Types of Cyclones and NDMA Guidelines on Management of Cyclones

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INTRODUCTION

About 8% of the area in the country is prone to cyclone-related disasters. Recurring cyclones account for large number of deaths, loss of livelihood opportunities, loss of public and private property and severe damage to infrastructure, thus seriously reversing developmental gains at regular intervals.

CYCLONES

  • Cyclone is a system of low atmospheric pressure and represents a circular fluid motion rotating in the same direction as the Earth.
  • This means that the inward spiralling winds in a cyclone rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere of the Earth.
  • Most large-scale cyclonic circulations are centered on areas of low atmospheric pressure. The cyclones can be tropical cyclones or temperate cyclones (extra-tropical cyclones).

Tropical Cyclones

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.

Temperate Cyclones

Temperate cyclones are generally called depressions. They have low pressure at the centre and increasing pressure outwardly. They are of varying shapes such as circular, elliptical. The formation of tropical storms as we read above are confined to oceans, the temperate cyclones are formed over land and sea alike.  Temperate Cyclones are formed in 35-65° North as well as South Latitudes. While the tropical cyclones are largely formed in summer and autumn, the temperate cyclones are formed in generally winter. Rainfall in these cyclones is low and continuous not as furious as in case of tropical cyclones.

Basic Difference between Tropical Cyclone and Extra-Tropical Cyclone

Basis Tropical Cyclone Extra-tropical Cyclone
Characteristics A low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain. A large scale low-pressure weather systems that occur in the middle latitudes of the Earth.
Formation It gets intensified over warm tropical oceans and required 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 and upper divergent above the sea level system. It gets intensified any part of extratropical regions of the Earth (usually between 30° and 60° latitude from the equator), either through cyclogenesis or extratropical transition.
Movement It moves from east to west. It moves from west to east.
Nature of Cyclone Violent storms Static, not violent
Type Warm Core Cold Core

Anticyclones

An ‘anticyclone’ is opposite to a cyclone, in which winds move into a low-pressure area. In an anticyclone, winds move out from a high-pressure area with wind direction clockwise in the northern hemisphere, anti-clockwise in the southern hemisphere.

Such a high-pressure area is usually spread over a large area, created by descending warm air devoid of moisture. The absence of moisture makes the dry air denser than an equal quantity of air with moisture. When it displaces the heavier nitrogen and oxygen, it causes an anti-cyclone.

Basic Difference between Tropical Cyclone and Anti-Cyclone

Storm Tides

Storm Tides refers to the combined effects of storm surge and astronomical tide. Storm surge is an abnormal rise in the level of water along a shore, primarily as a result of the high winds and low pressures generated with tropical cyclones; generally affects only coastal areas but may intrude some distance inland. Astronomical tides refers to tidal levels and character which would result from gravitational effects, e.g., of the Earth, Sun and Moon, without any atmospheric influences.

Requirements for formation

  • Water temperatures of at least 26.5 °C down to a depth of at least 50 m, so that it may cause the overlying atmosphere to be unstable enough to sustain convection and thunderstorms.
  • Rapid cooling with height, so that it may cause release of the heat of condensation that powers a tropical cyclone.
  • High humidity
  • Low amounts of wind shear as high shear is disruptive to the storm’s circulation.
  • A distance from the Equator is necessary, which should be at least 555 km or 5° of latitude, so that it allows the Coriolis effect to deflect winds blowing towards the low-pressure center and creating a circulation. Because the Coriolis effect initiates and maintains tropical cyclone rotation, tropical cyclones rarely form or move within about 5° of the equator, where the Coriolis effect is weakest.
  • A pre-existing system of disturbed weather.

Naming of Cyclones

Tropical cyclones are classified into three main groups, based on intensity: tropical depressions, tropical storms, and a group of more intense storms, whose name depends on the region. If a tropical storm in the North-western Pacific reaches hurricane-strength winds on the Beaufort scale, it is referred to as a typhoon. If a tropical storm passes the same benchmark in the Northeast Pacific Basin, or in the Atlantic, it is called a hurricane. Neither “hurricane” nor “typhoon” is used in either the Southern Hemisphere or the Indian Ocean. In these basins, storms of tropical nature are referred to simply as “cyclones”.

Reasons for Cyclone Vulnerability of India

There are several reasons for this vulnerability:

  • India has a long coastline of around 7,516 km, its coastal terrain is flat and continental shelf is shallow.
  • Further, most coastal cities have a high population density. Although the frequency of Tropical Cyclones (TCs) in the NIO covering the Bay of Bengal and the Arabian Sea is the least in the world (7% of the global total), their impact on the east coast of India, as well as the Bangladesh coast, is relatively more devastating.
  • This is evident from the fact that in the last 270 years, 21 of the 23 major cyclones (with a loss of about 10,000 lives or more) worldwide occurred over the area surrounding the Indian subcontinent (India and Bangladesh).
  • This is primarily due to the serious storm tide effect in the area.

Affected States and UTs

Thirteen coastal states and Union Territories (UTs) in the country, encompassing 84 coastal districts, are affected by tropical cyclones. Out of them, the most vulnerable on East coast are four states viz. Tamil Nadu, Andhra Pradesh, Orissa and West Bengal and one UT (Puducherry) while on the west coast is one state Gujarat.

National Cyclone Risk Mitigation Project

The National Cyclone Risk Mitigation Project (NCRMP), to be implemented with financial assistance from the World Bank, is envisaged to have four major components:

  • Component A: Improvement of early warning dissemination system by strengthening the Last Mile Connectivity (LMC) of cyclone warnings and advisories.
  • Component B: Cyclone risk mitigation investments.
  • Component C: Technical assistance for hazard risk management and capacity-building.
  • Component D: Project management and institutional support.

These components are highly interdependent and have to be implemented in a coherent manner. The planned framework of activities under this project provides end-to-end solutions for effective Cyclone Disaster Management (CDM) in all the 13 coastal states and UTs.

National Guidelines of Management of Cyclones