Cyclones are low pressure centres by which air-currents move upwards. Winds blow, towards the lowest pressure centre. These cyclones cause stormy weather and heavy rainfall along the path. By other locations of these origin, cyclones are classified broadly into two types: Temperate and tropical. Temperate cyclones are concentrated in the center latitudes b/w 35° to 65° in the hemispheres.

Tropical cyclones are notorious for violence and cause wide spread destruction. These develop over tropical oceans within a belt of 8° to fifteen° north and south latitudes. In northern hemisphere, the movement of winds in the cyclone is anti-clock wise though it may be clockwise in southern hemisphere.

Cyclones and Anti Cyclones

Tropical Cyclones

Tropical Cyclone includes all cyclonic disturbances originating over tropical oceanic waters.

Types of Tropical Cyclones

World Meteorological Organization (WMO) classification:

  1. Tropical Depression: Low pressure disturbances enclosed by isobars, wind speed in below 17 m/sec.
  2. Tropical Storm: Several closed isobars wind circulation 17 to 32 m/sec.
  3. Tropical Cyclones: Warm core circulation of tropical air with a small diameter often of an approximately circular shape, minimum surface pressure (<900 mb), with wind speed of at least 33 m/sec.

Atmospheric Moisture

Humidity refers to the condition of the air with regard to water vapour. Absolute Humidity refers to the actual amount of water vapour present per unit volume of air and is usually expressed in grams per cubic metre. It is usually, greater near the equator than the polar region.

Relative Humidity refers to the amount of water vapour in the air compared with the amount that would be present if the air were saturated at that temperature. It is expressesed in percentage. Saturated air’s relative humidity is 100 percent”.

Since the capacity of air for absorbing and retaining moisture varies with temperature, and drop in temperature drops the moisture holding ability of the air, there comes a point when the air cannot hold the moisture any more. This is saturated air and the temperature at which this condition is reached is the ‘dew point’. Any further cooling will result in condensation, Dew, fog, mist, frost and clouds are various forms of condensation.

Convectional Precipitation

Convectional Precipitation is typical of equatorial regions and is cause by heating from the ground surface. As a result of heating of the surface air, it expands and rises to great height. As the air rises it cools and finally it reaches the air of its own temperature and density and the further ascent now ceases. After the saturation point is reached torrential downpour occur.

Conditions necessary for convectional rainfall -

(a) intense heating of the surface.
(b) Abundant supply of moisture in the air.
(c) Turbulence in the atmosphere, and surface obstructions.

Distribution- Occurs throughout the year near the Equator, daily in the afternoon.
Away from the Equator the rainfall occurs in summer and both the total amount and duration of rainy season decrease in nearby hot deserts.
In middle latitudes, convectional rainfall occurs in early summer.

Cyclonic (Frontal) Rainfall

This type of precipitation is associated with a  cyclonic activity and occurs at zones of convergence particularly at the Inter Tropical Convergence Zone (ITCZ) and at Polar fronts in a zone of horizontal wind convergence, the warm air forced up and over the cooler air. This ascent is very slow. Condensation takes place and cooling produces a precipitation that its characteristically less violent than in thunderstorms but is continuous and covers and extensive area. These fronts are best developed in middle latitudes in cooler months. Most of the winter precipitation of lowlands in the middle latitudes is cyclonic in origin.

Distribution of Mean Annual Precipitation

  • The average annual precipitation over the whole Earth is about 80 centimeter’s (30 inches).
  • Equatorial regions and monsoon areas of Southeast Asia have the highest rainfall. Temperate regions, receive moderate amount of precipitation.
  • The subtropical high pressure belt and polar regions receive little precipitation.
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