Henri Peddington, coined the word ‘cyclone’ (Greek: cyclos, coils of a snake) alluding to the appearance of clouds in a tropical storm that frequent the sub continent. Types of cyclones include polar, extra tropical, sub tropical and tropical cyclones. Polar cyclone, also known as sub polar or arctic cyclone occupies a vast area of low pressure which strengthens in the winter and weakens in the summer. It is a low pressure weather system, usually spanning 1,000 to 2,000 kilometres. An extra tropical cyclone is a low pressure weather system with weather fronts – cold and warm, which are created with the meeting of two masses of air which may differ in temperature, humidity and densities. Strong cold fronts typically feature narrow bands of thunderstorms and severe weather – generally moving from west to east. Warm fronts form east of the cyclone centre and are usually preceded by precipitation and fog. They move polewards ahead of the cyclone path. A subtropical cyclone is a weather system that has few characteristics of a tropical cyclone and few of an extra tropical cyclone. They can form between the equator and the 50th parallel.
A tropical cyclone is a storm system characterised by a centre with low pressure and numerous thunderstorms that produce strong winds and profuse rain. A tropical cyclone feeds on heat released when moist air rises, resulting in condensation of water vapour contained in the moist air (Fig 1). They are fuelled by a different heat mechanism than other cyclonic windstorms leading to their classification as ‘warm core’ storm systems (Fig. 2).
According to their location and strength, tropical cyclones are also known as hurricane (West Indian islands in the Caribbean Sea and Atlantic Ocean), tornados (the Guinea lands of West Africa and the southern USA), typhoon (the China Sea and Pacific Ocean), willy-willies (north-western Australia), tropical storm, cyclonic storm, tropical depression, or simply as a tropical cyclone in the Indian Ocean.
While tropical cyclones can produce extremely powerful winds and torrential rain, they can also generate high waves and storm surges. Cyclones develop over large bodies of warm water and lose their strength as they move over land. Coastal regions can thus be significantly damaged by a tropical cyclone. Inland areas remain relatively safe although heavy rains can cause destructive floods. In fact extensive flooding up to 40 kilometres from the coastline can occur due to storm surges. Although their effects on human populations can be devastating, tropical cyclones relieve drought conditions – carry heat and energy away from the tropics to transport it towards temperate latitudes which constitutes an important part of the global atmospheric circulation mechanism. As a result, tropical cyclones help to maintain equilibrium in the Earth’s troposphere.
Traditionally, areas of tropical cyclone formation are divided into seven basins. These include the North Atlantic Ocean, the eastern and western parts of the Pacific Ocean (considered separately because tropical cyclones rarely form in the central Pacific), the south western Pacific, the south western and south eastern Indian Oceans and the northern Indian Ocean. The western Pacific is the most active while the north Indian is the least. On an average, 86 tropical cyclones of tropical storm intensity form annually worldwide with 47 reaching hurricane/typhoon strength and 20 becoming intense tropical cyclones. Worldwide, tropical cyclone activity peaks in late summer when the difference between temperatures aloft and sea surface temperatures are the greatest. However, each particular basin has its own seasonal patterns. On a global scale, May is the least active, while September is the most.
The Indian Case
In India, cyclones are classified on the basis of strength of the associated winds; storm surge and rainfall occurrences. The Indian Meteorological Department (IMD) has classified the low pressure systems in the Bay of Bengal and the Arabian Sea on the basis of the capacity to damage, which has also been adopted by the World Meteorological Organisation (Table 1 & 2).
There are 13 coastal states and UTs in the country with about 84 coastal districts affected by tropical cyclones. Four states (Tamil Nadu, Andhra Pradesh, Odisha and West Bengal) and one Union Territory (Puducherry) on the east coast; and one (Gujarat) on the west coast are the states that are more vulnerable to cyclone disasters. The frequencies of cyclones during 1891-2010 show that nearly 783 cyclones were formed over the Indian Ocean of which 596 cyclones were over the Bay of Bengal and 70 on the Arabian Sea. In all 117 cyclones landed on the coast during this period. As evident from the graph alongside (Fig. 3) recent years have shown a decline in cyclonic activity, especially marked from 1970 onwards.
A very severe cyclonic storm named Thane recently developed over the North Indian Ocean. Thane initially began as a tropical disturbance within the monsoon trough to the west of Indonesia, and gradually developed into a depression and moved towards the northwest. By December 28, 2011, Thane transformed into a very severe cyclonic storm prior to approaching the Indian states of Tamil Nadu and Andhra Pradesh. By the December 31 morning Cyclone Thane left at least 46 persons dead in the worst affected district – Cuddalore of Tamil Nadu, and in Puducherry, apart from ravaging infrastructure and fishing fleet.