Weather elements and agro-meteorological instruments
Weather elements that matter to crops and the instruments that measure them: Stevenson screen, rain gauge, Class A pan, anemometer, sunshine recorder.
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Weather decides when to sow, irrigate, spray and harvest, and much of how much a crop yields. A farmer judging whether to spray before rain, an advisory service issuing a frost warning, and a scientist working out a crop's water need all depend on measurements of the same handful of weather elements: temperature, humidity, rainfall, wind, radiation, sunshine, evaporation and air pressure.
Agricultural meteorology studies these elements in relation to crops, livestock and farming operations. In India, the India Meteorological Department (IMD) and the All India Coordinated Research Project on Agrometeorology run networks of agro-meteorological observatories. Their data feed the district-level Agromet Advisory Services bulletins sent to farmers.
Weather and climate
Weather is the state of the atmosphere at a given place and time, from hour to hour and day to day. Climate is the long-term pattern of weather, usually described by averages and extremes over at least 30 years. A single cold spell is weather; the number of frost days a district expects each winter is climate.
The observatory
An agro-meteorological observatory is a level, open, grassed plot, typically 55 m × 36 m for a full station, away from buildings and trees. Instruments are set out so that none shades or shelters another. Observations are taken at fixed times. In India, the main readings are at 0830 and 1730 IST, which correspond to 0300 and 1200 UTC.
Elements and instruments
Air temperature
Air temperature is measured in a Stevenson screen. This is a white, louvred wooden box on a stand, with its door opening away from the sun (facing north in the northern hemisphere). It shields thermometers from direct and reflected sunshine and from rain, while letting air flow freely. Inside are:
- a maximum thermometer (mercury, with a constriction) and a minimum thermometer (alcohol, with an index), which give the day's extremes;
- dry-bulb and wet-bulb thermometers for humidity;
- often a thermograph for a continuous record.
Grass minimum thermometers just above short grass record the lowest night temperature at plant level, which is useful for frost warnings. Soil thermometers at depths such as 5, 10, 20 and 30 cm show conditions for germination and roots.
Humidity
The psychrometer compares dry-bulb and wet-bulb readings. Evaporation cools the wet bulb, and the drier the air, the bigger the wet-bulb depression. Psychrometric tables convert the readings to:
- relative humidity (RH), the amount of water vapour as a percentage of what the air could hold at that temperature;
- dew point, the temperature at which the air would become saturated;
- vapour pressure and the vapour pressure deficit, which drives transpiration.
A hygrograph, traditionally using human hair, records RH continuously.
Rainfall
The ordinary (non-recording) rain gauge collects rain through a funnel of standard area into a bottle. The IMD standard gauge has a 200 cm2 collector. The rim is level and set about 30 cm above the ground to avoid splash. The collected water is measured with a graduated cylinder, and readings are in millimetres. Self-recording gauges, such as siphon and tipping-bucket types, record intensity and timing, which matter for erosion and runoff.
India uses a threshold of 2.5 mm in 24 hours to define a rainy day.
Evaporation
The USWB Class A pan evaporimeter is a galvanised iron pan 121 cm in diameter and 25.5 cm deep. It stands on a wooden frame about 15 cm above the ground and is filled to about 5 cm below the rim, often covered with wire mesh to keep birds out. The daily fall in water level, corrected for rain, gives pan evaporation (Epan). Pan evaporation is widely used to estimate crop water use.
Scheduling irrigation from pan data
Over a week, pan evaporation at a station averages 8 mm per day. With a pan coefficient of 0.75, ET0 = 0.75 × 8 = 6.0 mm per day. A wheat crop at heading has Kc ≈ 1.15, so ETc = 6.9 mm per day, or about 48 mm in the week. If the soil can supply 50 mm of readily available water, the crop needs irrigation about once a week. The IW/CPE approach uses the same idea: irrigate when cumulative pan evaporation reaches a set value.
Wind
The cup anemometer has three or four cups rotating on a vertical axis. It measures wind speed in km per hour or m per second. Standard height is 10 m, though agro-meteorological stations often use 2–3 m, which is closer to crop height. A wind vane shows direction, the direction the wind is blowing from. Wind affects evaporation, spray drift, lodging, pollination and the spread of pests.
Sunshine and radiation
- The Campbell–Stokes sunshine recorder is a glass sphere that focuses sunlight to burn a trace on a graduated card. The length of the burn gives the hours of bright sunshine.
- A pyranometer measures global solar radiation (direct plus diffuse) in W per m2 or MJ per m2 per day. A net radiometer measures incoming minus outgoing radiation, which is the energy available for evaporation and heating.
- PAR sensors (quantum sensors) measure photosynthetically active radiation (400–700 nm), which is used in crop growth studies.
Air pressure
A barometer, either the mercury Fortin type or an aneroid, measures atmospheric pressure, and a barograph records it. Falling pressure often signals approaching depressions and rain.
Dew
Dew gauges, such as the Duvdevani wooden block, estimate dew deposition. Dew matters for leaf wetness, which affects disease development, and for water supply to crops in dry regions.
Automatic weather stations
Automatic weather stations (AWS) use electronic sensors (thermistors, capacitive humidity sensors, tipping-bucket gauges, sonic or cup anemometers and pyranometers) that log readings every few minutes and transmit them by mobile network or satellite. IMD, state governments and crop insurance schemes have installed thousands of AWS and automatic rain gauges, many at block and gram panchayat level. They are used for weather-based crop insurance and village-level advisories. Regular calibration and maintenance are essential, because a drifting sensor can trigger wrong insurance payouts.
Key points
- Weather is the short-term state of the atmosphere; climate is its long-term pattern, averaged over at least 30 years.
- Temperature is measured with maximum, minimum, dry-bulb and wet-bulb thermometers in a Stevenson screen; grass minimum and soil thermometers are used near the ground.
- Humidity comes from the psychrometer (wet-bulb depression) or a hygrograph. Rainfall is measured with ordinary and self-recording gauges; a rainy day in India has at least 2.5 mm.
- The Class A pan is 121 cm wide and 25.5 cm deep; ET0 = Kp × Epan.
- The cup anemometer measures wind speed and the vane measures direction. The Campbell–Stokes recorder measures sunshine hours; the pyranometer measures solar radiation.
- Automatic weather stations support advisories and weather-based insurance.
What the exams ask
- Match weather elements with their instruments.
- Describe the Stevenson screen and the siting of an observatory.
- Give the dimensions and use of the Class A pan, and calculate ET0.
- Define relative humidity, dew point and rainy day.
- Explain how automatic weather stations support agro-advisories.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why is the Stevenson screen painted white?
White reflects most solar radiation, so the screen itself does not heat up and warm the thermometers. The louvres allow ventilation while blocking direct sun and rain, so the readings show the true air temperature in the shade.
Why do two nearby rain gauges show different rainfall?
Rainfall can vary sharply over short distances, especially in convective storms. Differences in gauge exposure, height and wind effects, along with reading errors, also matter. That is why rainfall networks need many gauges, and why insurance schemes are moving to denser station networks.
What is the difference between relative humidity and dew point?
Relative humidity depends on temperature: the same air has a lower RH in the afternoon than at dawn, even with the same moisture content. Dew point depends only on the moisture content, so it is a better measure of how much water vapour the air actually holds.
Sources
- World Meteorological Organization. Guide to Instruments and Methods of Observation (WMO-No. 8), 2023 edition. WMO, Geneva.
- World Meteorological Organization. Guide to Agricultural Meteorological Practices (WMO-No. 134), 2010 edition. WMO, Geneva.
- Allen, R. G., Pereira, L. S., Raes, D. and Smith, M. Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements. FAO Irrigation and Drainage Paper 56. FAO, Rome, 1998.
- Mavi, H. S. and Tupper, G. J. Agrometeorology: Principles and Applications of Climate Studies in Agriculture. Food Products Press, 2004.
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