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.
