Introduction

Agrometeorology = Agriculture + meteorologyMeteorology is generally defined as a branch of physics that

deals with the physical process in the atmosphere that produces weather and climate.

The physical process are influenced by solar radiation, temperature, humidity, wind movement, atmospheric pressure, precipitation and soil moisture.

Agrometeorology• Is the study of those aspects of meteorology which have

direct relevance to agriculture• Is an applied science which deals with the relationship

between agriculture (plants and animals) and prevailing conditions of weather and climate

Weather And ClimateWeather is a state of atmosphere at a given place and at a

given time.Is the condition of atmosphere in terms of temperature,

humidity, wind movement and atmospheric pressure for a short interval of time i.e. a part of day or complete day of small area

Climate is the summation of weather conditions over a given region during a comperatively larger period.

Denotes the measurement of temperature, humidity, wind movement, precipitation, intensity and duration of light in the specific time in months or greater then month

ScopeWeather Monitoring Department of Hydrology and Meteorology is established

in Nepal 282 agro-meteorological stations located in different parts

of the country to measure weather conditions Temperature, humidity, rainfall, direction and velocity of

wind and rate of evaporation are monitored World Meteorological Organization (WMO), the

international agency responsible for worldwide climatic data, is located in Geneva, Switzerland

Contd…Agrometeorological Forecasting Forecasting of weather condition and trend Forecasting of insects and disease to decide transplanting

of crops, spraying of insecticides and pesticides to control disease and harvesting the crops

Crop zonation and Crop Planning Environmental condition can be regionalized based on

latitude, altitude, day length etc. Nepal is divided into five crop zones- tropical, sub- tropical,

temperate, sub alpine and alpine

Contd… Weather Crop Relationship The relationship between weather and selection of crop

according to the prevailing weather conditionClimatic Control cannot control climate in huge scale Micro modification is possible Achieved by the use of greenhouse Important in the research to maintain environment of

certain place Helps in agro climatic classification and crop weather

modeling

Solar RadiationRadiation All matters above absolute zero emits and imparts

energy to the surrounding This transferring of energy and its mode of transfer is

known as radiation

Black body is a substance which emit the maximum amount of

radiation Absorbs completely all the radiation incident upon them Is thus a perfect radiator and absorber Sun is considered a black body at 5762K

Contd..

Solar radiation The ultimate source of all the energy is radiation

received from sun Is the result of molecular reactions taking place in the

sun its distribution in the atmosphere governs the weather

processes on the earth

Contd…Solar constant The sun radiates about 56*10^26 calories of energy every minute This energy incidents on a spherical shell with a radius of 1.5*10^13 cm

(the mean distance of the earth from the sun) and concentric with the sun.

This energy per unit area called solar constant (S) = total radiation/ total area of spherical shell

=56*10^26 cal/min 4π (1.5*10^13 cm)^2 =1.979 cal/cm^2/min≈ 2langley/min (1390 watt/m^2)

It fluctuates by 3.5% about its mean value, depending upon the distance of the earth from the sun

Contd…The total solar radiation intercepted by the earth in a

unit time =πr^2Swhere, r is the radius of earth and S is the solar constant

If this spread uniformly over the full surface of the earth the amount received per unit area and per unit time

Qs = πr^2s/4πr^2 =s/4

=0.5 ly/min =263kly/year

Contd…Actually, the distribution is not uniform and the annual

value at the equator is 2.4 times that near the polesThe solar energy incident upon a surface depends upon

theGeographic locationOrientation of the surfaceTime of the dayTime of the year, andAtmospheric conditions

Effect of Atmosphere on Incoming Solar Radiation

the atmosphere acts as a filter, and absorbs and diffuses a significant proportion of incoming solar radiation.

Mathematically,Qs= Cr+Ar +(Q+q)a +Ca+Aa+(Q+q)(1-a)where, Q and q are direct beam

and diffused solar radiation incident on the earth surface and a is albedo

Measurement of Solar Radiation

three properties of solar radiation that are measured

Intensity• Solarimeter• Bimetallic thermograph• Bellani PyranometerDuration• Cambell Stroke’s Sunshine Recorder

Quality• Spectrobolometer

Bellani Pyranometer gives a measure of total solar radiation

the glass sphere used to cover the sensor are made of quartz to insure that all the waves in solar radiation spectrum must reach the sensor

Solarimeter

measures the electric current generated by a series of thermocouple when exposed to solar radiation

is calibrated to give the intensity of solar radiation on the basis of voltage recorded

usually, a milivolt recorder or a digital integrator is connected to obtain, both instantaneous and time records of radiation

BimetallicBimetallic ActinographActinograph provides a measure of solar radiation

by recording temperature difference between black and white coated bimetallic strips.

Cambell Stoke's Sunshine Recorder

the length of the trace burnt on the specially treated card is measured as the duration of bright sunshinethe intensity of radiation required to burn the card is normally 0.2 to 0.4 cal/cm^2-min, below it no record is obtained

Spectrobolometer is used to measure the quality of solar radiation is in fact an arrangement of different instruments

that produces color bands in the solar radiation and provide mechanism to measure the energy

Significance of solar radiation in crop production

• provides light required for seed germination, leaf expansion, growth of stem and shoot, flowering, fruiting and thermal conditions necessary for the physiological functions of the plant.

• plays an important role as regulator and controller of growth and development.

• also influences assimilation of nutrient and dry matter distribution.

Atmospheric TemperatureAtmospheric Temperature Sun is the primary source of heat Heating of the atmosphere is an indirect process The earth surface converts the absorbed insolation into

heat, which in turn, heats the atmosphere by different process of heat transfer.

Process like evaporation and condensation play a significant role in the transfer of heat.

Ambient Temperature is the temperature of surrounding medium of a given location at a particular time

Diurnal Variation The rotation of earth on its axis produces alternating periods

of day and night, resulting the daily variations in temperature

The temperature difference between maximum and minimum during the day is called Diurnal Range of temperature

Daily variation decreses with increase in height above earth surface

Contd…Temperature Lag

Contd… Lapse Rate Temperature Inversion

Seasonal Variation

Measurement of air temperatureMercury / Alcohol Glass

Thermometer Bimetallic Thermograph

Maximum Minimum Thermometer

Significance of Air Temperature

The growth of higher plants is restricted to between 0 to 60 ˚C

But crop plants to a narrow range of 10 t0 40 ˚C Effect of low temperature on plant Effect of high temperature on plant

Soil Temperature Is the most important factor that controls

the intensity of biophysical, biochemical and microbiological process in soil

Heat is supplied to the soil by geothermal activity, atmospheric process and radiative transfer

Factors Affecting Soil Temperature

Amount of radiation received by soilAspect and slope of land

Albedo of soilTextureSoil moisture and air contentOrganic matter on soilRainfall characteristic of the areaFactors affecting rate of evaporation from soil

VegetationTillageMulchingIrrigation

Measurement Mercury/Alcohol in glass thermometer Bimetallic thermometer Electrical resistance thermometer

Significant of soil temperature in crop productionDirect effect in seed germination and root systemEffect in terms of respirationNitrificationOxidation of organic mattersMicro-biological process Indirect effect in growth of plants and its yield

Atmospheric PressureAtmospheric Pressure

Is defined as the force exerted by the air column above any given place

At sea level1034 gm/cm2760mm of Hg

Variation in Air Pressure

Vertical Variation Horizontal Distribution Pressure Gradient

Wind Two types of movement of air in the atmosphere Horizontal movement due to difference in pressure is

called windBalance out the uneven distribution of pressure over

the globeServes as means of transporting heat, moisture, and

other properties from one part of the earth to other Vertical movement of air column is called current

Significant for the formation of clouds, precipitations and various types of storms

Factors Affecting Wind Motion

Horizontal pressure gradient Rotation of the earth Frictional force Centrifugal action of wind

AnemometerWind Vane

Wind Measurement

Control of wind velocity

Wind Breaker Shelter belts

HumidityIs the water vapour content of air at any time and place But does not describe the droplets of fog, cloud or rainMay vary in atmosphere time to timeMoisture holding capacitySaturation

Depends on air temperatureIncreases slowly at lower and rapidly at higher

temperature

Types of HumidityAbsolute Humidity

Is the amount of water vapors in the air

Relative Humidity= actual vapour pressure of airsaturated vapour pressure at that temp

Dew PointThe temperature at which the air sample need to cooled to get saturatedAt dew point, RH is 100%

MeasurementDry and Wet Bulb Thermometer

Whirling or Sling Psychrometer

Hair Hygrometer

Significance of humidity in Agriculture

Plant water balanceIrrigation needsDisease dynamics

Soil MoistureSoil MoistureSoil Is a three state composition of matters; solid, liquid,

and gas The vacuum or free space between the soil solids are

called pores The water holding capacity of soil depends on its

porosityMoisture Content = weight of water (Ww)

weight of solid mass(Ws)

Classification

Soil Moisture Constants Saturation capacity Field capacity Permanent wilting point Temporary wilting point Ultimate wilting point

Precipitation Is the process of transfer of water either solid or liquid

to the earth surface from the atmosphere

Forms of PrecipitationRainSnowHailstonesleet

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