• What are Cyclones?

  • Cyclones in the Indian Seas

  • Destruction caused by Cyclones

  • Surge prone coasts of India

  • Cyclone Accounts

  • How to avoid the catastrophe?

  • Cyclone Operation in India

  • Disaster Prevention and Preparedness

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What are Cyclones? 

A "Cyclonic Storm" or a "Cyclone" is an intense vortex or a whirl in the atmosphere with very strong winds circulating around it in anti-clockwise direction in the Northern Hemisphere and in clockwise direction in the Southern Hemisphere.

The word "Cyclone" is derived from the Greek, word "Cyclos" meaning the coils of a snake. To Henri Peddington, the tropical storms in the Bay of Bengal and in the Arabian Sea appeared like the coiled serpents of the sea and he named these storms as "Cyclones".

Cyclones are intense low pressure areas - from the centre of which pressure increases outwards- The amount of the pressure drop in the centre and the rate at which it increases outwards gives the intensity of the cyclones and the strength of winds. 

The criteria followed by the Meteorological Department of India to classify the low pressure systems in the Bay of Bengal and in the Arabian Sea as adopted by the World Meteorological Organisation (W.M.O.) are:


Types of Disturbances

Associated wind speed in the Circulation

1. Low Pressure Area

2. Depression

3. Deep Depression

4. Cyclonic Storm

5. Severe Cyclonic Storm

6. Very Severe Cyclonic Storm

7. Super Cyclonic Storm

Less than 17 knots ( < 31 kmph)

17 to 27 knots ( 31 to 49 kmph)

28 to 33 knots ( 50 to 61 kmph)

34 to 47 knots ( 62 to 88 kmph)

48 to 63 knots ( 89 to 118 kmph)

64 to 119 knots ( 119 to 221 kmph)

120 knots and above ( 222 kmph and above)

1 knot - 1.85 km per hour

A full-grown cyclone is a violent whirl in the atmosphere 150 to 1000 km across, 10 to 15 km high. Gale winds of 150 to 250 kmph or more spiral around the center of very low pressure area with 30 to 100 hPa** below the normal sea level pressure. The central calm region of the storm is called the "Eye". The diameter of the eye varies between 30 and 50 km and is a region free of clouds and has light winds. Around this calm and clear eye, there is the "Wall Cloud Region" of the storm about 5O km in extent, where the gale winds, thick clouds with torrential rain, thunder and lightning prevail. Away from the "Wall Cloud Region", the wind speed gradually decreases. However, in severe cyclonic storms, wind speeds of 50 to 60 kmph can occur even at a distance of 600 km from the storm centre. The gales give rise to a confused sea with waves as high as 20 metres, swells that travel a thousand miles. Torrential rains, occasional thunder and lightning flashes - join these under an overcast black canopy. Through these churned chaotic sea and atmosphere, the cyclone moves 300 to 500 km, in a day to hit or skirt along a coast, bringing with it storm surges as high as 3 to 12 metres, as if splashing a part of the sea sometimes up to 30 km inland leaving behind death and destructions.

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Cyclones in the Indian Seas

Cyclones form in certain favourable atmospheric and Oceanic conditions. There are marked seasonal variations in their places of origin, tracks and attainment of intensities. These behaviours help in predicting their movements.

Cyclones affect both the Bay of Bengal and the Arabian Sea. They are rare in Bay of Bengal from January to March. Isolated ones forming in the South Bay of Bengal move west north westwards and hit Tamil Nadu and Sri Lanka coasts. In April and May, these form in the South and adjoining Central Bay and move initially northwest, north and then recurve to the northeast striking the Arakan coasts in April and Andhra-Orissa-West Bengal-Bangla Desh coasts in May. Most of the monsoon (June - September) storms develop in the central and in the North Bay and move west-north-westwards affecting Andhra-Orissa-West Bengal coasts. Post monsoon (October-December) storms form mostly in the south and the central Bay, recurve between 150 and 18O N affecting Tamil Nadu-Andhra Orissa-West Bengal-Bangla Desh coasts.

Cyclones do not form in Arabian sea during the months of January, February and March and are rare in April, July, August and September. They generally form in southeast Arabian Sea and adjoining central Arabian Sea in the months of May, October, November and December and in east central Arabian Sea in the month of June. Some of the cyclones that originate in the Bay of Bengal travel across the peninsula, weaken and emerge into Arabian Sea as low pressure areas. These may again intensify into cyclonic storms. Most of the storms in Arabian Sea move in west-north-westerly direction towards Arabian Coast in the month of May and in a northerly direction towards Gujarat Coast in the month of June. In other months, they generally move northwest north and then recurve northeast affecting Gujarat-Maharashtra coasts; a few, however, also move west north westwards towards Arabian coast.

Pre and Post-monsoon storms are more violent than the storms of the monsoon season. Life span of a severe cyclonic storm in the Indian seas averages about 4 days from the time it forms until the time it enters the land.

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Destruction caused by Cyclones

There are three elements associated with a cyclone, which cause destruction. They are explained in the following paragraphs:

  1. Cyclones are associated with high-pressure gradients and consequent strong winds. These, in turn, generate  storm surges. A storm surge is an abnormal rise of sea level near the coast caused by a severe tropical cyclone; as a result, sea water inundates low lying areas of coastal regions drowning human beings and live- stock, eroding beaches and embankments, destroying vegetation and reducing soil fertility.

  2. Very strong winds may damage installations, dwellings, communication systems, trees., etc. resulting in loss of life and property.

  3. Heavy and prolonged rains due to cyclones may cause river floods and submergence of low lying areas by rain causing loss of life and property. Floods and coastal inundation due to storm surges pollute drinking water sources causing outbreak of epidemics.

It may be mentioned that all the three factors mentioned above occur simultaneously and, therefore, relief operations for distress mitigation become difficult. So it is imperative that advance action is taken for relief measures before the commencement of adverse weather conditions due to cyclones.

The most destructive element associated with an intense cyclone is storm surge. Past history indicates that loss of life is significant when surge magnitude is 3 metres or more and catastrophic when 5 metres and above.

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Surge prone coasts of India

Storm surge heights depend on the intensity of the cyclone, i.e., very high-pressure gradient and consequent very strong winds and the topography of seabed near the point where a cyclone crosses the coast. Sea level also rises due to astronomical high tide. Elevation of the total sea level increases when peak surge occurs at the time of high tide.

Vulnerability to storm surges is not uniform along Indian coasts. The following segments of the east coast of India are most vulnerable to high surges

i) North Orissa, and West Bengal coasts.

ii) Andhra Pradesh coast between Ongole and Machilipatnam.

iii) Tamil Nadu coast, south of Nagapatnam.

The West coast of India is less vulnerable to storm surges than the east coast of India in terms of both the height of storm surge as well as frequency of occurrence. However, the following segments are vulnerable to significant surges :

i) Maharashtra coast, north of Harnai and adjoining south Gujarat coast and the coastal belt around the Gulf   of  Bombay.

ii) The coastal belt around the Gulf of Kutch.

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Cyclone Accounts

The accounts of some of the cyclones that struck the coasts of India are given below

  1. The oldest and the worst cyclone on record is that of October 1737 which hit Calcutta and took a toll of 3,00,000 lives in the deltaic region. It was accompanied by a 12 metre high surge. A violent earthquake coinciding with this storm enhanced the destruction.

  2. Midnapore Cyclone of October 1942 was accompanied by gale wind speed of 225 kmph

  3. Rameswaram Cyclone of 17th to 24th December 1964 wiped out Dhanuskodi in Rameswaran Island from the map. A passenger train which left Rameswaram Road station near about the midnight of 22nd was washed off by the storm surges sometimes later, nearly all passengers traveling in the train meeting water graves. The Pamban bridge connecting Mandapam and Rameswaram island was also washed away by the storm surges which could be 3-5 meters high.

  4. Bangla Desh Cyclone of 8-13 November 1970 which crossed Bangla Desh coast in the night of 12th was one of the worst in recent times, with storm surges of 4 to 5 metres height at the time of high tides, and with 25 cm of rain in the areas, the inundation took toll of about 3,00,000 people.

  5. Andhra Cyclone of 14-20 November 1977 that crossed coast near Nizampatnam in the evening of 19th, took a toll of about 10,000 lives. The Ship Jagatswamini, which went right into the eye of the storm in the evening of 17th experienced maximum wind speed of 194 kmph. As the storm approached the coast, gale winds reaching 200 kmph lashed Prakasam Guntur, Krishna, East and West Godavari districts. Storm surge of 5 meters high inundated Krishna estuary and the coasts south of Machilipatnam.

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How to avoid the catastrophe?

One thinking is fighting the storm and to subdue its violence; the other thinking is to learn to live with it.

Effective Cyclone Disaster Prevention and Mitigation Plan requires:

  • A Cyclone Forecast - and Warning Service.

  • Rapid dissemination of warnings to the Government Agencies, Marine interests like the Ports, Fisheries and Shipping and to General Public.

  • Organisations to construct Cyclone Shelters in the cyclone-prone areas and ready machinery for evacuation of people to safer areas.

  • Community preparedness at all levels to meet the exigencies.

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Cyclone Operation in India

For cyclone forecast and advance warning, the Government have strengthened the Meteorological Department, by providing Cyclone Surveillance Radars at Calcutta, Paradeep, Visakhapatnam, Machilipatnam, Madras and Karaikal in the east coast and at Cochin, Goa, Bombay and Bhuj in the west coast. Satellite picture receiving (APT) equipments at Delhi, Bombay, Pune, Madras, Visakhapatnam, Calcutta and Guwahati are receiving satellite pictures of the cyclones from the polar-orbiting Satellites of the U.S.A. and U.S.S.R. Since April 1, 1982 A.V.H.R.R. (Advance very High Resolution Radio-meter) ground receiving equipment is operative at New Delhi. At this Centre very High Resolution Cloud pictures (Resolution 1.1 km) in 5 channels as also T.I.P. data (Tiros Information Processor) are being regularly received and are being archived in Magnetic Tapes. Hard copies of the pictures in 2 or 3 channels are being obtained regularly. Distinct advantage of these pictures is due to their very High Resolution in all the 5 channels. Further improvements in the cyclone tracking and forecasting have taken place after the Meteorological application programme of the Indian Geo-Stationary Satellite INSAT-LB has become operational since October 1983. Monitoring of the cyclone by taking hourly pictures has helped the forecaster to improve his skill in issuing the timely warnings to the public. Satellite pictures received by the Meteorological Data Utilisation Centre (M.D.U.C.) at New Delhi are further disseminated to all the forecasting Offices through Radio Facsimile.

Cyclone operations are being done by the Meteorological Department through the Area Cyclone Warning Centres (ACWC) and the Cyclone Warning Centres (CWC). ACWC at Calcutta and Madras and the CWC at Bhubaneswar and Visakhapatnam are responsible for cyclone forecasting in the Bay of Bengal, ACWC at Madras and Bombay and CWC at Ahmedabad are responsible for the cyclones in the Arabian Sea - the National Forecast Centre or the WEATHER CENTRAL at Pune being the coordinator. Computerised Operational Advisory Forecasts on cyclone movements are being issued by the Numerical Weather Prediction (NWP) division of the Department at the H.Q. Office at New Delhi. Storm surge advisories are being issued to the ACWC/CWC by the Northern Hemispheric Analysis Centre (NHAC) at the H.Q. Office at New Delhi. Researches on scientific and operational aspects of Cyclones are being carried out at the Cyclone Warning Research Centre (CWRC) at Madras. Specialised researches on storm surges relating to surge height to storm intensity are done at the H.Q. Office at New Delhi.

It may be mentioned here that with the present knowledge about the cyclones and with the available aids, the average error in the predictions of the storm centre for a 24-hour forecast is about 200 km.

Cyclone warnings in appropriate formats are being disseminated to the various warnees (More).

Cyclone warnings issued to the Chief Secretaries, the Relief Commissioners and the District Collectors of the maritime states are the very basic information for cyclone distress prevention and mitigation. These are disseminated under "Two Stage Warning Scheme" i.e., in two stages whenever any coastal belt is expected to experience heavy rains, gales and tidal waves in association with a cyclonic storm or depression expected to intensify into a cyclonic storm. The first stage warning known as the "Cyclone Alert" is issued 48 hours in advance of the expected commencement of the adverse weather over the coastal areas. The second stage warning known as the "CycloneWarning" is issued 24 hours in advance. Both cyclone "Alert" and "Warning" messages are passed to the AIR stations for repeated broadcast. However, they are requested to broadcast cyclone warnings at hourly or half-hourly intervals, when the cyclone is nearer to the coast. It will be seen from the table that for dissemination of cyclone warnings, the Meteorological Department has to depend mostly on the Telecommunication channels of the Post and Telegraph Department consisting of landline telegrams, Teleprinter, Telex, Telephone, etc.. India Meteorological Department has been including some of the vital information like port signal advices in the cyclone warning bulletin issued to the AIR. Concerned warnees have also been advised to monitor the cyclone-warning bulletin on the AIR during cyclone seasons, which would help them to take appropriate action even in the case of not receiving the addressed warnings due to delay or failure of communications. During l987, INSAT based Cyclone Warning Dissemination System (CWDS) for south Andhra and north Tamil Nadu coasts was made operational, which is capable of circumventing failure of traditional communication Systems. Under this scheme, the cyclone warnings are directly sent to the users through INSAT. Such facilities have now been extended to all other cyclone prone areas.

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Disaster Prevention and Preparedness

(a) National

The Govt. of India suggested in 1969, to the governments of the maritime states to set up "Cyclone Distress Mitigation Committee" (CDMC) in the respective states with the objective of preventing loss of life and minimising damage to properties. CDMCs are to plan the communication systems in the state for quick dissemination of Meteorological warnings and prevention measures. Prevention measures include construction of storm shelters, connecting roads for evacuation of people, construction of wind breaks, dykes, bunds, flood storage reservoirs, afforestation along the coastal belts and improvement of drainage facilities. An advance warning will not be effective unless the public is enlightened about the destructive features and the actions to be taken by them to avoid sufferings. CDMCs have also, therefore, programmes for generating public awareness through information pamphlets, brochures, audiovisual materials, cyclone preparedness meetings, talks and discussions over the radio and television.

(b) International

The World Meteorological Organisation (WMO) has established in 1972, a Tropical Cyclone Project (TCP) with the objective of assisting the member countries to increase their capabilities to detect and forecast the approach and landfall of the tropical cyclones, evaluate and forecast, the storm surges, forecast the flooding arising from the cyclones and to develop schemes to organise and execute disaster prevention and preparedness measures. One such plan that is in operation for assisting the countries adjoining the Bay of Bengal and the Arabian Sea is the panel on the tropical cyclones of World Meteorological Organisation (WMO) and the Economic and Social Council for Asia and Pacific (ESCAP). The WMO/ESCAP panel has a technical support unit (TSU) and its office is rotated every four years and is at present located in Dhaka in Bangladesh. The present members of WMO/ESCAP panel on Tropical cyclone are Bangladesh, Burma, India, Maldives, Pakistan, Sri Lanka and Thailand.

The panel co-ordinates the Co-operative activities among the member countries in facing the cyclone problems. Guidelines have also been drawn up for disaster prevention and preparedness jointly by the WMO/ESCAP, League of Red Cross Societies (LRCS) and the United Nations' Disaster Relief Organisation (UNDRO), which are being, implemented by the concerned countries. Recently, a cyclone operational plan for the region has been formulated and accepted by member countries under this panel. RMC New Delhi is issuing advisory bulletin to the member countries, whenever, there is a cyclonic storm in North Indian Ocean.


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