TYPES OF RAINFALL

RAINFALL

There are three main types of rainfall. All have the common theme of air being forced to rise.

As air rises it cools it cannot hold as much moisture as it could when it was warmer. Eventually the rising air reaches a point where it is 100% saturated, in other words it cannot hold any more water. This is called dew point, and it is above this point that condensation occurs.

Condensation is the process by which the water vapour (a gas) held in the air is turned back into water droplets (a liquid), which fall as rain.

Types of Rainfall

a)      Relief/ orographic rainfall

b)     Convectional rainfall

c)      Frontal/cyclonic rainfall

Relief rainfall

Prevailing winds pick up moisture from the sea as they travel across it, making the wind moist and warm. The warm moist reaches the slope of a mountain or high hills and is forced to rise over along the mountain slope. As the warm moist wind rises, it is cooled due to a decrease in temperature until temperatures falls below due point.

The air condenses, forming clouds and raindrops starts to form.

More water vapour condense forming heavy raindrops, that later falls heavily on the windward side as relief rainfall.

The wind continues to be forced over the mountains and begin to descend on the leeward side/ rain shadow. The wind is warmed and causes little rain. The wind descends down the slope as cool dry wind.

Relief rainfall falls in light showers, rains for a long period of time and occurs in highland/mountainous areas.

Convectional Rainfall   

Very common in areas where the ground is heated by the hot sun, such as the Tropics. This is why those areas experience heavy rainfalls most afternoons.

Convectional rainfall occurs when:

Water in a lake/sea is heated by intense solar radiation. Water evaporates and the moist air over the water rises in convectional currents.

As the air rises up, it expands leading to cooling. The moist air condenses at a higher altitude/ level of the atmosphere. The condensed moist air form cumulo-nimbus clouds. These clouds cause heavy or torrential rainfall accompanied by lightning and thunder. At times it also contains hailstones.

Convectional rainfall mainly falls in the afternoon due to maximum heating of air and rains for a short period of time (20-30minutes).

Frontal rainfall

Frontal rainfall is associated with the movement of depressions over the country, which are described in more detail elsewhere in this topic.

Frontal rainfall occurs when:

Two air masses meet, one a warm air mass and one a cold air mass. The lighter, less dense, warm air is forced to rise over the denser, cold air. This causes the warm air to cool and begin to condense. As the warm air is forced to rise further condensation occurs and rain is formed. Frontal rain produces a variety of clouds, which bring moderate to heavy rainfall.

Frontal rainfall is characterized by thunder and lightning, and moderate to heavy rainfall.