic zones of Tamil Nadu2. North – Western Zone Salem, Dharmapuri, Krishnagiri andNamakkal (part) districts. The mean annual rainfall is 130
  • 875 mm and about 42 per cent of rainfall is receivedduring the SW monsoon period. The elevation rangesfrom 800 to 1000 m (MSL). The major soils are non-calcareous red and brown and calcareous black.Cultivation in drylands commences from June and twocrop sequences are followed. A significant practice oftransplanting finger millet, sorghum and tomato purelyunder rainfed condition is followed. Established mangogrooves in drylands are common.3. Western Zone Coimbatore, Erode, parts of Karur, Namakkal,Dindigul and Theni districts. The mean annual rainfall is715 mm with a contribution of 49 per cent from the NEperiod. The predominant soil types are red and blacksoils. Dryland sowings start in June / July in red soilswhile in black soils during September / October. Withthe receipt of early rain, groundnut is sown in red soils.In black soil areas, cotton for early rains andbengalgram for late rains are raised. Southern part ofthis zone, the rainfall is about 550 mm only and morearea is devoted to pastures. With the help of well andcanal irrigation crops like cotton, finger millet andsugarcane are raised. 131
  • 4. Cauvery Delta Zone Thanjavur, Tiruchirapalli, Thiruvarur,Nagapattinam, Perambalur, parts of Pudukottai andCuddalore districts. The mean annual rainfall varies from985 mm, out of which more than 50 per cent is receivedthrough NE monsoon period. The main source ofirrigation is the Cauvery river. The major soil type isalluvial in the old delta areas while red loamy andlateritic are also present in other areas. Rice occupiesthe major area and it is called as ‘rice bowl’ of TamilNadu. After the rice crop, pulses are raised with residualsoil moisture. In places with supplemental irrigationthrough filter points cotton, groundnut and sesamum areraised as summer crops.5. Southern Zone Madurai, Sivagangai, Virudhunagar,Ramanathapuram, Thoothukudi and Tirunelveli districts.The topography of the zone is undulating. This zone lieson the rain shadow area of the western ghats. The meanannual rainfall is 850 mm with a contribution of about470 mm from NE monsoon. The soils of this region fall 132
  • under major groups, viz., black, red, alluvial and lateritic.Saline coastal alluvial soils are also present in the coastalbelt. In black soils only one crop, either cotton orsorghum is raised. Direct seeded rice is cultivated underrainfed condition on light soils. On red soils, groundnutcrop is raised. Under garden land conditions, pearlmillet and chillies are raised.6. High Rainfall Zone Kanyakumari district -The mean annualrainfall is 1420 mm received in 64 rainy days, out ofwhich 38 and 36 per cent are respectively receivedduring SW and NE monsoon periods. The soils are deepred loam except the coastal areas where it is salinecoastal alluvium. Rice is the major annual crop underrainfed condition followed by tapioca. Plantation cropslike tea, pepper, clove, nutmeg, cardamom and coffeealso cultivated on the hills.7. Hilly Zone This zone comprises the hilly region of theNilgiris, the Shevroys, the Yalagiri, the Anamalai and thePalani. The mean annual rainfall is 2124 mm. The soilis mainly lateritic. The major crops are cole vegetables,potato, tropical and temperate fruits. At the foot of the 133
  • hills minor millets are raised. At higher altitudes wheatcultivation is common during winter season.Weather forecast The prediction of weather for the next few days tofollow. The Figure below depicts different weatherforecasting services normally practiced in a country. Agriculture including forestry and Animal General husbandry Shipping Mercantile Public & Naval Fishing Off shore drilling Weather forecasting Mountaineering Defence services Cyclones, Aviation Civil & floods and Military drought Government and Post officials 134
  • NEED / IMPORTANCE OF FORECAST  Basically weather has many social and economic impacts in a place.  Among different factors that influence crop production, weather plays a decisive role as aberrations in it alone explains up to 50 per cent variations in crop production  The rainfall is the most important among the required forecast, which decides the crop production in a region and ultimately the country’s economy. 135
  •  The planning for moisture conservation under weak monsoon condition and for flood relief under strong monsoon condition is important in a region.  A reliable weather forecasting when disseminated appropriately will pave way for the effective sustainability.  One can minimize the damage, which may be caused directly or indirectly by unfavourable weather.  The recurring crop losses can be minimized if reliable forecast on incidence of pest and diseases is given timely based on weather variables.  Help in holding the food grain prices in check through buffer stock operations. This means that in good monsoon years when prices fall, the government may step in and buy and in bad years when price tend to rise, it may unload a part of what it had purchased.  Judicious use of water can be planned in a region depending up on the forecast.Type of weather forecastTypes of Validity Main Predictionsforecast period users 136
  • 1 Short Up to 72 Farmers Rainfall range hours marine distribution, heavy a) Now 0-2 hours agencies, rainfall, heat and casting general cold waveb) Very short 0-12 public conditions, range hours thunder storms etc.2 Medium Beyond 3 Farmers Occurrence of range days and rainfall, upto 10 Temperature. days.3 Long Beyond Planners This forecasting is range 10 days provided for upto a Indian monsoon month rainfall. The out and a looks are usually season. expressed in the form of expected deviation from normal condition. Synoptic charts An enormous volume of meteorological data is being collected from all over the world continuously round the clock through various telecommunication channels. To assess, assimilate and analyse the vast data, they have to be suitably presented. For this purpose, the observations are plotted on maps in standard weather codes. These maps are called ‘Synoptic maps or charts’. 137
  • Synoptic charts display the weatherconditions at a specified time over a large geographicalarea. The surface synoptic charts plotted for differentsynoptic hours (00, 03, 06, 09, 12, 15, 18, 21 UTC)depict the distribution of pressure, temperature, dewpoint, clouds, winds, present and past weather. In placeof GMT, UTC (Universal Time Co-ordinate) is used. Theupper air charts are also prepared at the standardpressure levels of the atmosphere (different heights) ofthe atmosphere wherein the pressure, wind andtemperature are plotted. The surface charts together withthe upper air charts provide a composite three-dimensional weather picture pertaining to a given time.Thus it gives a birds eye view of the state of atmosphereat a time over a large area and is a important tool usedby operational meteorologists and scientists. The surface synoptic charts are the mostused charts. It contains the maximum number ofobservations with the largest number of parametersplotted and often forms the base on which the pressurelevel charts are built up. The pattern of the pressuredistribution is brought out by drawing isobars, troughs, 138
  • ridges, lows, highs, depressions, cyclones, cols, frontsand discontinuities. These systems are clearly markedand labeled using appropriate symbols and colours. In synoptic charts different weatherphenomena and atmospheric characters are marked withdifferent symbols as mentioned below.___________________________________________________________________________S.No Symbols Weather element/character/phenomenon 1. Narrow black lines lsobars 2. Numbers at ends of isobars Pressure values in hPa 3. Shading Precipitation 4. Arrows Winddirection 5. Feathers in the arrows Windvelocity 6. Small circles with shading Amountof clouds 139
  • In addition to the above, different symbols are used forrecording weather phenomena.Crop Weather calendar 140
  • In order to provide the farmers with an efficientweather service, it is essential that the weather forecastershould be familiar with the crops that are grown in aparticular agroclimatic zone. The type of forewarnings tobe given depends on the stages of the crop. In case offarmers, they should become familiar with weatherbulletins and learn how to interpret. To meet the aboverequirement, the detailed information collected from theagricultural departments has been condensed by the IMDand presented in a pictorial form known as crop weathercalendar. This calendar has three parts as follows. a) Bottom part b) Middle part c) Top parta) Bottom part provides the activities related to crop orinformation related to phenological stages of the crop andthe months.b) Middle part gives information regarding normalweather condition required for active crop growth. It isdivided into different sections according to rainfall, rainydays, minimum temperature, maximum temperature,pan evaporation and sunshine hours. 141
  • c) Top part gives information related to the weatherabnormalities or to take precautionary measures. Toppart is divided into different sections according to dryspell length, high wind, heavy rainfall and cloudyweather.Sample crop weather calendar prepared for cotton inTamilnadu for South Arcot district 142
  • 143
  • REMOTE SENSING AND CROP MODELING Remote Sensing is the science and art of acquiringinformation (spectral, spatial, temporal) about materialobjects, area, or phenomenon, without coming intophysical contact with the objects, or area, orphenomenon under investigation. Without direct contact,some means of transferring information through spacemust be utilised. In remote sensing, information transferis accomplished by use of electromagnetic radiation(EMR).Uses: Remote sensing techniques are used inagricultural and allied fields. 144
  • 1. Collection of basic data for monitoring of crop growth 2. Estimating the cropped area 3. Forecasting the crop production 4. Mapping of wastelands 5. Drought monitoring and its assessment 6. Flood mapping and damage assessment 7. Land use/cover mapping and area under forest coverage 8. Soil mapping 9. Assessing soil moisture condition, irrigation, drainage 10.Assessing outbreak of pest and disease11.Ground water explorationRemote Sensing platforms Three platforms are generally used for remotesensing techniques. They are ground based, air basedand satellite based. Infrared thermometer, Spectralradiometer, Pilot-Balloons and Radars are some of theground based remote sensing tools while aircrafts airbased remote sensing tools. Since the ground based and 145
  • air based platforms are very costly and have limited use,space based satellite technology has become handy forwider application of remote sensing techniques. Thedigital image processing, using powerful computers, isthe key tool for analyzing and interpretation of remotelysensed data. The advantages of satellite remote sensingare: Synoptic view – Wide area can be covered by a single image/photo (One scene of Indian Remote Sensing Satellite IRS series cover about 148 x 178 area). Receptivity – Can get the data of any area repeatedly (IRS series cover the same area every 16-22 days). Coverage – Inaccessible areas like mountains, swampyareas and thick forests are easily covered. Space based remote sensing is the process ofobtaining information about the earth from theinstruments mounted on the Earth ObservationSatellites. The satellites are subdivided into two classesand the types of satellite are as follows:Polar orbiting satellites These satellites operate at an altitude between 550and 1,600 km along an inclined circular plane over the 146
  • poles. These satellites are used for remote sensingpurposes. LANDSAT (USA), SPOT (FRANCE), and IRS(INDIA) are some of the Remote Sensing Satellites.Geostationary satellite These have orbits around the equator at analtitude of 36,000 km and move with the same speed asthe earth so as to view the same area on the earthcontinuously. They are used for telecommunication andweather forecasting purposes. INSAT series are launchedfrom India for the above purposes. All these satelliteshave sensors on board operating in the visible and nearinfrared regions of the electromagnetic spectrum.INSAT-3A was launched on 10th April, 2003.Role of Remote Sensing in agriculture Agricultural resources are important renewabledynamic natural resources. In India, agriculture sectoralone sustains the livelihood of around 70 percent of thepopulation and contributes nearly 35 percent of the netnational product. Increasing agricultural productivity hasbeen the main concern since scope for increasing areaunder agriculture is rather limited. This demands judiciousand optimal management of both land and water resources.Hence, comprehensive and reliable information on land 147
  • use/cover, forest area, soils, geological information, extent ofwastelands, agricultural crops, water resources both surfaceand underground and hazards/natural calamities likedrought and floods is required. Season-wise information oncrops, their acreage, vigour and production enables thecountry to adopt suitable measures to meet shortages, ifany, and implement proper support and procurementpolicies. Remote Sensing systems, having capability ofproviding regular, synoptic, multi-temporal and multi-spectral coverage of the country, are playing an importantrole in providing such information. A large number ofexperiments have been carried out in developing techniquesfor extracting agriculture related information from groundborne, air borne and space borne data.Indian Remote Sensing programme India, with the experience gained from itsexperimental remote sensing satellite missionsBHASKARA-I and II, has now established satellite basedoperational remote sensing system in the country withthe launch of Indian Remote Sensing Satellite IRS-IA in1988, followed by IRS-IB (1992), IRS-IC (1995) and IRS-ID (1997). The Department of Space (DOS) / IndianSpace Research Organisation (ISRO) as the nodal agency 148
  • for establishing an operation remote sensing system inthe country initiated efforts in the early 1970s forassessing the potentials of remotely sensed data throughseveral means. In order to meet the user requirement ofremote sensing data analysis and interpretation,ISRO/DOS has set up a system to launch remote sensingsatellites once in three or four years to maintain thecontinuity in data collection. The remote sensing andsome of its related institutes are depicted.Crop weather modelingCrop model It is a representation of a crop throughmathematical equations explaining the crops interactionwith both above ground and below ground environment. The increase in dry matter of the crop isreferred to as growth. The rate of growth of a healthycrop depends on the rate at which radiation isintercepted by foliage and / or on the rate at which waterand nutrients are captured by root systems and thereforeon the distribution of water and nutrients in the soilprofile. The crop development is described in terms ofvarious phenophases through which the crop completesits lifecycle. That is the progress of the crop from seeding 149
  • or primordial initiation to maturity. Finally the yield ofcrop stand is expresses as a product of threecomponents, viz., the period over which dry matter isaccumulated (the length of the growing period), the meanrate at which dry matter is accumulated and the fractionof dry matter treated as yield when the crop is harvested. It is understood that the crop growth,development and yield depend upon the mean dailytemperature (DTT), the length of the day and the amountof solar radiation (PAR) received by the crop.DTT = Max daily temperature + Min daily temperature -base temperature 2Where, DTT = Daily thermal time accumulation. The time needed for the crop to reach adevelopment stage depends upon temperature measuredabove a base value (DTT) and for photo periodicallysensitive phases such as flowering, the day length abovea fixed base. In the absence of stress, the harvest indexdoes not vary much from year to year for a specifiedcultivar / variety. Therefore, crop weather modeling isbased on the principles that govern the development ofcrop and its growing period based on temperature and / 150
  • or day length. They are used to quantify the rate of crop growth in terms of radiation interception, water use and nutrient supply which moderate harvest index when the crops experience stress condition. The basic information required to be generated for crop weather modeling includes.a) Crop phonology in relation to the temperature and day lengthb) Water use by the crop during different phenophases of crop growthc) The relationship between radiation interception, crop water use and total dry matter productiond) Partitioning of dry matter into various plant components as influenced by water and nutrient availability, ande) The effect of weather parameters on biotic interference to crop growth. Types of models• Statistical models• Mechanistic model• Deterministic models• Stochastic models• Dynamic models• Static models 151
  • • Simulation models• Descriptive models• Explanatory models Climatic normal The climatic normals are the average value of 30 years of a particular weather element. The period may be week, month and year. The crop distribution, production and productivity depend on the climatic normals of a place. If the crops are selected for cultivation based on the optimum climatic requirements it is likely that the crop production can be maximized. 152
  • WEATHER NORMALS FOR AGRICULTURAL CROPSSl Crops Optimum Day Rainf Altit. Temperature ° C lengt all udeN Germin Growth h (mm) abovo. ation stage e MSL (m)1 Rice Min 10 22-25 1500 <300 °C (flowerin 0 g) 20-21(gr ain formatio n) 20-25(ri pening)2 Maize 35-44 ° C3 Sorgh 7-10 25-30 Short um day4 Pearl 28-32 400- millet 7505 Finger 500- millet 10006 Kodo 400- millet 5007 Wheat 20-22 16-22 250- <350 1800 08 Barley 12-15 Long 400- (growth) day 500 30(repro duction)9 Oats 15-25 380- 153
  • 114010 Groun 27-30 24- 500- d nut 27 125011 Sesam 25-27 Short 500- <125 e day 650 012 Castor 20-26 Long 500- <300 day 600 013 Sunflo 20-25 500- <250 wer 700 014 Rape 18-25 Long 300- seed day 400 and Mustar d15 Safflow 15- 25-30 Day 600- er 16 neutr 900 al16 Soybea 15- 30-33 600- 1200 n 32 650 -200 017 Pigeon 20- pea 3018 Green 15 20-40 Short 600- gram day 100019 Black 1500 gram20 Cow 12- 21-35 Short 600 pea 15 day21 Bengal 15-25 600- gram 100022 Cotton 18 21-27 Day 500 neutr al23 Jute 27-40 Short 1500 154
  • day 24 Tobacc 28 25-35 500- o 1000 25 Sugar 24-30 Long 2000 cane day -250 0 26 Sugar 12- 22-30 Long beet 15 day 27 Potato 18- 18-20 20Incidence of pest and disease Considerable crop losses caused due to pestsand diseases in the humid and sub humid tropics. Manyof the restrictions on productivity and geographicaldistribution of plants and animals are imposed by pestsand diseases. The geographical distribution of pests ismainly based on climatic factors. The climatic conditionsshow a gradient from place to place and there is a relatedgradient in the abundance of a particular pest / disease.The periodic or seasonal nature of incidence and outbreaks of several pests and diseases of many crops canbe ascribed to weather conditions as the triggeringfactors. These epidemics of diseases are principallyweather dependent, either in terms of local weatherconditions being favourable for growth and developmentof the casual organisms or the prevailing winds helping 155
  • to disseminate airborne pathogens or spores of diseasessuch as mildew, rusts, scabs and blights. The migration and dispersion of insect pestsdepend on the wind speed and direction besides thenature of air currents. Some plant pathogenic virusessuitable for the development of these vectors favour thetransmission of such diseases. A surfeit of pests anddiseases, which infest plants are kept in chock byseasonal fluctuations in atmospheric temperature orrelative humidity and other weather factors. Insect pestoutbreaks occur as a result of congenial weatherconditions, which facilitate their un-interruptedmultiplication. The weather and climate greatly influencethe quantity and quality of food provided by the hostcrops to the associated species of pests. The abundanceor otherwise of the pestiferous species is thus dependenton climatic conditions, indirectly also. The surface air temperature, relativehumidity, dew fall, sunshine, cloud amount, wind,rainfall and their pattern and distribution are theprimary weather factors influencing the incidence oroutbreaks of pests and diseases of crops. In the humidtropics, the weather variables namely air temperature, 156
  • intermittent rainfall, cloudy weather and dewfall mayplay a crucial role in the outbreaks of pests and diseases.The impact of various weather components on pests anddiseases is experienced in a location and crop specificmanner. Among the major pests associated withcrops, insect, mite and nematode species are of a seriousnature in terms of their abundance and damagepotential. If the occurrence of pest / disease in time andspace can be predicted in advance with reasonableaccuracy on the basis of relevant weather parameters,appropriate and timely control measures can beprogrammed. Appropriate insecticide / fungicideinterventions can certainly reduce the pesticide load inthe environment and the related pollution and healthhazards. 157