Major Breeding Grounds

Major Breeding Grounds The major areas affected by
cyclones are-­
(i) South-east Carribean region where they are called
hurricanes.
(ii) Philippines islands, eastern China and Japan where
they are called typhoons.
(iii) Bay of Bengal and Arabian Sea where they are
called cyclones.
(iv) Around south-east African coast and Madagascar­
Mauritius islands.
(v) North-west Australia.
Characteristics The main features of tropical cyclones
are as follows.
Size and Shape Tropical cyclones have symmetrical
elliptical shapes (2:3 ratio of length and breadth) with steep pressure gradients. They have a compact size-80 km near centre, which may develop upto 300 km to 1,500 km.

Wind Velocity all~ Strength Wind velocity, in a tropical cyclone, is more in poleward margins than at centre and is more over oceans than over landmasses, which are scattered with physical barriers. The wind velocity may range from nil to 1,200 km per hour.

Orientation and Movement These cyclones start with a westward movement, but turn northwards around 20° latitude. They turn further north-eastwards around 25° latitude, and then eastwards around 30° latitude. They then lose energy and subside. Tropical cyclones follow a para­bolic path, their axis being parallel to the isobars.

Structure The centre is characterised by a patch of clear sky, called eye of cyclone. Here, because of the dry, descend­ing air, quiet conditions prevail. Outwards of the centre, there are cirrus clouds and still outwards, dark nimbus clouds, which cause occasional torrential downpour and thunderstorms. The right hand back comer of the cyclone gets heavy squalls and rainfall, but the left hand back comer gets clear weather as the trough passes. These cyclones are associated with destructive weather conditions, especially at the front of the cyclone. Also, these cyclones are highly unpredictable as the thermal effect over ocean and coastal areas changes very rapidly.

Associated Weather The arrival of a tropical cyclone at a particular place is marked by a sudden increase in air temperature and wind velocity, decrease in air pressure, appearance of cirrus or cirrostratus clouds in the sky, and emergence of high waves in the oceans. These clouds thicken and become cumulonimbus, and torrential rains begin. Giant waves spring up and the spray blows in continuous sheets which reduces visibility almost to zero.

This terrible storm continues for few hours and is abruptly followed by total calm and clear skies. This happens when the 'eye' of the cyclone arrives. Temperature rises, but the pressure is at its lowest. The eye is merely a hollow vortex produced by the rapid spiralling of air in the storm. The period of calm lasts for half an hour, and the weather suddenly changes with the arrival of the rear or the tail of the cyclone. The sky becomes overcast again; there is heavy downpour accompanied by lightning and thunder. Winds of high velocity again set in but this time in reverse direction to those of the first half of the storm. This situation continues for few hours, then gradually the winds abate, the clouds break and fair weather returns.