Tropical Cyclone

Recently in October 2013, Tropical cyclone Phailin struck the coast of Odisha and Andhra and destroyed many homes, lives and caused loss of property. It originated in the Gulf of Thailand and gradually grew in intensity as it moved over Andman Sea in Bay of Bengal. It was a Category 5 storm with 200 Kmph winds and rainfall.                      Tropical Cyclone

What is a Tropical Cyclone?

Tropical Cyclones are low pressure systems that are formed formed over the warm tropical waters due to gale force winds (gale refers to winds of tropical force). They generally extend hundreds of kilometer and persist for six hours. Most important thing to remember about them is that they die out when they travel through land & Cold Ocean or cold area.


Many people call tropical cyclone by various names; such as hurricanes, typhoons or cyclones. It depends on the location of the storm. But the scientific name is “tropical cyclone”. Storms that flow over Atlantic Ocean or eastern Pacific Ocean are called “hurricanes” (Hurricane in the Atlantic, Typhoon in the Pacific and Cyclone in the Indian Ocean).


  • Circular Eye: It is the centre of tropical cyclone. Owing to its round shape its size is generally expressed in diameter. Its diameter ranges from 10 km to over 100 km.
  • Eye wall: Circular eye is surrounded by thick ring of cloud. That thick ring is called eye wall. Area under it experiences extreme winds & heaviest rainfall.

Generally when the winds in the rotating storm have the speed of 39 mph then they are called tropical storm & when the speed reaches 74 mph it is called tropical cyclone.


Important feature of tropical cyclones is their degree of intensity. So tropical cyclones are classified according to their degree of intensity. Following are its types:

  • Tropical Disturbance: It has weak wind circulation & low velocities.
  • Tropical Depression: It has a definite wind circulation with velocities less than 63 kilometers per hour (kph)
  • Tropical Storm: It is a moderate tropical cyclone with maximum wind speed of 118 kph.
  • Typhoon: It is an intense tropical cyclone with maximum wind speed exceeding 118 kph.


Following things are required for tropical cyclones to develop:-

  • Very large ocean areas with surface temperature more than 26 C.
  • They need warm tropical oceans to feed them energy without which they will cease to exist.
  • Tropical cyclone can exist only when there is proper combination of circulation, divergence and convergence of air which is maintained over a considerable period of time on a proper scale.


Formation of tropical cyclones can be explained by convective theory or the frontal or counter-current theory.

  • According to convective theory: a large mass of air becomes convectively unstable and moist compared with its surroundings, which results in an upward motion of air. The air from the surroundings tend toward the low pressure area formed, so that, a cyclonic circulation is formed. The combined effects of the earth’s rotation and the centrifugal force, retards the movements of air towards the center causing further pressure fall. The process continues until a vigorous cyclonic wind system is developed. Likewise, the outward flow of air from the center at high levels also makes the pressure lower.
  • According to frontal or counter-current theory: many tropical cyclones form along the front between the trade winds and the equatorial air in the doldrums. Winds develop along this front and when conditions are favorable, forms into tropical cyclones. The convergence of the two air masses results in the upward motions which in addition to the deflective effect of the earth’s rotation, centrifugal force, and divergence at the upper levels results in allow pressure area with a spiral circulation toward the center.


Cyclones of severe category pass through four stages. But non-severe category cyclone, various constraints hamper its development such as movement over land or cooler place or making landfall, etc. Following are the stages of a typical severe tropical cyclone:-

Indian National Center for Ocean Information Services (INCOIS)

An autonomous organization of the Government of India, under the Ministry of Earth Sciences, located in Pragathi Nagar, Hyderabad. It provides ocean information and advisory services to different strata of the society such as industry, government and scientific community through sustained ocean observations and constant improvisation of its deliverables by means of systematic and focused research.


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