Introduction to agricultural meteorology

Definition

• Meteorology: The science that deals with the study of the atmosphere and weather processes.
• Climatology: Study of long-term average weather conditions (30 years or more).
• Agrometeorology (Agricultural Meteorology): It is the branch of meteorology that deals with the relationship between weather and agricultural production.
• In short: Agricultural Meteorology is the study of weather and climate and their influence on crop growth, yield, and agricultural operations.

Branches of Meteorology

• Synoptic Meteorology – Study of weather systems (cyclones, fronts).
• Dynamic Meteorology – Deals with atmospheric motion and forces.
• Physical Meteorology – Studies physical properties of atmosphere.
• Climatology – Study of average weather conditions.
• Agricultural Meteorology – Weather–crop–environment relationship.
• Father of Agricultural Meteorology: Angell James R.H. (USA)

Structure and composition of the atmosphere

Composition of Atmosphere

• Water vapour: varies from 0 to 4% depending on location.
• Dust particles: important for cloud condensation.

Structure of the Atmosphere

Fact: About 75% of the total atmospheric mass lies within the troposphere.

Solar radiation

Types of Solar Radiation

1. Short-wave radiation (incoming from the sun)
2. Long-wave radiation (emitted from the earth)

Instruments

• Pyranometer – measures total solar radiation.
• Pyrheliometer – measures direct solar radiation.
• Albedometer – measures reflected radiation (albedo).

Important Terms

• Albedo: % of solar radiation reflected by a surface.
  • Snow: 75–90%
  • Crops: 20–25%
  • Water: 5–10%

• Insolation: Incoming solar radiation at the Earth’s surface.
• Photosynthetically Active Radiation (PAR): 400–700 nm; about 45–50% of total radiation.
• Solar Constant: The amount of solar energy received per unit area at the top of the atmosphere when the Earth is at mean distance from the Sun = 2 cal/cm²/min or 1367 W/m².
• Albedo: Fraction of solar radiation reflected by a surface.
  • Earth’s average albedo = 30%
  • Fresh snow = 80–90%
  • Crops/Soil = 10–20%
•  Fact: About 47% of the total solar radiation reaching Earth is absorbed by the surface.

Temperature

Factors Affecting Temperature

1. Latitude
2. Altitude
3. Land and water distribution
4. Ocean currents
5. Cloud cover and winds

Temperature

• Definition: Measure of heat energy in a body, expressed in °C or °F.
  • °C = (°F − 32) × 5/9
  • °F = (°C × 9/5) + 32
• Lapse Rate: Temperature decreases with altitude — 6.5°C per km.

Temperature Measurement

• Thermometers:
  • Maximum thermometer: mercury-filled
  • Minimum thermometer: alcohol-filled
  • Soil thermometer: used for soil temperature

Temperature-based Indices

• Growing Degree Days (GDD): GDD = Tmax + Tmin2 – Tbase
• eat units – accumulated temperature above base temperature.

Optimum temperature for crop growth: