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Tropical Cyclone – a violently rotating wind system

Whenever we talk about extreme weather events, the first thing that immediately comes to our minds is cyclone. Cyclone refers to any spinning storm that rotates around a low pressure centre. Thus cyclone is a system of low atmospheric pressure in which barometric gradient is steep. It represents circular fluid motion rotating in the same direction as the earth rotation.

enawo

Cyclone Enawo – Photo Courtesy : NASA

Storm nomenclature  

When it comes to naming these rotating storms, location plays the key role, as location determines what name is to be ascribed to it. These are called hurricanes in the North Atlantic and north east Pacific Ocean. The same storm becomes typhoon if it occurs in the North West Pacific. And, if we find these storms in the South Pacific and Indian Ocean, they are referred to as the tropical cyclones.

Evolution of tropical cyclone from depression to violent storm

The first and foremost condition necessary for development of tropical cyclone is the availability of warm subsurface ocean temperature. As the warm, moist air over the ocean rises upward and away from ocean surface it leaves a vacuum near the surface, which is filled by the surrounding cool air. Then this cool air becomes warm too and rises. That is how the whole air circulation develops. The warm moist air after reaching a certain height  cools adiabatically and turns into clouds. This whole process of cloud formation and wind spin lead to formation of storm with an eye in the centre. The eye is in fact a very calm region with very low pressure. 

How tropical cyclone is different from temperate cyclone

The basic classification of cyclone comprise tropical and temperate. Both of these differs from each other in terms of their origin, location, factors contributing for development and direction of flow and area covered.

  • Difference in terms of origin of cyclone 

While tropical cyclone is restricted between 5 degree to 15 degree north and south latitude, temperate cyclones originates between 30 to 60 degree north and south latitude hence also called extra tropical cyclone.

  • Difference in factors contributing to development of cyclone 

While temperature and Coriolis force plays a crucial role in the development of tropical cyclone, the temperate cyclone develops due to the meeting of two different air masses i.e. hot air mass and cold air mass. When these two air masses of different characteristics meets a front is formed and the process is called front genesis.

  • Difference in terms of source of origin

While a tropical cyclone is thermally induced and originates over warm tropical ocean, the temperate cyclones form over mid latitude landmass.

  • Difference in terms of direction of flow of cyclones

While tropical cyclone takes a path from sea to land and moves from east to west, the temperate cyclones trajectory moves westward i.e from west to east.

  • Eye of the storm – presence and absence.

In case of tropical cyclone ‘eye’ is an important segment of a cyclone and is located at the centre of storm where as there is no such concept in the case of a temperate cyclone .

  • Types of cloud associated with cyclones

Wide range of clouds ranging from cumulonimbus, cirrus and cirrostratus are associated with extra tropical cyclone, whereas, in the case of tropical cyclone cumulonimbus clouds are generally formed which results in copious rains.

  • Distribution of vortex storms

These storms develops in six major regions of the world which includes,

  1. West Indies, Carribean Sea and Gulf of Mexico, where they are famous as hurricanes
  2. Philippines, China Sea and Japanese Islands also witness these as typhoons
  3. Arabian Sea and Bay of Bengal, called tropical cyclones
  4. Eastern coastal region of Mexico and Central America
  5. South Indian Ocean – Madagascar
  6. Region of Soma and Fiji island and east coast of Australia  

Temperate cyclones however, are mainly limited to North America and northern parts of  Europe

The Indian Meteorological Department (IMD) classifies this low pressure system in the Bay of Bengal and the Arabian Sea on the basis of its capacity to damage, which is adopted by the World Meteorological Department (WMO).

1 knot equals 1.85 km/hr

Type of Disturbances Wind Speed in km/h Wind Speed in Knots
Low Pressure Less than 31 Less than 17
Depression 31-49 17-27
Deep Depression 49-61 27-33
Cyclonic Storm 61-88 33-47
Severe Cyclonic Storm 88-117 47-63
Super Cyclone More than 221 More than 120


They are further divided into five different levels depending on its capacity to cause damage:-

Cyclone Category Wind Speed in km/h Damage Capacity
01 120-150 Minimal
02 150-180 Moderate
03 180-210 Extensive
04 210-250 Extreme
05 250 and above Catastrophic


Endnote

Tropical cyclone develops and is formed all around the world. Enjoy these breathtaking pictures of tropical cyclones captured by NASA satellites.

  • Hurricane  Matthew
tropical cyclone

Cyclone Matthew – Photo Courtesy : NASA

This was the strongest storm to hit the Caribbean in more than 50 years. The Category 4 storm, hurricane Matthew made landfall on south-western Haiti on October 4, 2016,

  • Four tropical cyclones in the North Western, Central and Eastern Pacific Ocean on September 1, 2015

Tropical Cyclone –a violent rotating wind storm

  • In the Western Pacific (far left) is Typhoon Kilo
  • Just east of Hawaii in Central Pacific is Hurricane Ignacio
  • Hurricane Jimena in Central Pacific .
  • The eastern-most storm is TD -14 (Tropical Depression 14E)  in the Eastern Pacific.

A look at the recent cyclone of the world:

  • Cyclone Yasi which struck the coast of Australia on February 2 
Yasi cyclone

Photo Courtesy : NASA

  • On February 14, 2017, Tropical Cyclone Dineo moved southwestward over the waters of the Mozambique Channel
Dineo

Photo Courtesy : NASA

  •  Tropical Cyclone Vardah which made landfall along the east coast of India
vardha cyclone

Photo Courtesy : NASA

  •  South Pacific island nation of Fiji witnessed the Winston cyclone 
winston

Photo Courtesy : NASA

  • On January 11, 2016, tropical storm Pali struck the Central Pacific Basin 
Pali

Photo Courtesy : NASA

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