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Tropical Cyclone

Tropical Cyclone

What Are Tropical Cyclones?

  • Tropical cyclones are regarded as one of the most devastating natural calamities in the world.
  • Tropical cyclones are violent storms that originate over oceans in tropical areas and move over to the coastal areas bringing about large scale destruction due to violent winds (squalls), very heavy rainfall (torrential rainfall) and storm surge.
  • They are irregular wind movements involving closed circulation of air around a low pressure center. This closed air circulation (whirling motion) is a result of rapid upward movement of hot air which is subjected to Coriolis force. The low pressure at the center is responsible for the wind speeds.
  • Squall == a sudden violent gust of wind or localized storm, especially one bringing rain, snow, or sleet.
  • Torrent == a strong and fast-moving stream of water or other liquid.
  • The cyclonic wind movements are anti-clockwise in the northern hemisphere and clockwise in the southern hemisphere (This is due to Coriolis force).
  • The cyclones are often characterized by existence of an anticyclone between two cyclones.

Coriolis Force (f)

  • The Coriolis force is zero at the equator (no cyclones at equator because of zero Coriolis Force) but it increases with latitude. Coriolis force at 5° latitude is significant enough to create a storm [cyclonic vortex].
  • About 65 per cent of cyclonic activity occurs between 10° and 20° latitude.

Conditions Favourable for Tropical Cyclone Formation

  • Large sea surface with temperature higher than 27° C
  • Presence of the Coriolis force enough to create a cyclonic vortex
  • Small variations in the vertical wind speed
  • A pre-existing weak low-pressure area or low-level-cyclonic circulation
  • Upper divergence above the sea level system

Names of cyclone in different regions

  • Cyclones in the Indian Ocean
  • Hurricanes in the Atlantic
  • Typhoons in the Western Pacific and the South China Sea
  • Willy-willies in Western Australia

Low-level Disturbances

  • Low-level disturbance (thunderstorms – they are the seeds of cyclones) in the form of easterly wave disturbances in the Inter-Tropical Convergence Zone (ITCZ) should pre-­exist.
  • Small local differences in the temperature of water and of air produce various low pressure centers of small size. A weak cyclonic circulation develops around these areas.
  • Then, because of the rising warm humid air, a true cyclonic vortex may develop very rapidly. However, only a few of these disturbances develop into cyclones.

Formation of Cyclone

  • The energy that strengthens the storm comes from the condensation process in the towering cumulonimbus clouds, surrounding the centre of the storm.
  • With an uninterrupted supply of moisture from the sea, the storm is again strengthened.
  • On reaching the terrestrial region the moisture supply is cut off and the storm dissipates.
  • The place where a tropical cyclone cuts the coast is called the landfall of the cyclone.
  • A landfall is frequently accompanied by sturdy winds, heavy rain and mounting sea waves that could threaten people and cause damage to properties.
  • Cyclones which cross 20 degrees North latitude are more destructive.
  • They cover a larger area and can originate over the land and sea whereas the tropical cyclones originate only over the seas and on reaching the land they dissipate.

Eye of Cyclone

  • A mature tropical cyclone is characterised by the strong spirally circulating wind around the centre which is called the eye.
  • The eye is an area with calm weather descending air.
  • It is characterized by light winds and clear skies.

Eye Wall

  • Around the eye is the eyewall, where there is a strong spiralling rise of air to a greater height reaching the tropopause.
  • The wind reaches maximum velocity in this region and torrential rain occurs here.
  • From the eyewall, rain bands may radiate and trains of cumulus and cumulonimbus clouds may drift into the outer region.

Vertical Structure of a Tropical Cyclone

  • There are three divisions in the vertical structure of tropical cyclones.
  • The lowest layer, extending up to 3 km and known as the inflow layer, is responsible for driving the storm.
  • The middle layer, extending from 3 km to 7 km, is where the main cyclonic storm takes place.
  • The outflow layer lies above 7 km. The maximum outflow is found at 12 km and above. The movement of air is anticyclonic in nature.

Characteristics of Tropical Cyclones

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 center, which may develop up to 300 km to 1500 km.
  • Wind Velocity and Strength
    • Wind velocity, in a tropical cyclone, is more in poleward margins than at center and is more over oceans than over landmasses, which are scattered with physical barriers. The wind velocity may range from nil to 1200 km per hour.
  • Path of Tropical Cyclones
    • 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.
    • Coriolis force or earth’s rotation, easterly and westerly winds influence the path of a tropical cyclone.
    • Tropical cyclones die at 30° latitude because of cool ocean waters and increasing wind shear due to westerlies.


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