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Cyclones, Formation

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Cyclone is a system of low atmospheric pressure in which the barometric gradient is steep. Winds circulate, blowing inwards in an anticlockwise direction in the Northern hemisphere and clockwise direction in the Southern hemisphere. Cyclones are classified into two parts:

Tropical Cyclone

A system of low pressure occurring in tropical latitudes, characterized by its very strong winds; found mainly in Indian Ocean, Indonesia, and Australia. Tropical Disturbances are classified into three parts:

  1. A Tropical Depression is a system with low pressure enclosed within few isobars and the wind speed is. 33 knot or 61 tens and it lacks a marked circulation.

  2. A Tropical Storm is a system with several closed isobars and a wind circulation of 115 Kms.

  3. A Tropical Cyclones is a warm core vortex circulation of tropical origin with a small diameter often of an approximately circular shape; they occur only in oceanic areas where the sea temperatures exceed 27 degree C.

Different Names of Tropical Cyclones

  • Hurricanes -N. America & Caribbean

  • Typhoons - Western North Pacific

  • Willywillies- Australia

  • Bagulo- Philippine Islands

  • Taifu- Japan

  • Cyclones- Indian Ocean

Structure of the Tropical Cyclones

Eye the innermost or central portion of the mature cyclone is the 'eye'. It is about 10 to 30 km in diameter, depending upon the size of the storm and is a more or less calm region with little or no clouds and some subsidence. The eye or the calm centre can be described variously as the: Pressure eye (where mean sea level pressure is lowest), Wind eye (light or calm wind conditions), Radar eye (the eye seen in radar echoes) and the Satellite eye (clear or dark spot seen in the cloud mass in satellite imagery).

Eye Wall or Inner Ring Surrounding the eye is a tight 'inner ring' of hurricane winds. This core of maximum winds is at the centre of a solid thick wall of towering Cumulonimbus clouds and is called the 'eye wall'.

Outer Ring An outer ring of cyclonic circulation lies beyond the eye wall, where the speed decreases steeply and clouds and rain diminish rapidly outwards.

  1. It is essentially radially symmetrical

  2. It has six regions

  1. The eye is the centre of the storm which is characterised by more or less circular with comparatively clear skies, lowest pressure, the highest temperature and highest relative humidities;

  2. The eye is surrounded by a wall of cumulonimbus known as eyeball. Strongest wind is found;

  3. Spiral bands or Rainbands or Feeder bands contain many individual thunderstorms which produce heavy rainfall;

  4. Annular zone is characterized by cloudiness and high temperatures and low humidities;

  5. Outer Convective band;

  6. Main cloudmass. Horizontal structure of tropical cyclone