TYPES OF PRECIPITATION
Depending upon the factors responsible for lifting and cooling of air, there are five types precipitation :
1) Convective precipitation :
Convective precipitation occurs due to heating of air. The air close to the earth surface gets heated, and its density decreases.
Consequently, the air rises upward in the atmosphere and it gets cooled adiabatically to form a cloud.
Precipitation caused by such clouds is called convective precipitation.
Convective precipitation is usually of very short duration. It occurs in the form of local whirling thunder storms and is typical of the tropical regions.
It is called tornado when accompanied by high velocity, destructive winds.
Convective precipitation covers a small area, usually less than 50 km². The rainfall intensity (i.e. rainfall per unit time) may be very high: sometimes it may even reach 10 cm/hour.
2) Orographic precipitation :
Orographic precipitation occurs due to lifting of moist air over mountains. It results in cooling, condensation and precipitation.
Heavy precipitation occurs on the windward side of the mountain, whereas on the leeward side, there is very little precipitation.
3) Cyclonic precipitation :
A cyclone is a large zone of low pressure which is surrounded by circular wind motion.
Air tends to move into the low pressure zone from surrounding areas and displaces low-pressure air upwards.
The winds blow spirally inward counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere.
Cyclonic precipitation occurs due to displacement of air in the upward direction. The normal extent of a cyclone is about 100 to 200 km in diameter.
The centre of the storm, called the eye, which may extend up to about 10 to 50 km, is relatively quite; but outside the eye, very strong winds blow with a speed as high as 200 km/hr.
The rainfall is usually quite heavy in the entire area occupied by the cyclone.
4) Frontal precipitation :
Frontal precipitation is a type of cyclonic precipitation. A frontal surface (or a front) is a surface which separates a warm air mass and a cold air mass.
A front is called a warm front when warm air displaces cold air. It is called a cold front when cold air displaces warm air.
Because the two types of fronts have different temperature and density, frontal precipitation occurs when they clash with each other. There is lifting of warm air over cold air, which results in cooling.
Frontal precipitation may be subdivided in to two types :
i) Warm front precipitation :
In this case, the warm air replaces the cold air mass. The warm air moves upwards over a relatively stationary wedge of cold air.
Warm front precipitation is spread over a large area, which may extend to 300 to 500 km ahead of the warm front.
However, the precipitation is usally light to moderate. The precipitation continues even after the passage of the warm front.
ii) Cold front precipitation :
In this type of precipitation, the cold air mass replaces a warm air mass. Thus the warm air is forced upwards by an advancing wedge of cold air.
Cold front precipitation usually occurs over a small area, which may extend to only 100 to 150 km ahead of the front. However, the precipitation is usually intense.
Non-frontal precipitation :
Non-frontal precipitation occurs when a low pressure zone develops in a region.
Air from an adjacent high pressure region flows into the low pressure zone and it causes the lifting of the air of the low pressure zone to higher altitudes.
Consequently, precipitation occurs due to cooling and condensation. In non-frontal precipitation, there are no fronts. The cyclonic precipitation discussed above is the non-frontal precipitation.
5) Precipitation Due to Turbulent Ascent : This type of precipitation occurs when an air mass is forced to rise up due to friction of the earth surface.
The friction of the earth surface is greater than that of the water surface.
The air mass, after its travel over ocean, rises up because of increased turbulence and friction.
Precipitation occurs after cooling and condensation. The winter rainfall in Tamil Nadu is mainly due to this type of turbulent ascent.
In nature, the various methods of cooling get intermixed and hence the resulting precipitation cannot be identified as being of any one particular type.