Why this matters now
This is a core climatology topic — the humidity terms, condensation forms and rainfall types are repeatedly tested in prelims, and underpin the monsoon and world climate.
Evaporation, condensation and humidity
Water enters the air by evaporation and transpiration and leaves by condensation. Humidity is the water vapour in the air: absolute humidity (actual mass of vapour per volume), relative humidity (percentage of the maximum the air can hold at that temperature), and the dew point (the temperature at which air becomes saturated). Warm air holds more moisture than cold air.
Forms of condensation
On cooling below dew point, vapour condenses into dew, frost, fog and mist (near the surface) or clouds (aloft). Clouds are classified by height and form — cirrus, cumulus, stratus and nimbus (and combinations like cumulonimbus, the thunderstorm cloud).
Types of rainfall
- Convectional rain — heated air rises, cools and rains (common in equatorial regions, afternoons);
- Orographic (relief) rain — moist air forced up a mountain rains on the windward side, leaving a dry rain shadow leeward;
- Cyclonic (frontal) rain — warm and cold air masses meet along a front.
World rainfall is highest near the equator and windward coasts, lowest in deserts and rain-shadow/continental interiors.
UPSC angle
Know absolute vs relative humidity and dew point, condensation forms (dew/frost/fog vs clouds), cloud types, and the three rainfall types (convectional/orographic/cyclonic) with the rain-shadow concept.
Frequently asked questions
What is relative humidity?
The percentage of water vapour the air holds relative to the maximum it can hold at that temperature.
What is the dew point?
The temperature at which air becomes saturated and water vapour begins to condense.
What are the three types of rainfall?
Convectional, orographic (relief) and cyclonic (frontal) rainfall.
What is a rain shadow?
The dry leeward side of a mountain, where descending air has lost its moisture as orographic rain on the windward side.