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What is a  Cyclone

Cyclone is defined as drastic atmospheric variation in the Earth System Science depending upon the amount of pressure variation & mode of propagation & circulation. It may be of different kinds as discussed( Read more).

It is an intense rotational low pressure system in tropics with huge mass of revolving moisture whirl in the atmosphere with very strong winds circulating around it, in an anti clockwise direction in the northern hemisphere and in clockwise direction on southern hemisphere (Fig. 1). It is a vast violent whirl of 150 to 800 km, spiraling around a centre and progressing along the surface of the sea at a rate of approx. 300 to 500 km a day.

Fig. 1. Cyclone Aila.

















Depending upon the pressure distribution in the atmospheric winds and its mode circulation in the Bay of Bengal and Arabian sea, South Asian countries experience all types of cyclones as described in Table 1 [World Meteorological Organisation (WMO)].

Table 1. Some of the most devastating Cyclones in the North Indian Ocean (Bay of Bengal) region. (Source:http://en.wikipedia.org/wiki/Cyclone#Structure.)

Place of landfall Date of landfall Maximum sustained winds (kmph) - estimated on the basis of satellite imageries
Chittagong 13 November, 1970 224
Chirala, Andhra Pradesh 19 November, 1977 260
Rameshwaram 24 November 1978 204
Sriharikota 14 November, 1984 213
BanglaDesh 30 November, 1988 213
Kavali, Andhra Pradesh 9 November, 1989 235
Machlipatnam, AP 9 May ,1990 235
Chittagong 29 April, 1991 235
Teknaf (Myanmar) 2 May, 1994 204
Teknaf 19 May, 1997 235
Paradip, Orissa 29 October, 1999 260
89.80E, Bangladesh 15 November, 2007 220
16.00N, Myanmar 02 May, 2008 200

History of Tropical Cyclones
Tropical Cyclones have existed for as long as the history of earth with water. So it is hard to say when the first cyclone was recorded. There are records of tropical cyclones as early as 9th century when Arab travelers to China noted the destructive effects of typhoon. One can get many of the records of conditions during tropical cyclones from the ship logs found during the eighteenth and nineteenth century.

How are Cyclone, Hurricane and Typhoon different- The terms "Cyclone", "hurricane" and "typhoon" are region specific names for a strong "tropical cyclone". Tropical cyclones are called "Hurricanes" over the Atlantic Ocean and "Typhoons" over the Pacific Ocean in the same way as they are called "Cyclone" in North Indian Ocean.

Meaning of the word Cyclone:The word Cyclone is derived from the Greek word "Cyclos" which means coiling of a snake. The word cyclone was coined by Henry Piddington who worked as a Rapporteur in Kolkata during British rule. Size of a cyclone over the north Indian Ocean- The size of a cyclone over Indian seas varies from 50 km radius to 2000 km with an average of 300 -600 km.

Impact of Cyclone

What causes disaster during cyclone?
The dangers associated with cyclonic storms are generally three fold.

  • Storm surge
  • Strong wind
  • Very heavy rains causing floods
Storm Surge





The severest destructive feature of a tropical storm is the storm surge popularly called tidal waves. The coastal areas are subjected to storm surge and are accentuated if the landfall time coincides with that of high tides. This is again more if the sea bed is shallow. Storm surge as high as 15 to 20 ft. may occur when all the factors contributing to storm surge are maximum. The storm tide is the combination of storm surge and the astronomical tide.

The storm surge has devastating consequences. The storm surges are by far the greatest killers in a cyclone. Majority of deaths occur during a cyclone when sea surge with great force, which sometime can travel deep inland and when the water is receding back, it can pull down heavy structures thus leaving no scope for survival. As sea water inundates low lying areas of the coastal regions it causes heavy floods, erosion of beaches and embankments, damage to vegetation and reducing soil fertility. Flooding due to storm surges pollute drinking water sources resulting in shortage of drinking water and causing out-break of epidemics, mostly water borne diseases In addition, the fertility of land is lost due to inundation by saline water for at least a few years to come.

Some of the significant storm surges over the region are mentioned below.

  • Hooghly river (WB), October, 1737 : 13 meter
  • Contai (WB), October, 1864 : 10-13 meter
  • Bangladesh cyclone, November, 1970 : 13 meter
  • Paradip, Orissa,October, 1971 : 4-5 meter
  • Balasore Orissa, May, 1989 : 3-6 meter
  • Orissa Super Cyclone, October, 1999 : 5-6 meter
Strong Wind











The strong wind speed associated with a cyclonic storm can result into severe damage to kutcha houses. Tree branches are likely to break off. The wind associated with a very severe Cyclonic storm and super cyclonic storm can uproot big trees, cause wide spread damages to houses and installations and total disruption of communications. The maximum wind speed associated with a very severe Cyclonic storm that hit Indian coast in the past 100 years was approx. 260 kmph in Oct., 1999 (Orissa Super cyclone).

Heavy Rain-











The rainfall associated with a storm varies from storm to storm even with the same intensity. Record rainfall in a cyclonic storm has been as high as 250 cm. It has been found that the intensity of rainfall is about 85cm/day within a radius of 50 km and about 35 cm/day between 50 to 100 km from the centre of the storm. Often such continuous heavy rainfall cause flooding which breach road and bridges making it difficult for evacuation.

When do coastal stations start experiencing bad weather associated with a Cyclone?
Coasts come under the influence of bad weather in the form of heavy rain, gale winds (exceeding 65 km/h) when the cyclone moves closer to the coast within 200km. Heavy rainfall generally commences about 9-12 hours before cyclone landfall. Gale force winds commence about 6-9 hours in advance of cyclone landfall. Maximum storm surge may appear at or near the landfall time.

Cyclone Warning Services

India Meteorological Department (IMD) Cyclone Warning Services
The cyclone warning organization of IMD has a three tier system to cater to the needs of the maritime States. These are : Cyclone Warning Division set up at IMD Head Quarters New Delhi to co-ordinate and supervise cyclone warning operations in the country and to advise the Govt. at the apex level; Area Cyclone Warning Centres at Chennai, Mumbai and Kolkata and Cyclone Warning Centers at Visakhapatnam, Ahmedabad and Bhubaneswar. The cyclone warning services is also coordinated with the Forecasting Division at Pune.

Monitoring of Cyclones by IMD
IMD has a well-established and time-tested organization for monitoring and forecasting tropical cyclones. A good network of meteorological observatories (both surface and upper air) is operated by IMD, covering the entire coastline and islands. The conventional observations are supplemented by observational data from automatic weather stations (AWS), radar and satellite systems. The satellite techniques (INSAT imagery obtained at hourly interval) are now used to find out the centre and intensity of the system. It is also used to find out various derived parameters which are useful in monitoring the development and movement of cyclones. The radar can be utilized to find out the location of the cyclonic storm more accurately when the system comes within radar range (approx. 400km). In addition it can find out convective cloud cluster, wind distribution, rainfall rate etc.

How track prediction is done in IMD
Tropical Cyclones move as a whole. The average speed is 15-20 kmph (360-480 km/day). They may change their direction of movement. When the speed of movement is 10-14 kmph, it is called as slow moving cyclone. If the speed of movement is more than 25 kmph, is called as fast moving cyclone. Various Techniques are available for Track Prediction of the storm as mentioned below:

  • Methods based on climatology, persistence and both Climatology & Persistence (CLIPER)
  • Synoptic Techniques - Empirical Techniques
  • Satellite Techniques
  • Statistical Techniques using climatology, persistence and synoptic
  • VAnalogue Techniques
  • Numerical weather prediction models

Nomenclature of Tropical Cyclone
Tropical cyclones are named to provide easy communication between forecasters and the general public regarding forecasts, watches, and warnings. Since the storms can often last a week or longer and that more than one can be occurring in the same basin at the same time, names can reduce the confusion about what storm is being described. The Southwest Indian Ocean tropical cyclones were first named during the 1960/1961 season.

Intensity Prediction
Subjective techniques like Climatology, Synoptic and Satellite (Dvorak) techniques and radar techniques are used for predicting intensity. Though the performance of Numerical Weather Prediction models in intensity prediction is not satisfactory, they provide valuable guidance in intensity prediction also.

4-stage warning system for Tropical Cyclones-
Since pre-monsoon cyclone season of 1999, IMD has introduced a 4-Stage warning system to issue cyclone warnings. They are as follows:

  • Pre-Cyclone Watch
    Issued when a depression forms over the Bay of Bengal irrespective of its distance from the coast and is likely to affect Indian coast in future. The pre-cyclone watch is issued by the name of Director General of Meteorology and is issued at least 72 hours in advance of the commencement of adverse weather. It is issued at least once a day.
  • Cyclone Alert
    Issued at least 48 hours before the commencement of the bad weather when the cyclone is located beyond 500 Km from the coast.
  • Cyclone Warning
    Issued at least 24 hours before the commencement of the bad weather when the cyclone is located within 500 Km from the coast. Information about time /place of landfall are indicated in the bulletin. Confidence in estimation increases as the cyclone comes closer to the coast.
  • Post landfall outlook
    It is issued 12 hours before the cyclone landfall, when the cyclone is located within 200 Km from the coast. More accurate & specific information about time /place of landfall and associated bad weather indicated in the bulletin. In addition, the interior distraction is likely to be affected due to the cyclone are warned in this bulletin.

Reference Time
The meanings of different reference times mentioned in the broadcasted cyclone warning bulletin are

  • EARLY HOURS 0000 - 0400 HRS. IST
  • MORNING 0400 - 0800 HRS. IST
  • FORENOON 0800 - 1200 HRS. IST
  • AFTERNOON 1200 - 1600 HRS. IST
  • EVENING 1600 - 2000 HRS. IST
  • NIGHT 2000 - 2400 HRS. IST
  • EARLY MORNING 0400 - 0600 HRS. IST
  • AROUNDNOON 1100 - 1300 HRS. IST)

Some abnormal characteristics associated with Tropical Cyclones-
Majority of Tropical Cyclones are associated with some sort of abnormal behavior such as

  • Rapidly changing trends in motion and intensity
  • Remaining quasi-stationary close to landfall
  • Development or intensification close to a populated coastline
  • Approaching a vulnerable coastline at an acute angle so that even minor forecast errors introduce large landfall uncertainties
  • Threatening the coastal community during high pitch of seasonal activity such as harvesting, festivals, holidays etc.

How accurate are cyclone landfall predictions?
Probability of correct forecast decreases with increasing forecast validity period. IMD’s mean forecast errors for 12, 24, 48 and 72 hours are about 50, 140, 300 and 500 km respectively, which are comparable to corresponding figures of other centers like National Hurricane Centre, Miami, which monitor Atlantic Hurricanes; Typhoon Warning Centre, Tokyo, which monitors Typhoons of Northwest Pacific etc.

Cyclones: the South Asian scenario

With a long coastline of more than 12,000 kilometers, most of it having very high population density, the South Asia has been perennially plundered by the fury of cyclones, most of which are Tropical cyclones. More than 95% of the major cyclone disasters experienced in the world have taken place in South Asia. The incidence of cyclones is more in the Bay of Bengal than in the Arabian Sea. Cyclones form in the south and central Bay of Bengal. Their initial course of movement is towards northwest, north and then they recurve towards northeast striking the Arakan coasts in April and Andhra-Orissa-West Bengal coasts of India and Bangladesh coasts in May. Most of the monsoon (June - September) storms develop in the central and northern Bay and move west-north-westwards affecting Andhra-Orissa-West Bengal coasts of India. Post monsoon (October-December) storms form mostly in the south and the central Bay, recurve between 150 and 180 N latitudes affecting Tamil Nadu-Andhra Orissa-West Bengal coasts of India and the coasts of Bangladesh.

In the Arabian sea, the cyclones form in southeast Arabian Sea and adjoining central Arabian Sea during the months of May, October, November and December and in east central Arabian Sea during the month of June. Some of the cyclones that originate in the Bay of Bengal travel across the peninsular India, weaken and emerge into Arabian Sea as low pressure areas. Some of these again intensify into cyclonic storms. Most of the storms in Arabian Sea move in west-north-westerly direction towards Arabian Coast during the month of May and in a northerly direction towards Gujarat Coast during the month of June. In other months, they generally move northwest, north and then recurve towards northeast affecting Gujarat-Maharashtra coasts; a few, however, also move west north westwards towards the Arabian coast. It has been generally observed that the pre and post monsoon storms/cyclones 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.

Cyclone mitigation measures

The cyclones are typical of the coastal areas and many of these areas have been witness to the fury of cyclones or disasters related to various low pressure systems. The first step in this direction is Preparedness. The impact of the damages caused by cyclones can be lessened to a considerable degree by taking co ordinate programmes. The first step is the issue of warning to the potentially vulnerable areas. In India the Indian Meteorological Department (IMD) has developed a four stage warning system:
Stage1: This warning is about the possibility of a cyclone when a low pressure depression develops in oceans.
Stage 2: The Alert stage, warning given 48 hours prior to the time when a cyclone is expected to hit the land.
Stage 3: The Warning stage, when a cyclone is formed. The warning is given 24 hours before the anticipated time of arrival of a cyclone.
Stage 4: Cyclone arrival, issued 12 hours before a cyclone is due to hit the land. The warning gives information about cyclone and continues until the winds subside. In sea ports, danger signals are hoisted about the impending cyclone5. Preparedness should be developed into a continuous cycle of planning, organizing, training, equipping, exercising, evaluation and improvement activities to ensure effective coordination and the enhancement of capabilities to prevent, protect against, respond to, recover from, and mitigate against the disaster

The step that follows Preparedness is the Response. The response phase includes the mobilization of the necessary emergency services and first responders in the disaster area. This is likely to include a first wave of core emergency services, such as firefighters, police and ambulance crews. They may be supported by a number of secondary emergency services, such as specialist rescue teams.

The final step in the cyclone rehabilitation is Recovery. The aim of the recovery phase is to restore the affected area to its previous state or in a realistic sense: as close as possible to the previous state. It differs from the response phase in its focus; recovery efforts are concerned with issues and decisions that must be made after immediate needs are addressed. Recovery efforts are primarily concerned with actions that involve rebuilding destroyed property, re-employment, and the repair of other essential infrastructure. Efforts should be made to "build back better", aiming to reduce the pre-disaster risks inherent in the community and infrastructure.

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