Humidity, Clouds & Precipitation
How water vapour becomes rain — humidity and dew point, the forms of condensation, cloud types, and the three mechanisms of rainfall.
Key Takeaways
- Relative humidity is the ratio of actual water vapour to the maximum the air can hold at that temperature.
- Condensation produces dew, frost, fog, mist and clouds.
- Rainfall is of three types by origin — convectional, orographic (relief) and cyclonic (frontal).
Core concept
Humidity is the water vapour present in the air. When air is cooled to its dew point, the vapour condenses into tiny droplets, forming clouds; when these droplets grow heavy enough, they fall as precipitation. This water cycle links the atmosphere, land and oceans.
Static foundation — measuring humidity
- Absolute humidity: the actual mass of vapour per unit volume of air.
- Relative humidity: the ratio (%) of actual vapour to the maximum the air can hold at that temperature — the most useful measure.
- Dew point: the temperature at which saturation (100% RH) is reached.
How Rain Forms
Evaporation
Heat turns water from oceans, rivers and soil into invisible water vapour, which rises.
Cooling & saturation
As the air rises it cools; on reaching the DEW POINT it becomes saturated.
Condensation
Vapour condenses onto tiny dust particles (condensation nuclei), forming cloud droplets.
Precipitation
Droplets coalesce and grow heavy, then fall as rain, snow, hail or sleet.
Three Types of Rainfall (by origin)
| Type | How it forms | Where |
|---|---|---|
| Convectional | Intense surface heating lifts moist air, which cools and rains | Equatorial regions; summer afternoons |
| Orographic (relief) | Moist winds forced to rise over mountains, cooling and raining on the windward side | Western Ghats, Himalayan foothills (leaves a rain shadow) |
| Cyclonic (frontal) | Warm and cold air masses meet; the warm air rises along the front | Temperate regions; along cyclones |
Forms of Condensation & Clouds
Tap to reveal.
Prelims trap zones
- Orographic rain falls on the WINDWARD side; the leeward side is the rain shadow.
- Frost forms only when the dew point is below 0°C; dew when it is above freezing.
- Cumulonimbus clouds bring thunderstorms and heavy rain — not cirrus (which are high, fair-weather clouds).
Knowledge Check
2 questions · check your understanding
1. The heavy rainfall on the western (windward) side of the Western Ghats is an example of:
2. The temperature at which air becomes saturated and condensation begins is called the:
Prelims Pointers
- Dew point is the temperature at which air becomes saturated and condensation begins.
- Clouds are classified as high (cirrus), middle (alto), low (stratus) and vertical (cumulus/nimbus).
- Orographic rainfall falls on the windward side of mountains; the leeward side is a rain shadow.
- Fog and mist differ in density and visibility (fog is denser).
Mains Angle
- 'Relief (orographic) rainfall shapes India's rainfall pattern.' Discuss.
- Explain the process of cloud formation and precipitation.
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