Water in the Atmosphere

Humidity, evaporation and condensation, the forms of condensation and the kinds of cloud, and the types and spread of rainfall.

Key ideas

Water vapour varies from zero to about 4% of the air; warmer air can hold more of it.
Air that has cooled to its dew point is saturated; further cooling makes vapour condense on dust, smoke and salt nuclei.
Dew forms when the dew point is above freezing and frost when it is at or below freezing; fog and mist are clouds at the ground.
The four basic clouds are cirrus (high, feathery), cumulus (cotton-wool, flat base), stratus (layers) and nimbus (dark rain clouds).
Precipitation falls as rain, snow, sleet or hail.
Rain is convectional (rising heated air), orographic (air forced up a mountain, with a dry rain shadow behind) or cyclonic (along fronts).
Rainfall is heaviest in the equatorial belt and on windward coasts, and falls from the equator to the poles and from coasts inland.

Relative humidity	vapour present ÷ vapour the air could hold × 100 (in %)
Absolute humidity	grams of water vapour per cubic metre of air
Vapour in a room	length × breadth × height × absolute humidity
To saturate	(capacity − present) × volume
Fog or mist?	fog: visibility under 1 km; mist: 1–2 km
Rainfall bands	over 200 cm heavy; 100–200 cm moderate; 50–100 cm; under 50 cm low

Thinking relative humidity stays the same when air warms: it falls, as warm air can hold more.
Mixing up absolute humidity (g per m³) and relative humidity (%).
Calling the leeward side the rainy side (it is the rain shadow).

On a calm, clear night, will moisture be deposited as dew or as frost if the dew point is (a) 7 °C (b) 2 °C?

Working: dew point above freezing: dew; at or below: frost

Answer: (a) dew (b) dew