Cyclones are caused by atmospheric disturbances around a low-pressure area distinguished by swift and often destructive air circulation. They are usually accompanied by violent storms and bad weather. The air circulates inward in an anticlockwise direction in the northern hemisphere and clockwise in the southern hemisphere. Cyclones are classified as: (i) extra tropical cyclones (also called  temperate cyclones); and (ii) tropical cyclones.

The World Meteorological Organisation (WMO, 1976) uses the term ‘tropical cyclone’ to cover weather systems in which winds exceed ‘gale force’ (minimum of 34 knots or 63 Kph). Tropical cyclones are the progeny of ocean and atmosphere, powered by the heat from the sea, driven by the easterly trades and temperate westerlies, the high planetary winds and their own fierce energy.
In India, cyclones are classified due to the:

Strength of the associated winds,

Storm surge and

Exceptional rainfall occurrences.

Extra tropical cyclones occur in temperate zones and high latitude regions, though they are known to originate in the polar regions. Cyclones that developin the regions between the tropics of Capricorn and Cancer are called tropical cyclones. Tropical cyclones are large-scale weather systems developing over tropical or subtropical waters, where they get organized into surface wind circulation. Cyclones are given many names in different regions of the world – they are known  as typhoons in the China Sea and Pacific Ocean; hurricanes in the West Indian islands in the Caribbean Sea and Atlantic Ocean; tornados in the Guinea lands of West Africa and the southern USA.; willy-willies in north-western Australia and tropical cyclones in the Indian Ocean. The word cyclone  is derived from the Greek word `Cyclos’ meaning the coils of a snake. It was coined by Henry Peddington because the tropical storms in the Bay of Bengal and in the Arabian Sea appeared like the coiled serpents of the sea.

The criteria below has been formulated by the Indian Meteorological Department (IMD), which classifies the low pressure systems in the Bay of Bengal and the Arabian Sea on the basis of the capacity to damage, which is adopted by the WMO.

Type of Disturbances Wind Speed in Km/h Wind Speed in Knots
Low Pressure Lees than 31 Less than 17
Depression 31-49 17-27
Deep Depression 49-61 27-33
Cyclonic Storm 61-88 33-47
Severe Cyclonic Storm 88-117 47-63
Very Sever Cyclonic Storm 117-220 63-119
Super Cyclone More than 221 More than 120

1 knot – 1.85 km per hour
Cyclones are classified into five different levels on the basis of wind speed. They are further divided into the following categories according to their damage capacity.

Cyclone Category Wind Speed in Km/h Damage Capacity
01 120-150 Minimal
02 150-180 Moderate
03 180-210 Extensive
04 210-250 Extreme
05 250 and above Catastrophic

Storm surges (tidal waves) are defined as the rise in sea level above the normally predicted astronomical tide. The major factors include:

A fall in the atmospheric pressure over the sea surface

The effect of the wind

The influence of the sea bed

A funnelling effect

The angle and speed at which the storm approaches the coast

The tides

The very high specific humidity condenses into exceptionally large raindrops and giant cumulus clouds, resulting in high precipitation rates. When a cyclone makes landfall, the rain rapidly saturates the catchment areas and the rapid runoff may extensively flood the usual water sources or create new ones.

How Cyclones are formed

The development cycle of tropical cyclones may be divided into three stages:
i) Formation and Initial Development Stage
The formation/ initial development of a cyclonic storm depends upon various conditions. These are:

A warm sea (temperature in excess of 26 degrees Celsius to a depth of 60 m) with abundant and turbulent transfer of water vapour to the overlying atmosphere by evaporation.

Atmospheric instability encourages formation of massive vertical cumulus clouds due to convection with condensation of rising air above ocean surface.

ii) Mature Tropical Cyclones
When the tropical storm intensifies, the air rises in vigorous thunderstorms and tends to spread out horizontally at the tropopause level. Once air spreads out, a positive perturbation pressure at high levels is produced, which accelerates the downward motion of air due to convection. With the inducement of subsidence, air warms up by compression and a warm ‘eye’ is generated. Generally, the ‘eye’ of the storms has three basic shapes: (a) circular; (b) concentric; and (c) elliptical. The main physical feature of a mature tropical cyclone in the Indian Ocean is a concentric pattern of highly turbulent giant cumulus thundercloud bands.

iii) Modification and Decay
A tropical cyclone begins to weaken in terms of its central low pressure, internal warmth and extremely high speeds, as soon as its source of warm moist air begins to ebb, or is abruptly cut off. This happens after the landfall or when it passes  over cold waters. The weakening of a cyclone does not mean the danger to life and property is over.

Indian Context

The Indian subcontinent is one of the worst affected regions in the world. The subcontinent with a long coastline of 8041 kilometre is exposed to nearly 10 per cent of the world’s tropical cyclones. Of these, the majority 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 severe) in a 50 km wide strip on the east coast. Less severe cyclonic activity has been noticed on the west coast, with 33 cyclones occurringin the same period, out of which 19  of these were severe.

Tropical cyclones occur in the months of May-June and October-November. The 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 at the time of 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 are the greatest killers of a cyclone, by which 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 320 kmph. Thus tropical cyclones, characterized by destructive winds, torrential rainfall and storm surges disrupt normal life with accompanying the 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 the coastal territories.

CYCLONES – Do’s & Dont’s
The actions that need to be taken in the event of a cyclone threat can broadly be divided into four classes, viz., (i) immediately before the cyclone season; (ii) when cyclone alerts and warnings are on;(iii) when evacuations are advised; and (iv) when the cyclone has crossed the coast.
(i) Before the Cyclone season:

Check the house; secure loose tiles, carry out repair works for doors and windows

Remove dead woods or dying trees close to the house; anchor removable objects like lumber piles, loose tin sheds, loose bricks, garbage cans, sign-boards etc. which can fly in strong winds

Keep some wooden boards ready so that glass windows can be boarded if needed

Keep a hurricane lantern filled with kerosene, battery operated torches and enough dry cells

Demolish condemned buildings

Keep some extra batteries for transistors

Keep some dry non-perishable food always ready for emergency use

(ii) When the Cyclone starts

Listen to the radio (All India Radio stations give weather warnings).

Keep monitoring the warnings. This will help you to prepare for a cyclone emergency.

Pass on the information to others.

Ignore rumours and do not spread them; this will help to avoid panic situations.

Believe in the official information

When a cyclone alert is on for your area continue normal working but stay alert to the radio warnings.

Remember that a cyclone alert means that the danger is within 24 hours. Stay alert.

When your area is under cyclone warning get away from low-lying beaches or other low-lying areas close to the coast

Leave early before your way to high ground or shelter gets flooded

Do not delay and run the risk of being marooned

If your house is securely built on high ground take shelter in the safer part of the house. However, if asked to evacuate do not hesitate to leave the place.

Board up glass windows or put storm shutters in place.

Provide strong suitable support for outside doors.

If you do not have wooden boards handy, paste paper strips on glasses to prevent splinters. However, this may not avoid breaking windows.

Get extra food, which can be eaten without cooking. Store extra drinking water in suitably covered vessels.

If you are to evacuate the house move your valuable articles to upper floors to minimize flood damage.

Have hurricane lantern, torches or other emergency lights in working conditions and keep them handy.

Small and loose things, which can fly in strong winds, should be stored safely in a room.

Be sure that a window and door can be opened only on the side opposite to the one facing the wind.

Make provision for children and adults requiring special diets.

If the centre of the cyclone is passing directly over your house there will be a lull in the wind and rain lasting for half and hour or so. During this time do not go out; because immediately after that very strong winds will blow from the opposite direction.

Switch off electrical mains in your house.

Remain calm.

(iii) When Evacuation is instructed

Pack essentials for yourself and your family to last you a few days, including medicines, special foods for babies and children or elders.

Head for the proper shelter or evacuation points indicated for your area.

Do not worry about your property

At the shelter follow instructions of the person in charge.

Remain in the shelter until you have been informed to leave

(iv) Post-cyclone measures

You should remain in the shelter until informed that you can return to your home.

You must get inoculated against diseases immediately.

Strictly avoid any loose and dangling wires from the lamp posts.

If you are to drive, drive carefully.

Clear debris from your premises immediately.

Report the correct loss to appropriate authorities.

Source : NDMA

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