Sustainability & Environment (Agri-Food Systems) · Climate Science & Agriculture · also Agricultural Meteorology, Introductory Agrometeorology & Climate Change (ICAR UG), Agroclimatology
Radiation, heat, moisture and motion in the atmosphere, and how weather and climate are measured and used on farms. ICAR AICE subject 60 Agricultural Meteorology examines it in depth.
In the exams: ICAR AICE-JRF/SRF subject 60 (whole paper); ICAR AIEEA-PG 3.1 Agricultural Meteorology; GATE ES Section 6 (meteorology for dispersion)
Atmospheric science explains how radiation, heat, moisture and motion behave in the air; agrometeorology applies that knowledge to farming. Together they explain why frost forms on clear, calm nights, why a crop's water use doubles on a hot, windy day, why the monsoon arrives when it does, how much sunlight a canopy intercepts, and how weather forecasts can be turned into advice that saves a crop. This manual covers the structure of the atmosphere, the radiation laws, the surface energy balance, humidity and stability, clouds and precipitation, pressure and winds, monsoons and weather systems, climatic classification, instruments and observatories, crop–weather relations, evapotranspiration and agricultural weather forecasting.
It is the foundation for several companion subjects and avoids repeating them. Climate System Science treats the global energy budget, greenhouse effect and climate change; Climate Models, Scenarios & Downscaling treats numerical models, seasonal forecasting and verification in depth; Climate Change Impacts on Agriculture covers heat thresholds, yield responses and crop models; and Climate Adaptation covers agromet advisory services and early warning as adaptation tools. Here the focus is the physical meteorology and the classical agrometeorological methods that underlie all of them.
Agricultural Meteorology is a full paper (subject 60) in the ICAR AICE-JRF/SRF PhD entrance and a sub-subject (3.1) of the ICAR AIEEA-PG Physical Science group, and it is examined in agricultural university courses worldwide. Those papers ask for definitions, laws and their constants, instrument specifications, the classifications of clouds and climates, the Indian monsoon, and numerical problems on radiation, humidity, lapse rates, heat units and evapotranspiration. GATE Environmental Science Section 6 uses the same meteorology for air-pollution dispersion: stability, lapse rates, inversions and plume behaviour. Formulas used here follow FAO Irrigation and Drainage Paper 56 where it applies, and variants are named where they exist.
Dry air near the surface is about 78.08 per cent nitrogen, 20.95 per cent oxygen and 0.93 per cent argon by volume, with carbon dioxide at about 0.04 per cent (over 420 parts per million and rising) and traces of neon, helium, methane, krypton, hydrogen, nitrous oxide and ozone. Water vapour is highly variable, from almost nil in polar air to about 4 per cent in humid tropical air. Aerosols, liquid and solid particles, are also variable and act as condensation nuclei. Below about 80–100 km the major gases are well mixed (the homosphere); above, gases separate by molecular weight (the heterosphere). Pressure and density fall roughly exponentially with height. Half the mass of the atmosphere lies below about 5.5 km, and the scale height, over which pressure falls by a factor of e, is about 8 km. Standard sea-level pressure is 1013.25 hPa. The atmosphere is divided into layers by the way temperature changes with height. The troposphere, from the surface to the tropopause, contains about three quarters of the mass and nearly all the water vapour and weather. Temperature falls with height at an average environmental lapse rate of about 6.5 °C per km. The tropopause is highest over the equator, about 16–18 km, and lowest over the poles, about 8 km, and it is higher in summer than in winter. In the stratosphere, up to about 50 km, temperature is constant at first and then rises with height, because ozone absorbs solar ultraviolet radiation; the ozone concentration peaks at about 20–25 km. The temperature inversion makes the stratosphere very stable, so vertical mixing is slow. In the mesosphere, up to about 80–85 km, temperature falls again to the coldest values in the atmosphere, near −90 °C at the mesopause. In the thermosphere temperature rises steeply as oxygen and nitrogen absorb very short-wave solar radiation, and the exosphere beyond merges into space.
Stratospheric ozone is measured in Dobson units (DU); one DU is a layer of pure ozone 0.01 mm thick at standard temperature and pressure, and the global average column is about 300 DU. The ozone layer absorbs most UV-B and all UV-C radiation, protecting plants, animals and people. Ozone depletion by chlorine and bromine from chlorofluorocarbons and halons, most severe over Antarctica in spring, led to the Montreal Protocol (1987), whose controls have begun to allow recovery. Ground-level ozone, in contrast, is a pollutant that damages crops, as described in the climate impacts subject. The ionosphere, from about 60 km into the thermosphere, contains layers of ions produced by solar radiation that reflect radio waves; it matters for communications, not for weather. For agriculture, the troposphere is where nearly all relevant processes occur, but the lowest few hundred metres, the atmospheric boundary layer, matters most. It is the layer directly influenced by the surface through friction and heating. By day, solar heating creates a deep convective mixed layer, often 1–2 km thick over land; at night, radiative cooling of the ground creates a shallow stable layer with a surface inversion. Within the boundary layer, the surface layer of a few tens of metres has strong gradients of wind, temperature and humidity, and the crop canopy itself creates its own microclimate. Spraying, frost, dew, evapotranspiration and the dispersion of pollutants and pollen are all controlled by boundary-layer conditions.
All bodies above absolute zero emit electromagnetic radiation. A black body is an idealised perfect absorber and emitter. Planck's law gives the energy a black body emits at each wavelength as a function of its temperature; integrating it over all wavelengths and differentiating it lead to the two laws most used in practice. The Stefan–Boltzmann law states that the total energy emitted per unit area by a black body is proportional to the fourth power of its absolute temperature: E = σT⁴, where σ = 5.67 × 10⁻⁸ W m⁻² K⁻⁴. For a real (grey) body, E = εσT⁴, where ε is the emissivity, between 0 and 1. Most natural surfaces, including leaves, soil, water and snow, have longwave emissivities of about 0.95–0.99. Because of the fourth power, a small change in temperature causes a large change in emission: a surface at 300 K emits about 459 W m⁻² as a black body, and one at 273 K about 315 W m⁻². Wien's displacement law states that the wavelength of maximum emission is inversely proportional to absolute temperature: λmax = b/T, with b ≈ 2898 μm K. The Sun, with an effective surface temperature of about 5800 K, emits most strongly near 0.5 μm, in the visible; the Earth, near 288 K, emits most strongly near 10 μm, in the thermal infrared. This is why meteorologists speak of shortwave (solar) and longwave (terrestrial) radiation, which barely overlap. Kirchhoff's law states that, for a body in thermal equilibrium, its emissivity at a given wavelength equals its absorptivity at that wavelength. A good absorber at a wavelength is a good emitter at that wavelength. It explains why snow, which reflects most visible light, is nonetheless nearly a black body in the infrared, and why greenhouse gases that absorb infrared also emit it.
The Beer–Lambert (Bouguer–Lambert–Beer) law describes the exponential weakening of a beam passing through an absorbing or scattering medium: I = I₀ exp(−kx), where I₀ is the incident intensity, k the extinction coefficient and x the path length or amount of absorber. In the atmosphere it describes attenuation of the solar beam along its path, which lengthens as the Sun gets lower (the optical air mass is about 1/cos of the zenith angle for moderate angles). In crop canopies, the same law, with leaf area index (LAI) as the path variable, gives the fraction of light transmitted to the ground: I/I₀ = exp(−k × LAI), so the fraction intercepted is 1 − exp(−k × LAI). The canopy extinction coefficient k is about 0.3–0.5 for erect-leaved crops such as cereals and 0.6–0.8 for horizontal-leaved crops such as soybean or cotton, varying with leaf angle and solar elevation. The inverse-square law states that radiation from a point source decreases with the square of distance. The Earth's orbit is slightly elliptical, so the solar irradiance at the top of the atmosphere varies by about ±3.4 per cent through the year; the mean value, the total solar irradiance or solar constant, is about 1361 W m⁻². Lambert's cosine law states that the irradiance on a surface is proportional to the cosine of the angle between the beam and the normal to the surface. A beam striking a surface at 60° from the vertical delivers half the energy per unit area that it would at normal incidence. This explains why higher latitudes receive less energy, why south-facing slopes in the Northern Hemisphere are warmer and drier, and why row orientation and leaf angle affect light interception. Together with day length, the cosine law determines the daily extraterrestrial radiation (Ra) at each latitude and date, which FAO-56 tabulates and which is used to estimate solar radiation and evapotranspiration.
Solar (shortwave) radiation reaching the ground, called global radiation, has two components: direct beam radiation from the Sun's disc and diffuse radiation scattered by air molecules, aerosols and clouds. On clear days direct radiation dominates; under overcast skies almost all is diffuse. Diffuse light penetrates canopies more evenly and can raise canopy photosynthesis per unit of radiation. About 45–50 per cent of global radiation lies in the photosynthetically active band of 400–700 nm (PAR). When global radiation is not measured, it can be estimated from sunshine duration with the Ångström–Prescott equation: Rs = (as + bs n/N) Ra, where n is actual and N maximum possible sunshine hours and Ra the extraterrestrial radiation. FAO-56 recommends as = 0.25 and bs = 0.50 where no local calibration exists, so a completely cloudless day passes about 75 per cent of Ra and a fully overcast day about 25 per cent. Albedo is the fraction of incoming shortwave radiation reflected. Typical values are 0.80–0.90 for fresh snow, about 0.20–0.25 for green crops (FAO-56 uses 0.23 for its reference grass), 0.10–0.20 for forests, 0.30–0.40 for dry light sand, 0.05–0.15 for dark wet soil, and about 0.06 for water with a high Sun, rising sharply at low sun angles. Net shortwave radiation is (1 − α)Rs. The surface also exchanges longwave radiation: it receives downward longwave from the atmosphere and clouds and emits its own εσT⁴. The difference is net longwave radiation, which is usually a loss, largest on clear dry nights and smallest under low cloud. Net radiation is Rn = (1 − α)Rs + L↓ − L↑. By day Rn is normally positive; at night it is negative, and the surface cools.
Net radiation at the surface is partitioned as Rn = H + LE + G (ignoring small storage and photosynthesis terms), where H is sensible heat flux, which warms the air by conduction and convection; LE is latent heat flux, the energy used to evaporate water, which is released later when the vapour condenses; and G is soil heat flux into the ground. The latent heat of vaporisation is about 2.45 MJ kg⁻¹ at 20 °C, so evaporating 1 mm of water (1 kg m⁻²) uses 2.45 MJ m⁻²; FAO-56 uses the factor 0.408 to convert MJ m⁻² day⁻¹ to mm day⁻¹ of evaporation. The Bowen ratio, β = H/LE, is small (about 0.1–0.3) over well-watered crops and irrigated fields, near 1 over dry grassland, and large (5 or more) over deserts. Over a day G is small for a cropped surface and is often neglected in daily evapotranspiration estimates. Because soil conducts heat slowly, soil temperature waves are damped and delayed with depth: the daily wave becomes negligible at about 30–50 cm, and the annual wave penetrates several metres. Mulches, residues and moisture change soil thermal properties and thus seedbed temperatures. Air temperature near the ground usually peaks in mid-afternoon, about two to three hours after solar noon, because the surface continues to gain energy after peak radiation, and reaches its minimum around sunrise. A temperature inversion is a layer in which temperature increases with height. Radiation (nocturnal) inversions form on clear, calm nights when the ground loses longwave radiation and cools the air above it; they are strongest in valleys where cold air drains and pools. Subsidence inversions form where air sinks and warms adiabatically in high-pressure systems; frontal inversions where warm air overrides cold air; and advection inversions where warm air moves over a cold surface. Inversions suppress vertical mixing, trapping pollutants, smoke and pesticide drift near the ground. Radiation frost occurs on clear calm nights under an inversion; advective frost occurs when a cold air mass moves in with wind. Frost protection methods, such as sprinkler irrigation (latent heat released on freezing), wind machines that mix warmer air from the inversion down to the crop, heaters and covers, exploit these physics; wind machines are effective only against radiation frost with a strong inversion.
The amount of water vapour in air is expressed in several ways. Vapour pressure (e or ea) is the partial pressure of water vapour, in kPa or hPa. Saturation vapour pressure (es) is the maximum vapour pressure at a given temperature over a flat water surface; it rises steeply and non-linearly with temperature, by about 6–7 per cent per °C. FAO-56 uses the Tetens-type equation es(T) = 0.6108 exp[17.27T/(T + 237.3)] kPa, with T in °C, which gives about 2.34 kPa at 20 °C and 4.24 kPa at 30 °C. Because es is non-linear, the daily mean es should be computed as the mean of es at Tmax and es at Tmin, not es at the mean temperature. Relative humidity (RH) is the ratio of actual to saturation vapour pressure, expressed as a percentage: RH = 100 ea/es. RH changes through the day even when the moisture content does not, falling in the afternoon as temperature rises and rising at night. Dew point temperature is the temperature to which air must be cooled at constant pressure and moisture content for saturation; ea = es(Tdew). Specific humidity is the mass of vapour per unit mass of moist air, mixing ratio the mass of vapour per unit mass of dry air, and absolute humidity the mass of vapour per unit volume. The vapour pressure deficit (VPD) is es − ea, the 'drying power' of the air. It drives transpiration and evaporation together with radiation and wind, and high VPD closes stomata in many crops. It is more directly related to plant water stress than RH is, because the same RH represents a much larger deficit at high temperature. Humidity is measured with psychrometers (wet- and dry-bulb thermometers), capacitive or resistive electronic sensors, hair hygrometers and dew-point hygrometers. With a psychrometer, ea = es(Twet) − γpsy(Tdry − Twet), where γpsy = apsy × P; FAO-56 gives apsy ≈ 0.000662 °C⁻¹ for ventilated (Assmann-type) psychrometers, about 0.000800 for naturally ventilated ones in a screen and about 0.0012 for non-ventilated ones indoors. The psychrometric constant γ used in evapotranspiration equations is about 0.067 kPa °C⁻¹ at sea level (0.665 × 10⁻³ P).
When a parcel of air rises it expands because pressure falls, and it cools without exchanging heat with its surroundings, an adiabatic process; sinking air is compressed and warms. Unsaturated air cools at the dry adiabatic lapse rate (DALR), about 9.8 °C per km. Once a rising parcel reaches saturation, condensation releases latent heat, so it cools more slowly at the saturated (moist) adiabatic lapse rate (SALR), about 4–7 °C per km depending on temperature, smaller in warm humid air. The height at which a lifted parcel becomes saturated, the lifting condensation level, can be estimated as about 125 m for each degree of difference between air temperature and dew point at the surface. Potential temperature, θ = T(1000/p)^0.286 with p in hPa, is the temperature air would have if brought dry-adiabatically to 1000 hPa; it is conserved in dry adiabatic motion. Stability compares the environmental lapse rate (ELR), the actual temperature profile, with the adiabatic rates. If the ELR is less than the SALR, the air is absolutely stable: a displaced parcel is colder than its surroundings when lifted and returns. If the ELR exceeds the DALR, the air is absolutely unstable. If the ELR lies between the two, the air is conditionally unstable: stable for unsaturated parcels but unstable once they are saturated, the common condition that allows thunderstorms to develop when moist air is lifted. An inversion (negative lapse rate) is extremely stable. The foehn (chinook) effect follows from these rates: air rising over a mountain cools at the SALR after condensing and dropping its rain on the windward side, then descends the leeward side warming at the DALR, arriving warmer and drier than it started. This produces rain shadows and hot, dry downslope winds. For air-pollution dispersion, stability is often classified with the Pasquill–Gifford classes A (very unstable) to F (moderately stable), determined from surface wind speed and daytime insolation or night-time cloud cover; D is neutral, typical of overcast or windy conditions. Plume shapes reflect stability: looping in unstable air, coning in neutral air, fanning in stable air, lofting when the plume is above a surface inversion, and fumigation when a morning inversion breaks up from below and brings a plume down to the ground.
Clouds form when air is cooled to saturation, most often by lifting, and vapour condenses on cloud condensation nuclei such as sea salt, sulphate and dust. The international classification published in the WMO International Cloud Atlas recognises ten genera, grouped by height of base. High clouds (bases typically above about 6 km in mid-latitudes) are cirrus (Ci), cirrocumulus (Cc) and cirrostratus (Cs), made of ice crystals; cirrostratus produces halos. Middle clouds (about 2–6 km) are altocumulus (Ac) and altostratus (As); nimbostratus (Ns), the thick grey layer cloud that gives prolonged steady rain or snow, usually has its base in the low or middle level and extends through great depth. Low clouds (base below about 2 km) are stratocumulus (Sc) and stratus (St), which may give drizzle. Clouds of vertical development are cumulus (Cu), the fair-weather heaped cloud, and cumulonimbus (Cb), the thunderstorm cloud with an anvil top, which produces heavy showers, hail, lightning and squalls. Height ranges are lower in polar regions and higher in the tropics. Cloud amount is reported in oktas (eighths of the sky covered). Fog is a cloud at the ground. Radiation fog forms on clear, calm nights as the ground cools air near it to the dew point, common in winter in the Indo-Gangetic Plains; advection fog forms when moist air moves over a colder surface; and upslope fog forms as air rises along terrain. Dew forms when surfaces cool below the dew point, and frost when they cool below freezing and vapour deposits as ice (hoar frost).
Cloud droplets, typically about 10–20 μm across, are far too small to fall as rain; a raindrop of about 2 mm contains roughly a million cloud droplets. Two processes grow precipitation particles. In the ice-crystal (Bergeron–Findeisen) process, which operates in clouds with temperatures below freezing containing both supercooled droplets and ice crystals, ice crystals grow at the expense of droplets because the saturation vapour pressure over ice is lower than over water at the same temperature; the crystals grow, fall, and melt into rain if they pass through warm air. In the collision–coalescence process, dominant in warm tropical clouds, larger droplets fall faster than small ones and collect them. Both operate in many clouds. Precipitation is classified by the lifting mechanism. Convectional precipitation comes from vertical currents in heated air, giving showers and thunderstorms, typical of tropical afternoons and summer. Orographic precipitation results from air forced up mountains; windward slopes are wet and leeward slopes lie in a rain shadow, as on the Western Ghats, where the windward coast receives very heavy monsoon rain while the Deccan plateau to the east is much drier. Cyclonic or frontal precipitation results from large-scale lifting in depressions and along fronts. Forms of precipitation include rain (drops larger than 0.5 mm), drizzle (fine drops smaller than 0.5 mm from stratus), snow (ice crystals and aggregates), sleet (a term used differently in different countries, for a mixture of rain and snow or for ice pellets), hail (layered ice balls from cumulonimbus with strong updrafts), graupel (soft hail) and freezing rain. Hail damages crops severely in a few minutes; its protection includes nets in orchards and crop insurance. Rainfall is described by its amount, intensity, duration and frequency. Intensity matters for erosion and infiltration; effective rainfall is the part that is stored in the root zone and used by the crop. Cloud seeding attempts to enhance precipitation by adding ice nuclei, such as silver iodide, to supercooled clouds or hygroscopic particles to warm clouds; its effectiveness is difficult to demonstrate statistically and remains contested for operational water supply.
Wind is air moving in response to forces. The pressure gradient force pushes air from high to low pressure and is proportional to the pressure difference divided by distance; isobars close together mean strong winds. The Coriolis force, an apparent force due to the Earth's rotation, deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere; its magnitude per unit mass is fV, where the Coriolis parameter f = 2Ω sin φ, Ω = 7.292 × 10⁻⁵ s⁻¹ and φ is latitude. It is zero at the equator, which is why tropical cyclones rarely form within about 5° of it, and maximum at the poles. Friction near the surface slows wind and so weakens the Coriolis force, causing surface winds to blow across isobars towards low pressure at an angle, larger over land than over sea. Above the friction layer, where the pressure gradient and Coriolis forces balance, the wind blows parallel to the isobars as the geostrophic wind. Buys Ballot's law follows: in the Northern Hemisphere, if you stand with your back to the wind, low pressure is on your left. Winds circulate anticlockwise around lows (cyclones) and clockwise around highs (anticyclones) in the Northern Hemisphere, and the reverse in the Southern. Surface air converges and rises in lows, giving cloud and rain, and sinks and diverges in highs, giving clear, settled weather. Local winds arise from local temperature contrasts. A sea breeze blows from sea to land by day as the land heats faster, and a land breeze from land to sea at night. In mountains, anabatic (upslope, valley) winds blow by day and katabatic (downslope, mountain) winds by night, draining cold air into valleys where frost pockets form. Wind speed is measured with cup or sonic anemometers and reported in m s⁻¹, km h⁻¹ or knots; the Beaufort scale describes wind force from 0 (calm) to 12 (hurricane force) by its observed effects.
Unequal heating between the tropics and poles drives the general circulation of the atmosphere, which carries heat poleward. The classical three-cell model describes a Hadley cell in each hemisphere between the equator and about 30° latitude: air rises in the equatorial low-pressure trough, the Intertropical Convergence Zone (ITCZ), with heavy convective rainfall; flows poleward aloft; sinks around 30° in the subtropical high-pressure belts (the 'horse latitudes'), where many of the world's deserts lie; and returns towards the equator at the surface as the trade winds, north-easterly in the Northern Hemisphere and south-easterly in the Southern. Between about 30° and 60° the Ferrel cell, a thermally indirect circulation driven by eddies, is associated with the surface westerlies, in which travelling mid-latitude cyclones and anticyclones dominate the weather. Between about 60° and the poles, the polar cell has sinking air at the pole, the polar easterlies at the surface and rising air at the subpolar low near the polar front. Jet streams are narrow bands of strong westerly winds near the tropopause: the subtropical jet at the poleward edge of the Hadley cell and the polar-front jet above the zone of strong temperature contrast. Their positions shift with the seasons and steer surface weather systems. In South Asia the subtropical westerly jet lies south of the Himalaya in winter and shifts north of the Tibetan Plateau before the monsoon onset, while a Tropical Easterly Jet develops over the Indian region in summer. The whole pattern migrates north and south with the Sun, lagging by one to two months. The seasonal migration of the ITCZ and the subtropical highs creates the wet and dry seasons of the tropics and the summer-dry Mediterranean climates on the western sides of continents near 30–40° latitude. Ocean currents, land–sea contrasts and mountains break the zonal belts into semi-permanent centres of action, such as the Azores and Pacific highs and the Icelandic and Aleutian lows.
A monsoon is a seasonal reversal of the prevailing wind accompanied by a marked change in rainfall. The basic driver is the difference in heating between land and ocean, amplified by the seasonal migration of the ITCZ and, in Asia, by the elevated heating of the Tibetan Plateau. In the South Asian summer monsoon, the land heats strongly in April and May, a heat low develops over north-west India and Pakistan, and moist south-westerly winds flow in from the Indian Ocean. A strong low-level cross-equatorial jet off the East African coast, the Somali or Findlater jet, carries much of this moisture. The monsoon trough, extending from north-west India to the head of the Bay of Bengal, and low-pressure systems and depressions forming over the Bay of Bengal and moving west-north-west bring much of the rain to central and northern India. The India Meteorological Department's normal date of onset over Kerala is 1 June, with a standard deviation of about a week. The monsoon then advances north-west and normally covers the whole country by about 8 July; withdrawal begins from north-west India in mid to late September and is complete by mid-October. These normal dates were revised in 2020 on the basis of recent data. Within the season, active spells alternate with breaks, when the monsoon trough shifts to the Himalayan foothills and rainfall over central India falls, a pattern linked to sub-seasonal oscillations. The north-east monsoon (October–December) brings the main rainy season to Tamil Nadu and adjoining areas. Monsoon rainfall varies from year to year, influenced by ENSO (El Niño years tend to be drier over India, though not always), the Indian Ocean Dipole and Eurasian snow cover. For agriculture, onset date, total, distribution, dry spells and withdrawal all matter; seasonal forecasting is covered in the climate models subject. Other monsoon systems include the West African monsoon, the East Asian monsoon and the Australian and North American monsoons.
An air mass is a large body of air with fairly uniform temperature and humidity at each level, acquired over a source region. Air masses are classified as continental (c) or maritime (m) and as arctic (A), polar (P) or tropical (T): for example cP (cold, dry), mP (cool, moist), cT (hot, dry) and mT (warm, moist). As an air mass moves, it is modified by the surface beneath. A front is the boundary zone between two air masses of different density. At a cold front, cold air advances and undercuts warm air, lifting it steeply; it brings a narrow band of cumulonimbus, heavy showers, a sharp temperature drop and a wind shift. At a warm front, warm air rises gently over retreating cold air, producing a broad sequence of cirrus, cirrostratus, altostratus and nimbostratus with prolonged light to moderate rain ahead of the front. An occluded front forms when the faster cold front overtakes the warm front, and a stationary front shows little movement. Extratropical (mid-latitude) cyclones form on the polar front and follow the life cycle described by the Norwegian school: a wave develops, warm and cold fronts form, the system deepens, occludes and decays. They bring most of the rain to mid-latitudes. In South Asia, western disturbances, extratropical systems travelling eastward from the Mediterranean region, bring winter rain and snow to north-west India and the Himalaya, which are important for rabi crops such as wheat, although intense ones can cause hail and lodging. Tropical cyclones are intense, warm-core low-pressure systems that form over warm oceans. Formation generally needs sea surface temperatures of about 26.5 °C or more through a deep layer, enough Coriolis force (so they form at least about 5° from the equator), high humidity in the middle troposphere, low vertical wind shear and a pre-existing disturbance. They are called hurricanes in the North Atlantic and north-east Pacific, typhoons in the north-west Pacific and cyclones in the Indian Ocean and South Pacific. Their hazards are wind, storm surge, heavy rain and flooding, as covered in the adaptation subject. In the North Indian Ocean, cyclone frequency peaks before and after the summer monsoon, in May and in October–November.
Climatic classification groups places with similar climates. The most widely used scheme was developed by Wladimir Köppen in the late nineteenth and early twentieth centuries and later refined with Rudolf Geiger (the Köppen–Geiger classification). It uses monthly mean temperature and precipitation and was designed to match the distribution of natural vegetation. Five main groups are defined. A (tropical): every month has a mean temperature of 18 °C or more. B (dry): annual precipitation is below a threshold that depends on mean annual temperature and the season of rainfall, so evaporation exceeds precipitation; BW is desert (arid) and BS steppe (semi-arid). C (temperate): the coldest month averages between 0 °C (or −3 °C in Köppen's original) and 18 °C. D (continental or cold): the coldest month is below 0 °C (or −3 °C) and the warmest above 10 °C. E (polar): the warmest month is below 10 °C, with ET (tundra) and EF (ice cap). Second letters describe the precipitation regime: f (no dry season), m (monsoon, with a short dry season compensated by heavy rains), w (dry winter) and s (dry summer). Third letters describe temperature, such as a (hot summer) and b (warm summer) for C and D climates, and h (hot) and k (cold) for B climates. Thus Am is tropical monsoon, as on the Kerala coast; Aw tropical savanna, covering much of peninsular India; BWh hot desert, as in western Rajasthan; Cwa humid subtropical with dry winters, as over much of the Indo-Gangetic Plains; and Csa Mediterranean. Updated high-resolution Köppen–Geiger maps exist for present and projected climates. The scheme is simple and widely understood, but its thresholds are empirical, it uses temperature and precipitation only, and it does not directly represent water balance or the needs of particular crops.
C.W. Thornthwaite's classification (1931, revised substantially in 1948) emphasised the water balance. Its central idea is potential evapotranspiration (PE), the water that would be lost by evaporation and transpiration from a vegetated surface with ample water, which he estimated from mean monthly temperature and day length. Comparing monthly precipitation with PE, with soil-water storage, gives monthly water surplus and deficit. From these Thornthwaite derived a humidity index, an aridity index and a moisture index (Im) that classify climates from perhumid through humid, subhumid and semi-arid to arid; positive Im indicates a humid climate with water surplus and negative Im a dry one. Several formulations of the moisture index exist, notably the 1948 version that weights the aridity index by 0.6 and the later Thornthwaite–Mather (1955) version, so boundary values must be read with the formula used. He also used a thermal efficiency index based on PE to classify temperature regimes, from megathermal to frost climates. The Thornthwaite–Mather water-balance bookkeeping remains widely used in agroclimatology to estimate soil-moisture regimes, the length of growing period and irrigation needs. FAO's agro-ecological zones methodology defines the length of growing period as the period when precipitation exceeds half of potential evapotranspiration, plus the time to use stored soil moisture, and uses it with temperature to match crops to land. Countries use agroclimatic zoning for planning. India's Planning Commission divided the country into 15 agro-climatic regions in the late 1980s, and the ICAR National Bureau of Soil Survey and Land Use Planning delineated 20 agro-ecological regions based on physiography, soils, bioclimate and length of growing period; the National Agricultural Research Project defined agro-climatic zones within states. Similar zonations exist elsewhere, for example the agro-ecological zones used in many African countries. Such zoning underpins research station networks, district contingency plans and the targeting of crop varieties.
Meteorological observations follow WMO standards so that data are comparable. Air temperature and humidity are measured in a ventilated shelter, the Stevenson screen, a white louvred wooden box that shields instruments from direct and reflected radiation while allowing air to flow; WMO recommends sensors at 1.25–2 m above ground over short grass. Inside are dry- and wet-bulb thermometers (forming a psychrometer), a maximum thermometer (mercury, with a constriction that stops the column returning) and a minimum thermometer (alcohol, with a glass index dragged down by the meniscus), and often a thermohygrograph recording on a chart. Grass minimum and soil thermometers at depths such as 5, 10, 20 and 30 cm record surface and soil temperatures. Rainfall is measured with a standard non-recording rain gauge, a cylindrical collector with a funnel and measuring glass, and with recording gauges: the siphon type, which records on a rotating drum, and the tipping-bucket type, which records each small fixed amount (commonly 0.2 or 0.5 mm) electronically. Gauges should be sited away from obstructions, at a distance of at least twice the height of nearby objects, with the rim level at a standard height. Tipping-bucket gauges undercatch in very intense rain, and all gauges undercatch in strong wind. Evaporation is measured with the US Class A pan, a circular galvanised pan 120.7 cm in diameter and 25 cm deep, set on a wooden frame 15 cm above the ground and filled to 5 cm below the rim; daily water loss, corrected for rain, gives pan evaporation. Wind speed is measured with cup anemometers, ideally at 10 m (the WMO standard) or at 2 m in agrometeorological stations, and direction with a wind vane. Sunshine duration is recorded with the Campbell–Stokes recorder, in which a glass sphere focuses sunlight to burn a trace on a card. Radiation is measured with pyranometers (global or diffuse radiation on a horizontal surface), pyrheliometers (direct beam, tracking the Sun) and net radiometers. Pressure is measured with mercury (Fortin) or aneroid barometers and recorded with barographs. A dew gauge measures dew deposition.
Agrometeorological observatories combine the standard meteorological observations with those of special agricultural interest, such as soil temperature and moisture, evaporation, leaf wetness, and crop phenology. Observations are made at fixed times so that data are comparable; national services set their observation schedules and observatory classes. Site selection matters: an observatory should be on level ground with short grass, representative of the surrounding area and free from obstructions, irrigation effects and urban heat. Changes of site, instrument or observation time introduce inhomogeneities into long records, which must be recorded in station metadata and corrected before trend analysis. Automatic weather stations (AWS) use electronic sensors (thermistors or platinum resistance thermometers, capacitive humidity sensors, tipping-bucket rain gauges, cup or sonic anemometers, pyranometers) connected to a data logger, with power from batteries and solar panels and data transmitted by mobile network or satellite. They record at short intervals, operate in remote areas, and provide the dense data needed for block-level advisories, crop insurance indices and disease forecasting. Their weaknesses are sensor drift, calibration needs, vulnerability to power failures, insects, dust and vandalism, and gaps if maintenance lapses. Data from AWS and manual stations should be compared during transition periods. Beyond ground stations, weather radars detect precipitation intensity and storm structure; radiosondes carried by balloons measure temperature, humidity, pressure and wind through the atmosphere, usually twice daily at standard times; and weather satellites, geostationary (such as the INSAT series over India) and polar-orbiting, observe clouds, temperature, moisture and vegetation. Eddy-covariance flux towers measure the turbulent exchange of heat, water vapour and carbon dioxide between crops and the atmosphere and are used to validate evapotranspiration estimates.
Crop development is governed mainly by temperature and, in sensitive crops, by day length. Growing degree days, GDD = Σ[(Tmax + Tmin)/2 − Tbase], accumulate thermal time above a base temperature, as explained in the climate impacts subject, which also treats cardinal temperatures and heat thresholds. Agrometeorologists use several related indices. Photothermal units (PTU) multiply daily GDD by day length (hours), to account for photoperiod effects. Heliothermal units (HTU) multiply daily GDD by actual bright sunshine hours, reflecting both temperature and radiation. Heat use efficiency (HUE) is the dry matter or grain yield produced per degree day (for example kg ha⁻¹ per °C day) and is used to compare sowing dates and cultivars: a sowing date that exposes a crop to heat during grain filling typically lowers HUE. Photoperiodism is the response of flowering to day length. Short-day plants, such as rice (many traditional cultivars), soybean, sorghum, cotton (many cultivars) and tobacco, flower when days are shorter than a critical length; long-day plants, such as wheat, barley, oats, mustard and lentil, flower when days exceed a critical length; day-neutral plants, such as many modern rice and wheat cultivars, tomato and cucumber, flower independently of day length. In fact many crops are quantitative rather than absolute in their response: flowering is hastened or delayed rather than prevented. Photoperiod insensitivity was a key trait of Green Revolution varieties, allowing them to be grown across latitudes and seasons. Vernalisation is the requirement of some crops, such as winter wheat and some brassicas, for a period of low temperature before they can flower. Light also matters through intensity and quality. Crops differ in radiation use efficiency: C4 crops convert intercepted radiation to biomass more efficiently than C3 crops under high light and temperature. Shade-tolerant crops suit intercropping and agroforestry.
Thermal stress includes heat stress, chilling injury and freezing injury. Heat stress at sensitive stages, especially flowering and grain filling, causes sterility and reduced grain weight; the thresholds and mechanisms are described in the climate impacts subject. Chilling injury occurs at low temperatures above freezing, typically below about 10–15 °C, in crops of tropical origin such as rice, maize, cotton, banana and many vegetables; in rice, cold at the booting and flowering stages causes spikelet sterility, a problem in high-altitude and high-latitude rice areas. Freezing injury occurs when ice forms within tissues; its severity depends on minimum temperature, duration and the crop's hardening. Crop microclimate, the climate within and just above the canopy, differs from screen-level weather. By day, canopy temperatures can be above or below air temperature depending on transpiration; by night, radiative cooling makes plant surfaces colder than the air, so frost can form on leaves when screen temperature is still above 0 °C. Wind breaks reduce wind speed for a distance of about 10–20 times their height downwind, cutting evaporation and mechanical damage. Mulches change soil temperature: straw mulch cools soil in summer, while clear plastic mulch warms it. Shade nets, greenhouses and polyhouses modify radiation, temperature and humidity for high-value crops. Irrigation cools canopies through evaporation and can be timed to protect against frost. Weather also controls pests and diseases through temperature, humidity and leaf wetness, and disease forecasting models use these variables; leaf-wetness duration is a key input for fungal diseases such as late blight of potato and apple scab. Weather influences field operations too: workable days for tillage and harvest, spraying conditions (wind below about 3–4 m s⁻¹ and no inversion or imminent rain), and drying of harvested produce. Agrometeorology turns these relations into calendars and advisories, discussed in the last chapter.
Evapotranspiration (ET) combines evaporation from soil and wet surfaces with transpiration from plants. FAO Irrigation and Drainage Paper 56 (Allen and colleagues, 1998) standardised its estimation through reference evapotranspiration (ETo), the ET rate from a hypothetical reference grass surface 0.12 m tall, with a fixed surface resistance of 70 s m⁻¹ and an albedo of 0.23, actively growing and well watered. ETo depends only on weather. The FAO Penman–Monteith equation for daily ETo is ETo = [0.408 Δ (Rn − G) + γ (900/(T + 273)) u₂ (es − ea)] / [Δ + γ (1 + 0.34 u₂)], where ETo is in mm day⁻¹, Rn and G are net radiation and soil heat flux (MJ m⁻² day⁻¹), T is mean daily air temperature at 2 m (°C), u₂ wind speed at 2 m (m s⁻¹), es − ea the vapour pressure deficit (kPa), Δ the slope of the saturation vapour pressure curve (kPa °C⁻¹) and γ the psychrometric constant (kPa °C⁻¹). The first term represents the radiation (energy) control and the second the aerodynamic control of evaporation. FAO-56 gives procedures for estimating missing data, such as Rs from sunshine hours and ea from Tmin, and for converting wind measured at height z to 2 m with u₂ = uz × 4.87/ln(67.8z − 5.42). Where data are limited, simpler methods are used. The Hargreaves–Samani equation, ETo = 0.0023 (Tmean + 17.8)(Tmax − Tmin)^0.5 Ra, with Ra expressed as equivalent evaporation in mm day⁻¹, needs only temperature and extraterrestrial radiation and is recommended by FAO-56 as a fallback. The pan method estimates ETo = Kp × Epan, where the pan coefficient Kp, typically about 0.35–0.85, depends on pan siting (fetch of green crop or dry fallow around it), wind and humidity. Older temperature-based methods, Thornthwaite and Blaney–Criddle, are less reliable outside the climates for which they were developed, especially in arid and windy areas where they underestimate demand.
Crop evapotranspiration under standard conditions is ETc = Kc × ETo, where the crop coefficient Kc integrates the differences between the crop and the reference grass in height, albedo, canopy resistance and soil evaporation. Kc follows the crop's growth stages: a low initial value (Kc ini), dominated by soil evaporation and depending on wetting frequency; a rise through the development stage; a peak mid-season value (Kc mid), around 1.0–1.2 for most full-cover field crops; and a late-season value (Kc end) that falls as the crop senesces, except for crops harvested fresh. FAO-56 tabulates typical values and stage lengths and adjusts Kc mid and Kc end for wind and humidity. The dual crop coefficient approach separates Kc into a basal coefficient for transpiration (Kcb) and a soil evaporation coefficient (Ke), Kc = Kcb + Ke, and is more accurate for daily scheduling with frequent wetting. Under water stress, actual ET falls below ETc; FAO-56 uses a water stress coefficient, Ks, so that ETc adj = Ks × Kc × ETo, with Ks falling below 1 when soil water depletion exceeds the readily available water, a fraction p of total available water. The yield consequences are described by the FAO-33 yield response factor, covered in the climate impacts subject. The net irrigation requirement is crop evapotranspiration minus effective rainfall and any change in soil water, and the gross requirement divides this by application efficiency. Irrigation scheduling decides when and how much to irrigate, using soil-water balance calculations with daily ETo, soil-moisture sensors, the ratio of irrigation water to cumulative pan evaporation (IW/CPE, widely used in India) or canopy temperature. Remote sensing estimates actual ET over large areas using surface energy balance models driven by thermal imagery, and eddy-covariance and lysimeter measurements provide ground truth. Weighing lysimeters, which measure the change in weight of an isolated soil block with its crop, are the reference method for measuring ET directly.
Weather forecasts are classified by lead time. Common definitions, such as those used by the India Meteorological Department, are nowcasts (the next few hours), short-range forecasts (up to about three days), medium-range forecasts (three to ten days), extended-range forecasts (ten to thirty days) and long-range or seasonal forecasts (a month to a season). Each supports different farm decisions: nowcasts for spraying and harvest operations, short and medium range for irrigation, fertiliser and pesticide timing and sowing, extended range for planning field operations, and seasonal outlooks for crop and variety choice and input planning. Modern forecasting is based on numerical weather prediction (NWP): computer models solve the equations of atmospheric motion and physics forward in time from an analysis of the current state built by data assimilation of observations from surface stations, radiosondes, aircraft, radars and satellites. Global models from centres such as ECMWF, NCEP and, in India, NCMRWF and IMD provide medium-range guidance; regional high-resolution models add detail. Ensemble prediction systems run a model many times from slightly perturbed initial conditions to estimate forecast uncertainty and give probabilities. Forecasters combine model output with observations, local knowledge and statistical post-processing to produce district- or block-level forecasts. Seasonal forecasting and its verification are developed further in the climate models subject. Forecasts are verified against observations. For yes/no events such as rain, a 2 × 2 contingency table counts hits (a), false alarms (b), misses (c) and correct negatives (d). Common scores are the probability of detection, POD = a/(a + c); the false alarm ratio, FAR = b/(a + b); the proportion correct (accuracy), (a + d)/n; and skill scores, such as the Heidke skill score, that compare accuracy with chance. Accuracy alone can mislead for rare events, because always forecasting 'no rain' in a dry season scores highly while having no value.
An agrometeorological advisory translates weather forecasts into crop- and location-specific recommendations. A typical bulletin contains the past week's weather, the forecast for the next few days (rainfall, maximum and minimum temperature, humidity, wind speed and direction, cloud cover), and advice on sowing, irrigation, fertiliser application, plant protection, harvest and post-harvest operations, livestock and fisheries. Advice is prepared by agrometeorologists with crop scientists, pest and disease specialists and extension staff, drawing on crop calendars, thresholds and disease forecasting models. Examples of weather-based advice include postponing irrigation or fertiliser top-dressing when heavy rain is forecast, to avoid waterlogging and nutrient loss; avoiding pesticide sprays before rain or in wind; advancing harvest of mature crops before a cyclone or heavy rain; light irrigation or smoke before a forecast frost night; and preventive sprays when forecast humidity and temperature favour a disease. Value depends on accuracy, timeliness, local relevance and farmers' ability to act; information about forecast uncertainty helps farmers judge risk. In India, the Gramin Krishi Mausam Sewa issues district- and block-level forecasts and advisories through agromet field units at agricultural universities and ICAR institutes and district agromet units at Krishi Vigyan Kendras, delivered by text messages, mobile apps, radio, television and newspapers; the service and its role in adaptation are discussed further in the adaptation subject. Other countries operate similar services through national meteorological agencies, often with the WMO Commission for Agricultural Meteorology's guidance (the WMO Guide to Agricultural Meteorological Practices). Evaluations of agromet services generally report benefits where advice is followed, through savings on irrigation and plant protection and avoided losses, but the size of reported benefits varies widely between studies and methods. Building two-way communication, feedback on forecast performance and attention to women farmers and smallholders increases the reach and value of advisories.
A student uses this subject to master the agrometeorology paper's laws, constants and numericals: Wien's and Stefan–Boltzmann laws, net radiation and Bowen ratio, saturation vapour pressure, relative humidity, dew point and VPD from psychrometer readings, lifting condensation level and stability, Köppen letters, Class A pan and other instrument specifications, GDD, PTU, HTU and HUE, and ETo by Penman–Monteith, Hargreaves and pan methods, followed by ETc = Kc × ETo. Distractors in exams often come from mixing up DALR and SALR, short- and long-day crops, or using es at mean temperature.
A five-year practitioner, such as an agromet field unit scientist, an irrigation engineer or an extension specialist, uses the subject to run and maintain an observatory or AWS network, quality-check data, compute ETo and crop water requirements for irrigation schedules, prepare weekly district advisories from medium-range forecasts, issue frost and heat alerts, and advise on spraying windows and inversion risk. Typical decisions are whether to recommend irrigation before a forecast hot spell and how to word an advisory when forecast uncertainty is high.
A senior professional with 25 years' experience, such as a head of a state agromet service, a research station director or a water resources planner, uses the subject to design observation networks, set data standards and homogenisation procedures, oversee crop-weather calendars and contingency plans, evaluate the economic value of advisory services, and choose ET methods and crop coefficients for regional irrigation planning. Such a person ensures that instrument changes and station moves do not corrupt long records used for climate analysis.
A researcher with a PhD and 25 years' experience works on boundary-layer and canopy micrometeorology, eddy-covariance and remote-sensing estimation of evapotranspiration, crop–weather modelling, monsoon variability and its agricultural predictability, frost and heat forecasting, and impact-based advisories. Such a researcher develops locally calibrated crop coefficients and Ångström constants, mentors observatory and modelling teams, and connects classical agrometeorology to climate change science and services.
This manual covers classical and physical meteorology at the level of agrometeorology courses; it does not derive the equations of atmospheric dynamics, radiative transfer through the atmosphere or boundary-layer similarity theory. National observation schedules, observatory classes and instrument specifications other than the Class A pan and WMO siting standards differ between countries and are not listed. Thornthwaite moisture-index class boundaries are not given because they differ between the 1948 and 1955 versions. Values such as albedos, extinction coefficients, pan coefficients and crop coefficients are typical ranges that vary with conditions and should be taken from FAO-56 tables or local calibration. Numerical weather prediction, seasonal forecasting and verification beyond the contingency table are left to the climate models subject; agromet advisory services as adaptation tools are covered in the adaptation subject.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions.
These notes are free to read here, with no account.
What the Laisha app adds:
Free for ten days, then a subscription. We would rather write that here than have you find it later.