Precipitation & Climate Classifications

1. Humidity and Precipitation

Water exists in the atmosphere primarily as invisible water vapor. The capacity of air to hold water vapor increases exponentially with temperature.

Types of Humidity

• Absolute Humidity: The actual mass of water vapor present in a given volume of air (e.g., grams per cubic meter).
• Specific Humidity: The mass of water vapor per unit mass of air (e.g., grams per kilogram). Not affected by volume changes (expansion/compression).
• Relative Humidity (RH): The ratio of the actual amount of water vapor in the air to the maximum amount the air could hold at that temperature, expressed as a percentage. At 100% RH, the air is saturated. If temperature drops, RH increases until saturation is reached at the Dew Point.

Forms of Condensation

Condensation occurs when air is cooled below its dew point around hygroscopic nuclei.

• Dew & Frost: Moisture condenses to form water droplets (dew) or ice crystals (frost) on solid surfaces (grass, car roofs) during clear, cold nights.
• Fog & Mist: Microscopic water droplets suspended near the ground. Fog limits visibility to less than 1 km.
• Clouds: Classified by height and form into Cirrus (feathery ice clouds, high), Cumulus (puffy, cotton-like), Stratus (low, layered), and Nimbus (dark, rain-bearing). Cumulonimbus clouds produce severe thunderstorms.

Types of Precipitation

1. Convectional Rainfall: Common in the equatorial region. High heat causes rapid evaporation and rising convection currents. The air cools rapidly, causing heavy downpours with lightning in the afternoon.
2. Orographic (Relief) Rainfall: When a moist wind encounters a mountain barrier, it is forced upward. It expands, cools adiabatically, and precipitates heavily on the windward slope (e.g., Western Ghats facing the Arabian Sea). As the air descends on the other side (the leeward slope or rain shadow region), it compresses and warms, bringing little to no rain (e.g., Deccan Plateau, central Maharashtra).
3. Frontal (Cyclonic) Rainfall: Occurs in mid-latitudes when a cold and warm air mass converge, causing the lighter warm air to rise, cool, and condense.

2. Climate Classifications

A. Köppen's Empirical Classification (1918)

The most widely used system. It is based on empirical data of mean monthly and annual Temperature and Precipitation, explicitly linking climate types to native vegetation boundaries. It categorizes five major climate groups (A-E) plus Highland (H):

• A - Tropical climates: All months have average temperatures > 18°C. Subdivisions: Af (Tropical Rainforest, no dry season), Am (Tropical Monsoon), Aw (Tropical Savanna/Wet-Dry).
• B - Dry climates: Evaporation exceeds precipitation. Subdivisions: BW (Arid/ Desert), BS (Semi-arid/Steppe). Further divided by h (hot) and k (cold). e.g., BWh (Hot Sahara).
• C - Warm Temperate (Mesothermal) climates: Mild winters. Subdivisions: Cs (Mediterranean, dry summer), Cfa (Humid Subtropical, no dry season), Cfb (Marine West Coast).
• D - Cold Snow Forest (Microthermal) climates: Severe winters. Only found in the Northern Hemisphere (e.g., Taiga).
• E - Cold (Polar) climates: No true summer. Subdivisions: ET (Tundra), EF (Ice Cap, permanent ice like Antarctica).

B. Thornthwaite's Classification (1931, revised 1948)

A more complex, rational approach focusing on the water balance of the soil. He introduced concepts of Precipitation Effectiveness (the P/E index, determining vegetation limits) and Thermal Efficiency (T/E index). His crucial 1948 concept was **Potential Evapotranspiration (PET)—**the amount of moisture that would be evaporated/transpired if unlimited water were available. Climate types are delineated by comparing actual precipitation with PET.

C. Trewartha's Classification

A simplified version of Köppen's system designed to overcome its extreme complexities and geographical anomalies while keeping the basic A, B, C, D, E framework intact. Emphasizes the length of the growing season.