All about Tropical Cyclones

• Chiefly Tropical cyclones are violent storms that originate over oceans in tropical areas and move over to the coastal areas bringing about large scale destruction caused by violent winds, very heavy rainfall and storm surges.
• Tropical Cyclones are one of the most devastating natural calamities in the world.
• These Tropical cyclones originate and intensify over warm tropical oceans. The conditions favourable for the formation and intensification of tropical storms are:
  • Large sea surface with temperature higher than 27° C.
  • Presence of the Coriolis force.
  • 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.

Stages of Formation: Tropical Cyclones

The development cycle of tropical cyclones divided into three stages:

Formation and Initial Development Stage

• The formation and initial development of a cyclonic storm depends upon the transfer of water vapour and heat from the warm ocean to the overlying air, primarily by evaporation from the sea surface.
• It encourages formation of massive vertical cumulus clouds due to convection with condensation of rising air above the ocean surface.

Mature Stage

• When a tropical storm intensifies, the air rises in vigorous thunderstorms and tends to spread out horizontally at the tropopause level. Once air spreads out, a positive pressure at high levels originates, which accelerates the downward motion of air due to convection.
• With the inducement of subsidence, air warms up by compression and a warm ‘Eye’ (Low pressure centre) originates. The main physical feature of a mature tropical cyclone in the Indian Ocean is a concentric pattern of highly turbulent giant cumulus thundercloud bands.

Modification and Decay

• A tropical cyclone weakens in terms of its central low pressure, internal warmth and extremely high speeds, as soon as its source of warm moist air begins to ebb or abruptly cut.
• This happens after its landfall or when it passes over cold waters.

Nomenclature of Tropical Cyclones

• However The naming of tropical cyclones is a recent phenomenon. The process of naming cyclones involves several countries in the region and is done under the aegis of the World Meteorological Organization (WMO).
• For the Indian Ocean region, a formula for naming cyclones- 2004 .
  
  Eight countries in the region – Bangladesh, India, Maldives, Myanmar, Oman, Pakistan, Sri Lanka and Thailand – all gave a set of names, given sequentially whenever a cyclonic storm develops.
• Hudhud, Titli, Phethai, Fani, Vayu and Amphan are among the names of cyclones in the Indian Ocean region.

Worldwide Terminology of Tropical Cyclones

• However they have Different names in different regions of the world – eg.they are known as Typhoons in the China Sea and Pacific Ocean; Hurricanes in the West Indian islands in the Caribbean Sea and Atlantic Ocean; Tornados in the Guinea lands of West Africa and southern USA.; Willy-willies in north-western Australia and Tropical Cyclones in the Indian Ocean.

Extratropical Cyclone

• Extratropical cyclones are mid-latitude depressions, temperate cyclones, frontal depressions and wave cyclones.
• These are active above the mid-latitudinal region between 35° and 65° latitude in both the hemispheres. The direction of movement is from west to east and more pronounced in the winter seasons. It is in these latitude zones the polar and tropical air masses meet and form fronts.

Air Mass

• Air Mass is an extremely large body of air whose properties of temperature and moisture content (humidity), at any given altitude, are fairly similar.
  • It can cover hundreds of thousands of square miles of area.
  • It may have only a little horizontal variation in temperature and moisture throughout the air mass.
• When an air mass remains over a homogenous area for a sufficiently longer time, it acquires the characteristics of the area. The homogenous regions can be the vast ocean surface or vast plains.

Fronts

• When two different air masses ( different properties) meet, the boundary zone between them is a front.
• However There are four types of fronts:
  • Stationary front: When the front remains stationary,
  • Cold front: When the cold air moves towards the warm air mass, its contact zone is called the cold front,
  • Warm front: If the warm air mass moves towards the cold air mass, the contact zone is a warm front.
  • Occluded front: If an air mass is fully lifted above the land surface, it is called the occluded front.
• The fronts occur in middle latitudes and have steep gradient in temperature and pressure. They bring abrupt changes in temperature and cause the air to rise to form clouds and cause precipitation.

Cyclones in India

Tropical Cyclones

• Tropical cyclones originate over the Bay of Bengal, Arabian Sea and the Indian ocean. These tropical cyclones have very high wind velocity and heavy rainfall and hit the Indian Coastal states.
• Most of these cyclones are very destructive due to high wind velocity and torrential rain that accompanies it.
• There are three elements with cyclones which cause destruction during its occurrence. These are-
  • Strong Winds/Squall: It damages installations, dwellings, communications systems, trees etc., resulting in loss of life and property.
  • Torrential rains and inland flooding: Rain is a serious problem for the people who become shelter less due to the cyclone.
  • Storm Surge: It is an abnormal rise of sea level near the coast caused by a severe tropical cyclone. Due to storm surge sea water inundates low lying areas of coastal regions drowning human beings and livestock.

Management of Cyclones

However There are many structural and non-structural measures for effective disaster management of cyclones.

• The structural measures include construction of cyclone shelters, construction of cyclone resistant buildings, road links, culverts, bridges, canals, drains, saline embankments, surface water tanks, communication and power transmission networks etc.
• Non-structural measures like early warning dissemination systems, management of coastal zones, awareness generation and disaster risk management and capacity building of all the stakeholders involved.

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