2. What areCyclones?
• "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.
• Cyclones are intense low pressure areas - from
the centre of which pressure increases
• 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.
4. • 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.
5. • Cyclones that developin the regions between
the tropics of Capricorn and Cancer are called
• Tropical cyclones are large-scale weather
systems developing over tropical or
subtropical waters, where they get organized
into surface wind circulation.
6. • 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.
7. • 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.
10. • 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 tides
11. Exceptional rainfall occurrences
• The very high specific humidity condenses into
exceptionally large raindrops and giant
cumulus clouds, resulting in high precipitation
• 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.
12. How cyclones are formed
• i) Formation and Initial Development Stage
• ii) Mature Tropical Cyclones
• iii) Modification and Decay
13. i) Formation and Initial Development
• 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
• Atmospheric instability encourages formation
of massive vertical cumulus clouds due to
convection with condensation of rising air
above ocean surface.
14. ii) Mature Tropical Cyclones
• When the tropical storm intensifies, the air rises in vigorous
thunderstorms and tends to spread out horizontally at the
• 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.
15. 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.
17. Mechanismof cyclones
• A full-grown cyclone is a violent whirl in the
atmosphere 150 to 1000 km across, 10 to 15
• 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.
• 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
• When two cyclones exist near to each other, they
inter-act and move anti-clockwise with respect to
• In India, when cyclones recur they get broken up
over the Himalayas and their further eastward
20. 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
• On an average, five to six tropical cyclones
form every year, of which two or three could
21. • 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.
22. • 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
• 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.
23. • 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.
24. • 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.
25. • 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;
• The most fatalities come from storm surges
and the torrential rain flooding the lowland
areas of the coastal territories
26. The principaldangersof 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
28. Surgeprone coastsofIndia
• Vulnerability to storm surges is not uniform along Indian
• East coast of India are most vulnerable to high surges
– i) North Orissa, and West Bengal coasts.
– ii) Andhra Pradesh coast between Ongole and
– 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.
29. How to avoidthe 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
– 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
30. 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.
31. 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
32. 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
– Ignore rumours and do not spread them; this will
help to avoid panic situations.
33. 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
– Provide strong suitable support for outside doors.
– Keep torches handy
34. 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
• When your area is under cyclone warning get away from
low-lying beaches or other low-lying areas close to the
• 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
35. When the Cyclone starts
– Be sure that a window and door can be opened
only on the side opposite to the one facing the
– 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
– Switch off electrical mains in your house.
– Remain calm
36. 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
• Remain in the shelter until you have been informed to
• 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
38. When choosing asite foryour house,consider
• 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.
No shielding from high
wind due to absence of
Shielding from high wind by
permeable barriers such as
• In hilly regions,
–construction along ridges should be avoided
since they experience an increase of wind
–whereas valley experiences lower speeds in
41. 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.