Rain is grace; rain is the sky descending to the earth. — John Updike
10.1 Humidity and Evaporation
Humidity measures water vapour in the air:
- Absolute humidity — actual water vapour per unit volume
- Relative humidity — ratio of actual to maximum possible at that temperature (%)
Evaporation increases with temperature, wind, and exposed water surface. Condensation occurs when air cools to dew point.
10.2 Forms of Condensation
| Form | Condition | Example |
|---|---|---|
| Dew | Surface cooling at night | Winter mornings in plains |
| Frost | Dew point below 0°C | Himalayan foothills |
| Fog | Condensation near surface | Indo-Gangetic winter fog |
| Mist | Less dense than fog | Hill stations |
| Cloud | Condensation at altitude | Cumulus, stratus, cirrus |
Adiabatic cooling — air rises and cools; if lifted enough, condensation produces clouds and precipitation.
10.3 Precipitation
Precipitation — water released from atmosphere (rain, snow, sleet, hail).
Mechanisms:
- Convectional — intense heating (summer thunderstorms in India)
- Orographic — air forced over mountains (Western Ghats, Himalayas)
- Cyclonic/frontal — along weather fronts and low-pressure systems
Rainfall types: trace, showers, continuous. Snowfall occurs above the snow line in Himalayas.
10.4 Indian Monsoon Context
The monsoon is the seasonal reversal of winds bringing ~75% of India's annual rainfall (June–September).
Key processes:
- Differential heating of land and sea
- ITCZ shift over the subcontinent
- Somali jet and cross-equatorial flow
Breaks in monsoon and El Niño events can cause drought; excess rain causes floods (Kerala 2018, Uttarakhand 2013).
Chapter Summary
- Humidity, evaporation, and condensation control atmospheric moisture
- Dew, fog, and clouds are forms of condensation
- Precipitation types: convectional, orographic, cyclonic
- Monsoon delivers most of India's rainfall; vulnerable to variability
- Understanding moisture processes explains floods and droughts
Exercises (NCERT)
- Exercise 1 — humidity, evaporation, and condensation
- Exercise 2 — types of precipitation and cloud formation
- Exercise 3 — monsoon mechanism and rainfall variability in India