CLIMATE 1.3 Variations in precipitation across Earth's surface

Water cycle

1. water vapour produced from evapo-transpiration (evaporation from sun’s heat in water bodies, transpiration from plants)

2. water vapour rises and cools and condenses into tiny water droplets at dew point temp, clouds are formed

3. when water droplets in clouds become big and heavy, they fall to ground as precipitation

4. water infiltrates soil and rock on ground surface or flows over ground surface as surface runoff and enter water

Factors affecting movement of water

• type of soil and ground surface

• how built up an area is (high infiltration rate, low speed of surface runoff)

• presence of natural vegetation

• gradient of ground surface

Relative humidity

Ratio of amount of water vapour in air to maximum amount of water vapour air can hold at a given temp

High relative humidity, low temp

Relative humidity at 100% means air is saturated, condensation occurs

Formation of clouds

As air rises up in atmosphere, it cools, reducing amount of water vapour it can hold

When amount of water vapour in air exceeds maximum amount of water vapour air can hold

RH becomes 100%, this point is dew point temperature, air condenses into water droplets on condensation nuclei

Formation of rain

Water droplets in clouds collide and coalesce, becoming bigger and heavier, falling to Earth as raindrops

Convectional Rain

1. Sun’s heat is absorbed by land, and air above land gains heat

2. Warm air rises and cools, condensing on condensation nuclei at dew point temperature, forming clouds

3. Water droplets in clouds collide and coalesce, and when they are large and heavy enough, they fall to ground as rain

Afternoon thunderstorms in Singapore

Relief Rain

1. Prevailing winds pick up moisture over sea, pushing moist air up windward side of mountain

2. The rising moist air cools and condenses on condensation nuclei at dew point temperature, forming clouds

3. Water droplets in clouds collide and coalesce, when they become large and heavy enough, they fall to ground as rain on windward side

4. Most moisture has fallen on windward side, so leeward side experiences dry, descending air.