March 16, 2014

Different Types of Rainfall - Explained

Types of Rainfall: When the air rises it expands and gets cooled. When the air is cooled beyond the dew point, condensation takes place and results in rainfall. Rainfall occurs in three different ways and as such it is known by three different names, i.e.. the relief rainfall, the convectional rainfall and the cyclonic rainfall.

1. Relief Rainfall — Air is forced to rise, when it meets a mountain and is therefore cooled. If enough water vapour is present it is deposited on the windward side of the mountains. This is the most widespread form of rainfall. When the moisture-laden winds strike against the mountain ranges, they rise up. expand and down and bring heavy rainfall on the wind-ward side. Such a rain by the uprising winds striking against the mountains is called the Relief Rainfall.

Example of Relief Rainfall in India: The rain on the Western Ghats of India or on the southern slopes of the Himalayas is the relief rainfall. As winds cross over to the other side (leeward side) they lose most of their moisture. While descending they become warm and dry and hence there is less rain on the leeward side. The rainless area on the leeward side of a mountain is called the Rain Shadow Area. The Deccan region in India lies in the rain shadow of the Western Ghats.

Relief Rainfall occurs when:
  • The prevailing winds pick up moisture from the sea as they travel across it, making the air moist.
  • The moist air reaches the coast and is forced to rise over mountains and hills.
  • This forces the air to cool and condense, forming clouds.
  • The air continues to be forced over the mountains and so it drops its moisture as relief rain.
  • Once over the top of the mountain the air will usually drop down the other side, warming as it does so. This means it has a greater ability to carry water moisture and so there is little rain on the far side of the mountain. This area is called the rain shadow.

Windward and Leeward Sides Compared:
a)        Windward side is that side of the mountain which faces the rain-bearing winds. On the other hand, the leeward side is that side of the mountain which is opposite to the windward side.
b)      When the rain bearing winds climb the windward side of the mountain, they cool down and bring heavy rainfall. On the other hand, when they cross over to the other side (i.e. the leeward side) they have already lost much of their moisture. While descending they further become warm and dry so they give less rainfall on the leeward side.
c)        On the windward side, the air ascends and becomes cool. On the other hand, on the leeward side the air descends and gets warmed.
d)        The western slopes of the Western Ghats get more rain because they form the windward side. On the other hand, the eastern slopes of the Western Ghats get lesser rain because they form the leeward side.

2.        Convectional Rainfall — The condition of rising currents of warm air separated by more wide-spread areas of slowly sinking air is referred to as CONVECTION. The unequal heating of the earth’s surface causes convection. In the equa­torial region the air becomes hot It picks up a great deal of moisture and when it begins to rise it produces a convectional air current. In the upper part of the atmosphere it cools down. As the moisture laden air gets cooled down in the upper region, condensation takes place and there is a heavy rainfall accompanied by lightning and thunder. Rain caused by the rising currents of air is known as the convectional rainfall.

Convectional rainfall occurs when:
  • The surface of the earth is heated by the sun.
  • The warm surface heats the air above it. Hot air always rises so this newly heated air does so.
  • As it rises the air-cools and begins to condensate.
  • Further rising and cooling causes a large amount of condensation to occur and rain is formed.
  • Convection tends to produce towering cumulo-nimbus clouds, which produce heavy rain and possible thunder and lightning.

3. Cyclonic Rainfall/ Frontal Rainfall— This rain is caused by depressions or Lows. Cyclonic rain originates where warm tropical air meets cold polar air. The warm air overrides the cold air.
In a cyclone, winds from all sides try to rush towards the low pressure in the centre; consequently there is circular motion which makes the whirling air at the centre to rise up and up.
When contrasting air masses make contact, an abrupt zone or boundary is formed. This boundary is called a FRONT and is accompanied by rather abrupt changes in temperature, pressure and humidity.      
When a mass of warm air moves into a region of cold air, the warm air overrides the cold air mass, forcing the cold air to retreat. This situation is called a WARM FRONT, which are characterized by several days of rain.       
A cold air mass moving into a warm air mass produces a frontal surface, which is more vertical than that of a warm front. This situation produces a COLD FRONT. Cold air masses advance rapidly and force the warm air mass upward where it becomes cooled. The movement of the air mass is rapid enough to produce cumulonimbus clouds. Rainfall is heavy but brief in duration.

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.
  • It is common in temperate zones where the air from the polar high pressure belts meets the air from the sub-­tropical high pressure belts. 


1 comment:

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