188 ENVIRONMENTAL SCIENCE
(cyclogenesis) to its termination (proteolysis or occlusion) is called the ‘life cycle of cyclone’;
which is completed through six successive stages.
(a) The first stage involves the convergence of two air masses of contrasting physical
properties and directions. Initially, the air mass (warm and cold) move parallel to
each other and a stationary front is formed. This is called initial stage.
(b) The second stage is also called as ‘incipient stage’, during which the warm and
cold air masses penetrate into the territories of each other and thus a wave-like
front is formed.
(c) Third stage: This is the mature stage when the cyclone is fully developed and
isobars become almost circular.
(d) Fourth stage: Warm sector is narrowed in extent due to the advancement of cold
front than warm front, as cold front comes nearer to warm front.
(e) Fifth stage: Starts with the occlusion of cyclone when the advancing cold front
finally overtakes the warm front and an occluded front is formed.
(f) Sixth stage: Warm sector completely disappears, occluded front is eliminated and
ultimately cyclone dies’ out.(II) Tropical Cyclones
(i) General Characteristics
Cyclones developed in the regions lying between the tropics of Capricorn and Cancer
are called Tropical Cyclones which are not regular and uniform like extra tropical or temperate
cyclones. There are numerous forms of these cyclones, which vary considerably in shape,
size, velocity and weather conditions. The weather conditions of low latitudes mainly rainfall
regimes are largely controlled by Tropical Cyclones.
(a) Size of tropical cyclones varies considerably. On an average their diameters range
between 80 km and 300 km.
(b) Weak cyclones move at the speed of about 32 km per hour while hurricanes attain
the velocity of 180 km per hour or more.
(c) Tropical cyclones become more vigorous over the oceans but become weak and
feeble while moving over land areas. This is why these cyclones affect only the
coastal areas e.g. Tamil Nadu, Orissa and West Bengal coasts of India.
(d) The centre of the cyclone is characterized by extremely low pressure.
(e) Tropical cyclones are not characterized by temperature variations in their different
parts because they do not have different fronts.
(f) There are no different rainfall cells hence each part of the cyclones yields rainfall.
(g) Tropical cyclones are not always mobile. Normally, they move from east to west
under the influence of trade winds
(h) Tropical cyclones are confined to a particular period of the year (summer season).(ii) Types of Tropical Cyclones
Generally they are divided into 4 major types: