With the aid of diagrams, describe the formation of each of the following: (a) Orographic rainfall (b) Convectional rainfall

Answer Details

(a) Orographic (relief) rainfall

Orographic rainfall forms when moist air is forced to rise over a highland or mountain barrier. Warm, moisture-laden winds (often from the sea) meet a mountain range and are compelled to rise up the windward slope. As the air rises it cools at the lapse rate; on reaching its dew point the water vapour condenses to form clouds, and heavy rain falls on the windward slope. After crossing the summit, the air descends the leeward slope, warms and dries; this drier region is the rain shadow, which receives little rain.

(b) Convectional rainfall

Convectional rainfall forms through the strong heating of the ground surface. Intense insolation heats the land, which in turn heats the layer of air above it. This warm, moist air becomes lighter and rises rapidly in strong upward (convection) currents. As it rises it cools and condenses, building tall cumulonimbus clouds. When condensation is complete, heavy rain falls, usually in the afternoon and often with thunder and lightning. It is common in the equatorial regions and in the tropics during hot afternoons.