A “Cyclonic Storm’ or a “Cyclone” is an intense vortex or a whirl in the atmosphere with very strong winds circulating around it in anti-clockwise direction in the Northern Hemisphere and in clockwise direction in the Southern Hemisphere. The word “Cyclone” is derived from the Greek word ‘Cyclos” meaning the coil of a snake. To Henri Paddington, the tropical storms in the Bay of Bengal and in the Arabian Sea appeared like the coiled serpents of the Sea and he named these storms as “Cyclones”. Tropical cyclones are also referred to as ‘Hurricanes’ over Atlantic Ocean, ‘Typhoons’ over Pacific Ocean, ‘Willy-Willies’ over Australian Seas and simply as ‘Cyclones’ over north Indian Ocean (NIO).
Classification of Cyclonic Disturbances
Cyclones are intense low pressure areas – from the center of which pressure increases outwards. The amount of the pressure drop in the center and the rate at which it increases outwards gives the intensity of the cyclones and the strength of winds. The criteria followed by the India Meteorological Department (IMD) to classify the low pressure systems in the Bay of Bengal and in the Arabian Sea as adopted by the World Meteorological Organisation (W.M.O.).
Structure of Tropical Cyclone
Tropical Cyclones are warm core low pressure systems having a large vortex in the atmosphere, which is maintained by the release of latent heat by convective clouds that form over warm oceans. In the northern hemisphere, the winds in a cyclone blow anticlockwise in the lower troposphere and clockwise in the upper troposphere. However, in the southern hemisphere, the winds of the cyclone blow in the opposite direction i.e. clockwise in the lower levels and anticlockwise in the upper levels.
A full-grown cyclone is a violent whirl in the atmosphere with 150 to 1000 km diameter and 10 to 15 km height. Gale winds of 150 to 250 kmph or more spiral around the center of very low pressure area with 30 to 100 hPa below the normal sea level pressure. In a fully developed cyclonic storm, there are four major components of horizontal structure viz. Eye, Wall cloud region, Rain/Spiral bands and Outer storm area.