The term 'Cyclone' derived from a Greek word meaning the coil of a snake, is used for

tropical revolving storms occurring in the Bay of Bengal and the Arabian Sea. These storms are confined

to definite regions of the earth and occur, for the most part, in the tropical portions of the western

sides of the great oceans. The term cyclone is applied to all tropical storms although they are

known as 'hurricanes' in the Atlantic and Eastern Pacific, as 'typhoons' in the Western Pacific, as 'willy

willias' in the Australian Seas and as 'Baguios' in the Philippines

Kaliaperumal, N

Kalimuthu, S

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rr •I } Mandapam And Its Environs SOUVENIR Edited DR. R.S. LAL MOHAN. M.Sc, Ph.D., President Recreation Club of Regional Centre of CMFRI, Mandapam Camp. Published On the Occasion of 35th Anniversary of this Recreation Club of CMFRI Mandapam Camp Tamllnadu IS Tropical C/clones {Dr. N. Kaliaperumal, M.Sc Ph.D., and Mr.S. Kalimuthu, M.Sc) INTRODUCTION The term 'Cyclone' derived from a Greek word meaning the coil of a snake, is used for tropical revolving storms occurring in the Bay of Bengal and the Arabian Sea. These storms are confined to definitfe regions of the earth and occur, for the most part, in the tropical portions of the western sides of the great ocfeans. The term cyclone is applied to all tropical storms atthough they are known as 'hurricanes' in the Atlantic and Eastern Pacific, as'typhoons'in the Western Pacific, as 'willy willias' in the Australian Seas and as 'Baguios' in the Philippines. MAIN FEATUf^ ES OF CYCLONE A tropical cyclone, is a vast violent whirl 150 to 800 km across, 6 to 10 km high, spiralling around a Centre and progressing along the surface of the sea at a rate of 300 to 500 km a day. The speed of a storm can be occasionally 160 km an hour or even more, although in the storms of the Indian seas the winds are generally less. Heavy rainfall, very heavy sea and swell are experienced, in association with storms. The average life span of tropical cyclones in the Indian sea is about 6 days, from the time they form until they dissipate. Some storms last only a few hours, while others may last as long as two-weeks. The evolution of the cyclone can be broadly divided into four phases, the formative phase, the developing phase, the mature phase and the dissipating phase. In the formative phase, the atmospheric pressure falls gradually, the wind freshens and even-ually takes on a cyclonic circulation. In the developing phase the pressure fall continues, the wind speed increases, taking; up a definite anti-clockwise direction of motion around a central region, the squalls i-ncrease, skies over extensive areas become heavily overcast and rainfall becomes heavier. In the mature phase a tropical cyclone is said to be'severe' with a calm central area of 10-30 km in diameter, an inner ring of hurricane winds (90 km per hour or more) 50 to 150 km in width, an outer storm area and the outermost area of weak cyclonic circulation. Soon after the ring of hurricane wfnds enters a land area, the dissipating phase sets in. The increased frictional effects of the land and the lack-ofmbisfyire supply from a warm oceanic surface rapidly decrease the energy suppf-y, the winds weaken aritf the atmijspheric pressure at ths centre registers a rapid rise, although the rainfall continues to be heavy for another day or two. AREAS OF FORMATION. Tropical cyclones originate over comparatively warm oceanic surface as feeble anti • clockwise circJulationsf^ ^ fairly close to the doldrums. In the tropics the prevailing winds to the north of the equator blow from the north - east, to the south of the equator they blow from the southeast. Between these two lies the belt of calm, so-called 'doldrums'. This belt of calm swings northward and southward wffli the advance and retreat of the northern summer and is more or less located over the thermal ecjaator, thfe area of maximum sea surface temperatures. This thermal equator lies well in the northern hemisphere during the northern summer. 35^ DYNAMICS OF A CYCLONE According to the current concepts, a, large mass of warm moist stagnant air becomes yet warmer and more moist and begins to rise. This rising air is replaced by air flowing in from alt sides out because of the rotation of the earth, the inflowing air is deflected, and a whirling system, in which the aLr ascends spirally, is formed. The earth's rotation and the centrifugal force developed by the air motion along a curved path augment the process until a vigorous wind system is developed, ENERGY SOURCE To maintain a cyclonic circulation for a SEibstantiaily long time the emerging Cyclone should: be supplied with energy. The cyclone picks up this energy from the oceanic surface. The energy is released when water vapour picked up from the warm oceanic surface is condensed into rain in the storm. The heat released warms the afr around: it, tha wairmad m rises and winds rush in from all sides towards the centre. CYCLONES IN THE INDIAN SEAS Over Bay of Bengal and Arabian sea only 2 or 3 cyclones a year are observed. The sea-sonal frequency course shows two maxima, one in May and the second in November. The cyclonic storms which form during the premonsoon and post-monsoon months, April-May and October- November are generally of great intensity and usually accompanied by very dangerous and rapid shifts of winds, torrential rains and terrific squalls. One of the destructive features of the storms in Bay of Bengal is the "storm tide" which occurs when they strike the coast. The cyclonic disturbances during the mon-soon period, June fo September, are usually erf snrati intensity. The damage that these cause is mainly due to heavy rain, rather than to strong winds. CYCLONES OF MANDAPAM AREA Mandapam area is very often affected by storms. In the recent past, the havoc caused- by the cyclones of 1964 and 1978, was very severe. During 1984 cyclone, the Pamban bridge was damaged and rail traffic to Rameswaram Island was discontinued for -64 days. One train along with passengers; was washed off near Dhanushkodi., Rati traffic between Pamban and Dhanushkodj was cut off after; this, cyclone. The loss of property was enormous due to these two cyclones. ; ^ WARNING TO THE PUBLIC Occurrence of cyclones cannot tie prevervted but precautionary measures taken in, time Qgffl,, eliminate practically all loss of life and significantly reduce damage to property. With the-gifowth ,o,f , meteorology, the process of detection of cyclones and forecasting their intensity and movement has steadily developed on scientific lines. Ports, Railways, Posts, Telecommunication, Irrigation, Public Works Dep-artments are warned by high priority telegrams. Ships at sea are informed by broadcasts from coastal radio stations. The public in general are warned through All India Radio, Doordharshan and newspapers. These messages are disseminated through police wireless network also. When the storm strikes the coast, the coastal districts fee! the full fury of the associated gales and heavy rain and unless proper precautions are taken in time, heavy loss of life and property is unavoidable. ROLES OF PORTS AND METEOROLOGICAL OBSERVATORY Port Office and Meteorological observatory are situated at Pamban. On receipt of information about cyclone, the port office hoist appropriate visual signals prominently, both by day and night-Mariners and fishermen are generally aware of the meaning of these signals. Signals I to IV are for 36 Cautionary warning. Signals V to Vll are for indicating awaiting danger by cyclone, Signals Vltl TO X indicate great danger to that port area by cyclone and signal XI indicates that the communication with the meteorological centre has broken down and the local officer considers that there is danger of bad weather, The meteorological observatory collects information every 3 hours regarding wind velocity, wind direction, atmospheric pressure, maximum and minimum temperature rainfall, humidity and type of clouds. These data are sent to the Regional Director, Regional Meteorological Centre. Nungambakkam, Madras-6. PRECAUTIONARY MEASURES Some ot the important precautions that can be observed by individuals as soon as official in-formation regarding impending cyclones are given below. 1. Evacuate people, cattle and other live stock from low-lying areas. 2. Tie up boats and other crafts safely in sheltered places. 3. If the house is not safe against high winds and is situated in lowlying area, it is better to move to a safer area. 4. Protect doors, windows and fastening them securely. 5. Objects lying In the open and loose objects on the roof are dangerous missiles in a high wind. Take them inside the house and fasten them well to strong supports. 6. Store drinking water and food which requires little preparation or cooking and which can be preserved. 7. Do not go out in the storm. When the lull occurs and in case of emergency to go out, stay, near shelter; the wind may begin again from the opposite direction all of a sudden. 8. Heed official warnings and do not believe rumours. RELIEF MEASURES The Government of Tamilnadu has built cyclone shelters along the coastline for providing shel-ter to the fishermen during cyclones. After the cyclones. Government makes all arrangements to feed the stranded people until they are resettled. The roads, power supply lines, telegraphic lines and rail-way lines are repaired Immediately on war footing to bring the life to normal. Grants and loans are provided to those who have lost the house and other properties. Fishermen are supplied with free fishing equipments and nets. The Government employees serving in the affected area are granted in-terest free advance amount as a relief measure. Inspite of the Governments efforts in taking precau-tions and remedies, the public have their own responsibilities for taking precautions to safeguard their own properties from destruction. 

Tropical Cyclones

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