TYPES OF RAINFALL
Water occurs in three states on the Earth viz, the gaseous(vapor or steam), liquid(water) and the solid(ice).
It can change its state from one to another by absorption or release of heat and energy.
The process of liquid water passing into gaseous or vapor form is called ‘Evaporation’.
Water vapor passing into liquid is called ‘Condensation’.
Water vapor passing into solid-state without the intervening liquid state is called ‘Sublimation’.
Rainfall is the natural process of condensation through which gaseous form of water is converted into liquid water droplets.
It occurs due to cooling of saturated air mass, warm and moist air mass rising upward, warm air rising over cold air, sufficient humidity in the air and condensation etc.,
Types of Rainfall
According to the process of formation, nature and features, three types of rainfall are recognized.
The rain caused by the process of convection is called convectional rainfall.
The, air on being heated, becomes light and rises up in convection currents. As it rises, it expands and loses heat and consequently condensation takes place and cumulous clouds are formed.
This process releases latent heat of condensation which further heats the air and forces the air to go further up.
Convectional precipitation is heavy but of short duration, highly localized and is associated with the minimum amount of cloudiness.
It occurs mainly during summer and is common over equatorial doldrums in the Congo Basin, the Amazon basin and the islands of south-east Asia and Tropical regions.
In the areas of high temperature, air rises up due to heating. This rising air cools, gets saturated, as a result condensation takes place and later rainfall occurs.
This rain is accompanied by thunder and lightning.
It is the most common and widespread form of rainfall in the world. It is also called ‘Mountain Rainfall’ or ‘Relief Rainfall’.
During this rain the moisture-laden winds are forced to ascend over the mountains in their path. As the wind rises, it expands and loses temperature. This results in condensation, leading to rainfall.
This rainfall is found in the windward side of the mountain and is heavy.
In the leeward side of the mountain, as the wind begins to descend, the temperature steadily increases resulting in dry air by forming ‘Rain Shadow Region’.
The cyclonic rainfall is most common in the temperate region.
The rainfall caused with a cyclone or depression is known as cyclonic rainfall.
The winds take a circular movement in the regions where warm and cold air masses meet.
The amount of rainfall received, is measured by an instrument called Rain gauge.
Lines drawn on the map or globe to show the places having the same amount of rainfall.
Cyclone is a small low-pressure area in the center surrounded by high pressure.
The winds blow spirally towards the low-pressure area and form a convergence of winds.
In the northern hemisphere the direction of cyclonic winds is anti-clockwise and in the Southern hemisphere it is clockwise.
Types of Cyclones:
Cyclones are classified into two types. These are
Temperate Cyclones or Extra-Tropical cyclone
1. Tropical Cyclones
Tropical cyclones are violent storms that originate over oceans in tropical areas and move over to the coastal areas bringing about large-scale destruction caused by violent winds, very heavy rainfall and storm surges.
Tropical Cyclones are one of the most devastating natural calamities in the world.
Tropical cyclones originate and intensify over warm tropical oceans.
The origin of tropical cyclones are much related to intensive pressure gradient with the temperature variation.
The rise of convectional currents develops a low-pressure convergence zone.
The high-pressure zone in the water bodies develops the movement of winds towards tropical low-pressure land masses.
These tropical convergent winds form tropical cyclones in the eastern coasts of many countries.
They cause heavy rainfall with high-velocity winds.
Tropical cyclones are highly dangerous and devastating.
The conditions favorable for the formation and intensification of tropical storms are:
Large sea surface with a temperature higher than 27° C.
Presence of the Coriolis force.
Small variations in the vertical wind speed.
A pre-existing weak low-pressure area or low-level cyclonic circulation.
Upper divergence above the sea level system.
Structure of a tropical cyclone
It is the center of a cyclone around which strong spirally winds circulate in a mature tropical cyclone. It is a region of calm with subsiding air.
The eye is surrounded by the “eye wall”, the roughly circular ring of deep convection, which is the area of highest surface winds in the tropical cyclone. Eye Wall region also sees the maximum sustained winds i.e. fastest winds in a cyclone occur along the eye wall region.
2. Temperate or Extra-Tropical Cyclones
In the temperate region, cyclones are produced by the meeting of warm air mass of the tropical region and cold air mass of the polar region.
The tropical air mass is lighter and it is pushed up by the advancing dense cold air mass.
The process of mixing of these two air masses takes place in the form of cyclones. They are associated with heavy rainfall.
The Temperate region thus gets maximum rainfall from the cyclones.
There is a great degree of variation in the shape and size of extra-tropical cyclones.
Generally, the isobars are almost circular or elliptical. However, in certain depressions, the isobars take the shape of the letter ‘V’. Such storms are called V-shaped depression.
At times, the cyclones become so broad and shallow that they are referred to as troughs of low pressure.
As the name itself indicate the Anti-cyclones are contrasting to Cyclones.
Anti-cyclone is a high-pressure area in the center with winds blowing outwards, towards the low-pressure areas.
In the northern hemisphere the direction of the anti-cyclone wind is clockwise and in the southern hemisphere it is anti-clockwise.
These are common in sub-tropical high-pressure areas and absent in equatorial region.
The weather condition during anti-cyclone is generally fine and dry.