TYPES OF RAINFALL – Convectional, Orographic, Convergence


There are three important types of rainfall.

Convectional Rainfall

Excessive heating of the earth’s surface in the tropical region results in the vertical air currents.

These currents lift the warm moist air to higher strata of the atmosphere.

When the temperature of such a humid air starts falling below dew point continuously, clouds are formed.

These clouds cause heavy rainfall which is associated with lightning and thunder.

This type of rainfall is called conventional rainfall.

It is very common in the equatorial region where it is a daily phenomenon in the afternoon.

With thunder and lightning, heavy rainfall takes place but this does not last long.

Such rain is common in the summer or in the hotter part of the day.

It is very common in the equatorial regions and interior parts of the continents, particularly in the northern hemisphere.

 

Orographic or Relief Rainfall

Orographic rainfall is formed where air rises and cools because of a topographic barrier.

When their temperature falls below the dew point, clouds are formed.

These clouds cause widespread rain on the windward slopes of the mountain range.

This type of rain is called orographic rainfall.

It is also known as the relief rain.

The chief characteristic of this sort of rain is that the windward slopes receive greater rainfall.

When these winds reach the other slope, they descend, and their temperature rises. Then their capacity to take in moisture increases and hence, these leeward slopes remain rainless and dry.

The region lying on the leeward side of the mountain receiving little rain is called rainshadow area.

A famous example of orographic rainfall is Cherrapunji on the southern margin of the Khasi Hills in Meghalaya.

 

Convergence or Cyclonic Rainfall

Convergence rainfall is produced where air currents converge and rise.

In tropical regions where opposing air currents have comparable temperatures, the lifting is more or less vertical and is usually accompanied by convention.

Convection activity frequently occurs along fronts where the temperature of the air mass concerned is quite different.

Mixing of air along the front also probably contributes to condensation and therefore to the frontal rainfall.

When two large air masses of different densities and temperature meet, the warmer moist air mass is lifted above the colder one.

When this happens, the rising warm air mass condenses to form clouds which cause extensive downpour.

This rainfall is associated with thunder and lightning.

This type of rainfall is also called frontal rainfall.

This type of rainfall is associated with both warm and cold fronts.

It is generally steady and may persist for a whole day or even longer.

Cyclonic rainfall occurs in both tropical and temperate regions.

Tropical cyclones cause heavy rainfall accompanied by high-velocity winds. Parts of Seemandhra, Odisha and West Bengal are highly prone to such cyclones.

Temperate cyclones are less intense. They also cause precipitation. The winter rains in northern parts of India are caused by temperate cyclones called westerly disturbances in India.