CYCLONE

What are Cyclones? 

• "Cyclone" is an intense 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.

• Word "Cyclone" is derived from the Greek, word "Cyclos" meaning the coils of a snake.

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• 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.

• Cyclones are classified as: (i) extra tropical cyclones (also called temperate cyclones); and (ii) tropical cyclones.

• 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 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.

Criteria followed to classify cyclones

• As adopted by Meteorological Department of India

• 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

Exceptional rainfall occurrences

• 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

• i) Formation and Initial Development Stage• ii) Mature Tropical Cyclones• iii) Modification and Decay

i) Formation and Initial Development Stage

• 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.

Mechanism of cyclones

Mechanism of cyclones

• A full-grown cyclone is a violent whirl in the atmosphere 150 to 1000 km across, 10 to 15 km high.

• 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 winds, thick clouds with torrential rain, thunder and lightning prevail.

• Away from the "Wall Cloud Region", the wind speed gradually decreases.

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• Once the cyclones reach higher latitudes they often change their direction and move north and then north-east (south and south east hemisphere). The process is known as recurreature.

• When two cyclones exist near to each other, they inter-act and move anti-clockwise with respect to each other.

• In India, when cyclones recur they get broken up over the Himalayas and their further eastward movement ceases.

Cyclone Map of India

Indian context

• The subcontinent with a long coastline of 8041 kilometre is exposed to nearly 10 per cent of the world’s tropical cyclones.

• 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.

• 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.

• 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

The principal dangers of a cyclone

• Gales and strong winds– damage installations, dwellings, communication systems, trees.,

etc. resulting in loss of life and property. • Torrential rain– may cause river floods

• Storm surges or high tidal waves– 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.

What is Storm Surge?

Surge prone coasts of India

• Vulnerability to storm surges is not uniform along Indian coasts.

• 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– 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.

How to avoid the catastrophe?

• 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.

Cyclone warning

• Two Stage Warning Scheme“– 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 "Cyclone Warning" is issued 24 hours in advance.

– Both cyclone "Alert" and "Warning" messages are passed to the AIR stations for repeated broadcast.

CYCLONES - Do's & Dont's

• 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

– Demolish condemned buildings – Keep some dry non-perishable food always ready

for emergency use

CYCLONES - Do's & Dont's

• When the Cyclone starts– Listen to the radio about weather warnings – Keep monitoring the warnings. This will help you

to prepare for a cyclone emergency.– Pass on the information to others. Believe in the

official information – Ignore rumours and do not spread them; this will

help to avoid panic situations.

When the Cyclone starts

– 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.

– 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.

– Provide strong suitable support for outside doors. – Keep torches handy

When the Cyclone starts

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

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

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

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

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

• Make provision for children and adults requiring special diets.

When the Cyclone starts

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

– 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

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

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• Post-cyclone measures – You should remain in the shelter until informed

that you can return to your home. – 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

When choosing a site for your house, consider the following

• In cyclonic regions close to the coast, a site above the likely inundation level should be chosen. In case of non availability of high level natural ground, construction should be done on stilts with no masonry or cross bracings up to maximum surge level, or on raised earthen mounds to avoid flooding/inundation but knee bracing may be used.

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No shielding from high wind due to absence of barriers

Shielding from high wind by permeable barriers such as strong trees

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• In hilly regions, – construction along ridges should be avoided

since they experience an increase of wind velocity –whereas valley experiences lower speeds in

general

Damaging Effects of Cyclone on Houses

Due to the high wind pressure and improper connection of the house to the footings it can be blown away.

Damaging Effects of Cyclone on Houses

Roofing materials not anchored can be blown away

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Light weight verandah roofs are more susceptible to damage due to high wind speed.

THANK YOU