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Tropical Cyclone

Introduction


Tropical cyclones are violent storms that form over tropical oceans and move to coastal areas, causing widespread devastation due to high winds, heavy rainfall, and storm surges. This is one of nature's most disastrous  natural calamities. They are known as Cyclones in the Indian Ocean, Hurricanes in the Atlantic, Typhoons in the Western Pacific and South China Sea, and Willy-willies in the Western Australia.


In the northern hemisphere, cyclones circulate in an anticlockwise direction. In the southern hemisphere, cyclones circulate clockwise.


Tropical cyclones form and strengthen over warm tropical oceans. Tropical storms form and intensify when the following conditions exist:


  • a large sea surface with a temperature greater than 27° C


  • the presence of the Coriolis force


  • small variations in vertical wind speed; wind shear refers to the difference in wind speeds at various heights. When the wind is uniform, tropical cyclones form. Cyclone formation processes are limited to equatorial latitudes to weak vertical wind shear. Wind shear is high in temperate regions due to westerlies, which inhibits the formation of convective cyclones.


  • a pre-existing weak-low-pressure area or low-level-cyclonic circulation; and


  • upper divergence above the sea level system.


  • ITCZ plays an important role here, the convergence of air masses of different temperatures and the resulting instability are the prerequisites for the origin and growth of violent tropical storms.




The energy that intensifies the storm is produced by the condensation process in the towering cumulonimbus clouds that surround the storm's centre. The storm is gaining strength as a result of the constant supply of moisture from the sea. When the storm reaches land, the moisture supply is cut off, and the storm dissipates. The location where a tropical cyclone crosses the coast is referred to as the cyclone's landfall. Cyclones that cross 20’N latitude in general recurve and are more destructive.



Eye


The eye of a tropical cyclone is a region of mostly calm weather located at the centre of the storm. A storm's eye is usually circular and 25–40 miles (40–65 km) in diameter. The eye is a calm area with settling air. It is surrounded by the eyewall, which is where the cyclone's most severe weather occurs. The eye is the region with the lowest surface pressure and the warmest temperatures aloft (in the upper levels) – the eye temperature may be 10°C or more warmer than the surrounding environment at an altitude of 12 km, but only 0-2°C warmer at the surface in a tropical cyclone.


Eye Wall


The eye wall is the area around the eye where there is a strong spiralling ascent of air to greater heights, eventually reaching the tropopause. The wind reaches its maximum velocity in this region, reaching speeds of up to 250 km/h. Torrential rain falls in this area. Rain bands may radiate from the eye wall, cumulus and cumulonimbus clouds may drift into the surrounding area. The storm's diameter over the Bay of Bengal, Arabian Sea, and Indian Ocean ranges between 600 and 1200 km. The system travels at a slow pace of 300-500 km per day. Storm surges are generated by the cyclone and inundate the coastal lowlands. The storm dies away on the ground.




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