Cyclones are usually located approximately 30 degrees above and below the equator. They vary in diameter from 50 kms to 320 kms, but their effects dominate thousands of sq. kms of ocean surface and the lower atmosphere.
The perimeter may measure 1,000 km but the powerhouse is located within the 100 kms radius. Nearer the eye, winds may hit 320 km ph. Tropical cyclones are intensifying swirls of cloud and rain which then progress into tropical storms. They spin clockwise in the northern Hemisphere and anti-clock wise in the southern hemisphere.
Between five and 20 kms tall when fully formed, they become self- sustaining and bloat until they hit cool land or ocean being of oceanic origin, they generally hit the east coast of the continents. The Indian subcontinent is the worst cyclone affected part of the world as a result of a low-depth ocean bed topography and coastal configuration. Stretches along the Bay of Bengal Coast-line have the world’s shallowest waters.
The relatively dense population and poor economic condition completes the picture. The population density in some of the coastal districts is as high as 670 person per square km compared to the state average of 26 persons per sq km. Cyclone strike here in May-June and October- November, with the monsoon’s onset and retreat. Cyclones of a diameter of 600 kms or more is one of the most destructive and dangerous atmospheric storms on the earth.
With about 6 per cent of the world wide cyclones, the Indian sub-continent is the worst cyclone affected areas. No universally acknowledged theory of occurrence of tropical cyclone is known today.
A tropical cyclone can form when the horizontal temperature gradients are exceedingly high around a weakly developed area of low pressure. The cyclone is the heat engine whose heater is the oceanic surface. The released heat after condensation converts it into kinetic energy for the cyclone. The following are the stages in the formation of a cyclone:
(a) Temperature of the oceanic surface over 26°C
(b) Appearance of a closed isobar
(c) Low pressure dropping below 1000 mb
(d) Areas of circular movement, first spreading to a radius of 30-50 kms then increasing gradually to 100-200 kms and even to 1000 kms. And
(e) Vertically the wind speed first rising to a height of 6 km, then much higher.
Structure of Tropical Cyclones:
Tropical cyclones have large pressure gradients 14-17 mb/ 100 kms; in some cyclones it is as high as 60 mb/ 100 kms. The wind belt can spread to a distance of 10 to 150 kms from the centre and at times even further. Cyclonic circulation at the surface converts into anti-cyclonic at the higher level. The tropical cyclones have a warm core. In the centre of the cyclone there is generally a cloudless spot known as the eye of the storm.
The eye is encircled by a cloud of great vertical extent. The average rainfall in a tropical cyclone amounts to over 50 cm, some time rising to over 100 cm. The cyclone moves forward at an average speed of about 20 kms per hour.
As soon as the cyclone moves over the land, its energy, because of the absence of sea water, starts decreasing. This leads to the death of the cyclone. The cyclone lasts for five to seven days. The damaging impact of a tropical storm is due to winds of hurricane force, hurricane waves and floods caused by heavy showers.
Majority of the storms inflict heavy losses because of either strong wind or storm waves. In mountainous areas, heavy damages are caused by powerful surface runoffs smashing everything on their way. Intensity of storm waves depends on wind speed, pressure gradient, and topography of sea bottom and profile of coastline. Tropical cyclones entail casualties and losses to life and property in spite of warning system introduced in several areas.
The number of storms in the Bay of Bengal is much greater than in the Arabian Sea. The maximum number of storms both in the Bay of Bengal and the Arabian Sea occur in the months of October and November. Also, early part of the monsoon season is favourable for the formation of tropical storms in both the Bay of Bengal and the Arabian Sea.
Most of the cyclones have their origin between 10°N and 15°N during the monsoon season. Almost all storms in the Bay of Bengal have their genesis between 16°N and 21 °N and west of 92E in June. By July, the Bay storms form north of 18°N and west of 90E. It is also noteworthy that most July storms move along a westerly track. They are generally confined to the region between 20°N and 25°N and recurvature to the Himalayan foothills is comparatively rare.
Reducing Impact of Damage:
Most damage from cyclones is caused by the strong winds, torrential rain and high storm tides. Floods generated by cyclonic rainfall are more destructive than the winds. Today, due to the significant improvement in the cyclone warning system and adequate and timely steps taken, loss of human lives is comparatively less. Other measures like construction of cyclone shelters, embankments, dykes, reservoirs and coastal afforestation help immensely.
The insurance of crops and cattle also a long way in helping people cope with the losses. The warning about the path of cyclone is possible by the satellite images that are available; the computer generated models can predict with fair accuracy the direction and intensity of the winds and direction of the cyclone.
India’s Efforts Cyclone Warning System:
India, according to the World Meteorological Organisation, suffers only six per cent of the total cyclones worldwide. China and Japan face upto 30 per cent (calling them typhoons), and the Americas 23 per cent (hurricanes). But these regions do not suffer such massive devastation. Clearly, there is a way to prevent it. But that needs a comprehensive disaster-management policy.
India has an efficient cyclone warning system. Tropical cyclones are tracked with the help of:
(i) Regular observation from weather network of surface and upper air observation stations;
(ii) Ships reports;
(iii) Cyclone detection radars;
(iv) Satellites, and
(v) Reports from commercial aircraft.
About 280 ships of merchant fleet have meteorological instruments for taking observations at sea. A network of ten cyclone detection radars have been set up along the coast at Kolkata, Paradip, Visakhapatnam, Machilipatnam, Chennai, Karaikal, Cochin, Goa, Mumbai and Bhuj. The range of coastal radars, its intensity and movement is monitored with weather satellites.
Warnings are issued by the area cyclone warning centres located at Kolkata, Chennai and Mumbai, and cyclone warning centres at Bhubaneswar, Visakhapatnam and Ahmedabad. The IMD still largely depends on DoT’s telegraph and telecommunication channels for transfer of data to area cyclone centres as well as dissemination of warnings to various users like district collectors, state government officials, etc. As the storm approaches the coasts, many of these channels completely breakdown.
To overcome these difficulties, IMD has developed a system known as Disaster Warning System (DWS) to transmit cyclone warning bulletins through INSAT-DWS to the recipents. This consists of the following elements:
(i) The cyclone warning centre for originating the area code of the districts and disaster warning message.
(ii) The earth station located near the cyclone warning centre with uplink facility in C-bank and suitable communication links.
(iii) The C/S bank transponder on board INSAT.
(iv) The INSAT-DWS receivers located in cyclone prone areas. The IMD has also published the tracks of the cyclonees since 1981 and updates that every months in its quaterly Journal “Mausam”.