🌦️ Atmospheric Moisture and Precipitation

Atmospheric moisture is much more than just “water in the air.” It controls cloud formation, rainfall, dew, fog, frost, crop water demand, disease spread, and the success of many farm operations such as irrigation, spraying, and storage.

Atmospheric Moisture

Water is present in the atmosphere in three forms:

• vapour,
• liquid droplets,
• ice crystals.

The amount of water vapour in the air changes with temperature and location. Warm air can hold more moisture than cold air, which is why humidity behavior changes strongly between morning and afternoon.

Measures of Humidity

Absolute Humidity

The actual mass of water vapour present in a unit volume of air, usually expressed in g/m³.

Specific Humidity

The mass of water vapour per unit mass of moist air, usually expressed in g/kg.

Relative Humidity (RH)

Relative humidity is the ratio of actual vapour pressure to saturation vapour pressure at the same temperature.

RH = (e / eₛ) × 100

Why RH matters:

• it influences transpiration,
• it affects disease development,
• it helps decide spray timing,
• it changes during the day as temperature changes.

Usually:

• maximum RH occurs in the early morning,
• minimum RH occurs in the afternoon.

Dew Point Temperature

The dew point is the temperature at which air becomes saturated if cooled at constant pressure. When air or a surface cools to the dew point, condensation begins.

• If temperature stays above 0°C, dew forms.
• If temperature falls below 0°C, frost may form.

Forms of Precipitation

Rainfall

Rainfall is the most important form of precipitation for Indian agriculture.

• India’s average annual rainfall is about 1170 mm.
• Roughly 75% is received during June to September through the southwest monsoon.

Rainfall intensity is often described as:

• light: < 2.5 mm/hr
• moderate: 2.5-7.5 mm/hr
• heavy: > 7.5 mm/hr

Drizzle

Very fine droplets with low intensity. Drizzle wets the surface but contributes less total water than normal rain.

Snow

Occurs in the form of ice crystals and is common in high-altitude Himalayan regions.

Hail

Hail consists of hard ice pellets, usually more than 5 mm in diameter. It can cause severe mechanical damage to standing crops, especially during Rabi.

Dew

Dew is the condensation of water vapour on cool surfaces during the night. In some dry regions, it gives a small but useful moisture contribution.

Types of Rainfall

1. Convective Rainfall

Caused by local heating of air. Warm air rises, cools, condenses, and produces rain. It is common in hot tropical afternoons.

2. Orographic Rainfall

Occurs when moist air is forced to rise over mountains. The Western Ghats are a classic Indian example.

3. Cyclonic or Frontal Rainfall

Associated with low-pressure systems, cyclones, and frontal disturbances.

Importance in Agriculture

Atmospheric moisture and precipitation affect farming in many ways:

• Rainfed farming depends directly on rainfall quantity and distribution.
• Crop water requirement must match rainfall pattern, not just seasonal total.
• High humidity often favors fungal and bacterial diseases.
• Low humidity increases evapotranspiration and crop water stress.
• Excess rainfall can cause waterlogging, nutrient leaching, and root damage.

Practical Example

A season may receive “normal” total rainfall, but if most of it falls in a few heavy events followed by long dry spells, crops can still face drought stress between rains. This is why distribution is as important as total rainfall.

Summary Cheat Sheet