Important instruments in meteorological observatory

Meteorological observatory

Purpose; Meteorological observatories collect data on weather elements such as temperature, rainfall, humidity, wind, and sunshine, which are vital for agricultural planning, forecasting, and research.

Components of a Standard Meteorological Observatory

• Stevenson’s Screen – houses thermometers at 1.25 m height.
• Rain gauge station – installed in open area.
• Wind instruments – anemometer and wind vane on a 10 m mast.
• Evaporation pan – kept on a wooden platform (Class-A pan).
• Soil thermometers – at 5, 10, 20, and 30 cm depths.
• Sunshine recorder – measures duration of bright sunshine hours.

Standard Measurement Times (as per IMD)

• 08:30 hrs IST – morning observation
• 17:30 hrs IST – evening observation
• Rainfall is measured at 08:30 hrs IST daily.

Solar radiation balance

• Radiation Balance Equation: Rn = (S−R) + (Ai−Ao)
• Where:
  • Rn​: Net radiation
  • S: Incoming shortwave solar radiation
  • R: Reflected shortwave radiation
  • Ai​: Incoming longwave radiation
  • Ao: Outgoing longwave radiation

• Net Radiation (Rₙ)
  • Governs heat energy availability for evapotranspiration, plant growth, and soil processes.
  • Positive Rₙ → heating of surface
  • Negative Rₙ → cooling of surface (nighttime radiation loss)

Radiation Use Efficiency (RUE); The ability of plants to convert solar radiation into dry matter (biomass).

Typical RUE values:

• C₃ plants: 1.2–2.0 g/MJ
• C₄ plants: 2.5–3.5 g/MJ

Heat and energy balance in agriculture

Components of Heat Balance; Rn =H + LE + G

Where:

• H: Sensible heat flux (heating the air)
• LE: Latent heat flux (evaporation and transpiration)
• G: Soil heat flux

Wind systems and atmospheric pressure

Pressure Measurement

• Instrument: Barometer (Mercury / Aneroid type).
• Unit: millibar (mb) or hectopascal (hPa).
• Standard Atmospheric Pressure: 1013.25 mb at sea level.

Global Wind Belts

Local Winds and Agricultural Significance