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Chapter: 11th Geography : Chapter 6 : Atmosphere

Atmospheric Disturbances (Cyclone and Anti Cyclone)

The atmospheric disturbances which involve a closed circulation of air around a low pressure at centre and high pressure at periphery, rotating anticlockwise in northern hemisphere and clockwise in southern hemisphere is called ‘Cyclones’.

Atmospheric Disturbances (Cyclone and Anti Cyclone)

The atmospheric disturbances which involve a closed circulation of air around a low pressure at centre and high pressure at periphery, rotating anticlockwise in northern hemisphere and clockwise in southern hemisphere is called ‘Cyclones’ (Figure 6.31). Cyclones may be classified into two types based on latitude of its origin.

They are:

A. Tropical cyclone B. Temperate cyclone.

A. Tropical Cyclone

Cyclone formed in the low latitudes is called as Tropical cyclone. They form over warm ocean waters in the tropical regions. The warm air rises, and causes an area of low air pressure.

Stages of Development of Tropical Cyclone

As per the criteria adopted by the World Meteorological Organisation (W.M.O.),

India Meteorological Department classifies the low pressure systems in to vary classes based on wind speed.


1.        Tropical Disturbances


2.        Tropical depressions Low winds with a speed between 31 and 61 km ph.


3.        Tropical cyclone wind speed from 62 to 88 km ph and it is assigned a name.


4.        Severe Cyclonic Storm (SCS) wind speed is between 89 to 118 km ph


5.        Very SCS wind speed between 119 to 221 km ph and


6.        Super Cyclonic Storm when wind exceeds 221 km ph.

 Origin of Tropical Cyclone

Tropical cyclones have certain mechanism for their formation. These are

A source of warm, moist air derived from tropical oceans with sea surface temperature normally near to or in excess of 27 °C (Figure 6.32)

Wind near the ocean surface is blowing from different directions converging and causing air to rise and storm clouds to form.

Winds which do not vary greatly with height are known as low wind shear. This allows the storm clouds to rise vertically to high level;

Coriolis force is induced by the rotation of the Earth. The mechanisms of formation vary across the world, but once a cluster of storm clouds starts to rotate, it becomes a tropical depression. If it continues to develop it becomes a tropical storm, and later a cyclone/ super cyclone.

Characteristics of the Tropical Cyclone

The centre of the cyclone where the wind system converges and vertically rises is called as Eye. The eye is a Calm region with no rainfall and experiences highest temperature and lowest pressure within the cyclonic system (Figure 6.32).

Cyclone wall is made up of Cumulo Nimbus clouds with no visibility, higher wind velocity and heavy rain fall with lightning and thunder.

Tropical cyclones mostly move along with the direction of trade wind system. So they travel from east to west and make land fall on the eastern coast of the continents (Figure 6.33).

Landfall: The condition at which the eye of the tropical cyclone crosses the land is called ‘Land fall’ of the cyclone (Figure 6.34).

Naming of Tropical Cyclones

The practice of naming storms (tropical cyclones) began years ago, in order to help in the quick identification of storms in warning messages because names are presumed to be far easier to remember than numbers and technical terms (Figure 6.35).

In the pursuit of a more organized and efficient naming system, meteorologists later decided to identify storms using names from a list arranged alphabetically. Since 1953, Atlantic tropical storms have been named from lists originated by the National Hurricane Centre. They are now maintained and updated by an international committee of the World Meteorological Organization (WMO).

Large scale destruction caused by Odisha cyclone in 1999, triggered the issue of naming tropical cyclones developed in the North Indian ocean. As a result, naming conventions for storms that develop in the Indian Ocean began in 2004. WMO (World Meteorological Organisation)had informed each of the eight South Asian member countries to submit a list of their own eight names for the cyclones.

Condition of Super Cyclone Formation

Longer travel or stay of low pressure system over warm ocean water.

The speed of jet stream may influence the formation of super cyclone.

Tornado and Water Spouts

It is a very small intense, funnel shaped very speed whirl wind system. Its speed and direction of the movement are erratic (Figure 6.36). The winds are always as fast as 500 km ph. The fast moving air converges in the middle and rises up. The uplift is capable of rising dust, trees and other weaker objects in its path. South and western part of Gulf States of USA experiences frequent tornados.

Water spouts are formed over water body similar to tornados in the formation and structure. This sometimes leads to fish rain, if the mass of fish comes under the water spout.

B. Temperate Cyclone

The cyclone formed in the mid latitudes is called as temperate cyclone. As they are formed due to movement of air masses and front, they are called as ‘Dynamic cyclone’ and ‘Wave cyclone’. This cyclone is characterised by the four different sectors, which are varied with their weather patterns (Figure 6.37).

Stages in the Formation of Temperate Cyclone

1.        Frontogenesis –Formation of front due to collision of two contrasting air masses (Figure ).

2.        Cyclone genesis – Formation of cyclone due to conversion of fronts into various sectors.

3.        Advancing Stage – The stage where cold front advances towards warm front.

4.        Occlusion stage - The stage where the cold front over takes warm front

5.        Frontalysis – The last stage where fronts disappear and cyclone ends its life.


Unlike tropical cyclone, temperate cyclone forms over both land and water in all seasons. It covers larger area than tropical cyclone and stays for a longer period.


Temperate cyclone moves along with the westerly wind system from west to east.

Anti Cyclones

Anti cyclone is a whirlwind system in which high pressure area at the centre and surrounded by low pressure at periphery rotating clockwise in northern hemisphere and anti clock wise in southern hemisphere(Figure 6.39).

This is the largest among the whirl wind systems. Normally, they are associated with high pressure belts of sub tropical and polar region.

Anti cyclones are classified as warm core and cold core, based on their temperature, which are resulted in aridity and cold waves respectively.

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