Cyclones are one of the natural hazards that affect India almost every year causing large loss of lives and properties. Tropical Cyclone (TC), also known as ‘Cyclone’, is the term used globally to cover tropical weather systems in which winds equal or exceed the minimum of 34 knot (62 kmph). These are intense low pressure areas of the earth atmosphere coupled system and are extreme weather events of the tropics. Hazards associated with tropical cyclones are long duration rotatory high velocity winds, very heavy rain and storm tide (the combined effect of storm-surge and astronomical tide). Out of these, the storm surge is the greatest killer hazard associated with cyclone.

The Indian subcontinent is one of the worst affected regions in the world. The subcontinent with a long coastline of 8041 kilometers is exposed to nearly 10 per cent of the world’s tropical cyclones. Of these, the majority of them have their initial genesis over the Bay of Bengal and strike the East coast of India. On an average, five to six tropical cyclones form every year, of which two or three could be severe. More cyclones occur in the Bay of Bengal than the Arabian Sea and the ratio is approximately 4:1. Cyclones occur frequently on both the coasts (the West coast - Arabian Sea; and the East coast - Bay of Bengal). An analysis of the frequency of cyclones on the East and West coasts of India between 1891 and 1990 shows that nearly 262 cyclones occurred (92 of these severe) in a 50 km wide strip above the East coast. Less severe cyclonic activity has been noticed on the West coast, where 33 cyclones occurred the same period, out of which 19 of were severe.

Tropical cyclones occur in the months of May-June and October-November. Cyclones of severe intensity and frequency in the North Indian Ocean are bi-modal in character, with their primary peak in November and secondary peak in May. The disaster potential is particularly high during landfall in the North Indian Ocean (Bay of Bengal and the Arabian Sea) due to the accompanying destructive wind, storm surges and torrential rainfall. Of these, storm surges cause the most damage as sea water inundates low lying areas of coastal regions and causes heavy floods, erodes beaches and embankments, destroys vegetation and reduces soil fertility.

Cyclones vary in diameter from 50 to 320 km but their effects dominate thousands of square kilometers of ocean surface and the lower atmosphere. The perimeter may measure 1,000 km but the powerhouse is located within the 100-km radius. Nearer the Eye, winds may hit at a speed of 320 km. Thus, tropical cyclones, characterized by destructive winds, torrential rainfall and storm surges disrupt normal life with the accompanying phenomena of floods due to the exceptional level of rainfall and storm surge inundation into inland areas. Cyclones are characterized by their devastating potential to damage structures, viz. houses; lifeline infrastructure-power and communication towers; hospitals; food storage facilities; roads, bridges and culverts; crops etc. The most fatalities come from storm surges and the torrential rain flooding the lowland areas of coastal territories.

NDMA constituted a steering committee, headed by G. S. Mandal and M Mohapatra, to carry out the upgraded Hazard Profile Map of India. A Report on the same is available online that maps India for the cyclone hazard risk based on:

• Frequency of cyclones affecting the districts,

• Frequency of severe cyclones affecting the districts,

• Probable maximum precipitation over the districts,

• Maximum wind strength over the districts due to the cyclone and

• Probable maximum storm surge over the district due to cyclone.

Based on above categorization, the consolidated lists of districts touching the coast with different intensity of proneness, like very high (P1), high (P2), moderate (P3) and low (P4) are shown in tables in the report.

In general, the coastal districts of West Bengal, Orissa, Andhra Pradesh and Tamil Nadu are more prone and are in the high to very high category. The Proneness factor is very high for the districts of Nellore, East godawari, in Andhra Pradesh; Balasore, Kendrapara, in Orissa; and South 24 Parganas and Midnapore, in West Bengal.

The consolidated lists of districts not touching the sea (but close to the coast) with different degree of proneness are shown in separate tables. In this list the interior districts of West Bengal, like north 24 Praganas and Howrah and Kolkata have been classified as very high cyclone hazard prone districts.