Humidity and evapotranspiration

Humidity

• Absolute humidity: mass of water vapour per volume of air.
• Relative humidity (RH): percentage of moisture present in air compared to maximum possible at that temperature.
• Relative humidity (RH) = (Actual vapour pressure / Saturation vapour pressure) × 100

• Absolute humidity (g/m³)

Dew Point: temperature at which air becomes saturated.

Evaporation and Transpiration

• Evaporation: loss of water from soil/water bodies.
• Transpiration: loss of water from plant leaves.
• Evapotranspiration (ET): total water loss from soil + plant surface.

Instruments:

• Psychrometer – measures RH
• Hygrometer – measures humidity
• Evaporimeter (Class A Pan) – measures evaporation 

Fact:

• Evaporation rate increases with high temperature, low RH, and high wind speed.
• Kharif crops experience high evapotranspiration compared to Rabi crops.

Wind

Definition

• Movement of air due to difference in pressure.
• Wind direction: from where it blows.
• Wind speed: measured by anemometer.

Instruments

• Wind vane: measures wind direction.
• Anemometer: measures wind speed.
• Cup anemometer: most common.

Precipitation

Forms of Precipitation

1. Rain
2. Drizzle
3. Snow
4. Hail
5. Dew
6. Frost

Rainfall Measurement

• Rain gauge – standard instrument (Non-recording / Recording type).
  • Non-recording type: Symon’s rain gauge (India standard).
  • Recording type: Tipping bucket, Weighing bucket, Float type.

Rainfall Distribution in India

• Highest rainfall: Mawsynram, Meghalaya (~11,000 mm).
• Lowest rainfall: Western Rajasthan (<100 mm).
• Annual average: ~1200 mm.

Weather and climate

Agro-Climatic Classification of India

• By Planning Commission: 15 agro-climatic regions.
• By ICAR: 127 agro-climatic zones.
• By NBSS & LUP: 20 agro-ecological regions.

Weather forecasting

Types of Weather Forecasts

Agromet Advisory Services (AAS)

• Provided by IMD (India Meteorological Department) and ICAR.
• Issued twice a week at district level through Agro-Met Field Units (AMFUs).
• Helps farmers take weather-based decisions (sowing, irrigation, fertilizer use).

Agroclimatic concepts and crop-weather relation

Climatic Factors Affecting Crop Growth

1. Radiation – energy for photosynthesis.
2. Temperature – affects germination, growth, yield.
3. Rainfall – affects soil moisture and crop selection.
4. Humidity – affects evapotranspiration and disease.
5. Wind – influences lodging, pollination, transpiration.

Climatic Requirements of Major Crops