2. "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.
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
known as Hurricane in the Atlantic and Eastem Pacific,
Typhoon in Western Pacific, Willy-Willies in
Australian sea, Baguis in the Philippines.
3. 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.
4. As adopted by
Department of India
S.No. Disturbance Wind Speed (Knots)
1. Low Less than 17.
2. Depression 17-27(32-50 km/h)
3. Deep Depression 28-33 (51-62 km/h)
4. Cyclonic storm 34-47 (63-88 km/h)
Severe cyclonic storm with
a core of Hurricane
1 knot - 1.85 km per hour
7. 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 gale winds, thick clouds with
torrential rain, thunder and lightning prevail.
Away from the "Wall Cloud Region", the wind speed gradually
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
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
8. 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.
Before it recurves, the speed decreases and the system remains
stationary for a day or so.
When two cyclones exist near to each other, they inter-act and
move anti-clockwise with respect to each other.
In the Atlantic, tracks often execute a parabola.
In India, when cyclones recur they get broken up over the
Himalayas and their further eastward movement ceases.
9. Cyclones derive their names through a systematic procedure laid down
by the World Meteorological Organisation (WMO) and the United
Nations Economic and Social Commission for Asia and the Pacific
naming of cyclones began in September 2004
Eight north Indian Ocean countries - Bangladesh, India, the Maldives,
Myanmar, Oman, Pakistan, Sri Lanka and Thailand - have prepared a list
of 64 names.
Since 2004, the eight countries have faced 20 cyclones.
The countries take turns in naming the cyclones. The last six were: Nargis
(Pakistan), Rashmi (Sri Lanka), Khai-Muk (Thailand), Nisha
(Bangladesh), Bijli (India) and Aila (Maldives).
11. Gales and strong winds
damage installations, dwellings, communication systems,
trees., etc. resulting in loss of life and property.
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.
13. Every cyclone that affects the coast produces a storm surge. But
not all storm surges rise to dangerous levels. The height of the
surge depends on:
The intensity of the cyclone - as the winds increase, the sea water is
piled higher and the waves on top of the surge are taller.
The forward speed of the cyclone - the faster the cyclone crosses the
coast, the more quickly the surge builds up and the more powerfully it
The angle at which the cyclone crosses the coast - local zones of
enhanced surge in areas such as narrow inlets and bays.
The shape of the sea floor - the surge builds up more strongly
if the slope of the sea bed at the coast is shallow.
Past history indicates that loss of life is significant when surge
magnitude is 3 metres or more and catastrophic when 5 metres
14. 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.
15. 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
16. "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.
17. Contrary to popular belief, few houses are blown over. Instead,
they are pulled apart by winds moving swiftly around and over
the building. This lowers the pressure on the outside and creates
suction on the walls and roof, effectively causing the equivalent of
Whether or not a building will be able to resist the effects of wind
is dependent not so much upon the materials that are used but the
manner in which they are used.
common belief: that heavier buildings, such as those made of
concrete block, are safer.
Truth: well-built and properly-engineered masonry house offers a
better margin of safety than other types of buildings
18. Due to the high
the house to the
footings it can
be blown away.
21. When cyclones
with heavy rain
for a long
buildings can be
damaged due to
22. Though cyclonic storms always approach from the direction of the sea towards the
coast, the wind velocity and direction relative to a building remain random due to
the rotating motion of the high velocity winds.
In non-cyclonic region where the predominant strong wind direction is well
established, the area behind a mound or a hillock should be preferred
Similarly a row of trees planted upwind will act as a shield.
The influence of such a shield will be over a limited distance, only for 8 – 10 times
the height of the trees.
A tree broken close to the house may damage the house also hence distance of tree
from the house may be kept 1.5 times the height of the tree.
23. No shielding from high wind due to
absence of barriers
Shielding from high wind by
permeable barriers such as strong
24. In hilly regions,
construction along ridges should be avoided since
they experience an increase of wind velocity
whereas valley experiences lower speeds in general
25. 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.
26. Shape is the most important single factor in determining the
performance of buildings in cyclones.
Simple, compact, symmetrical shapes are best.
The square plan is better than the rectangle since it allows high
winds to go around them.
The rectangle is better than the L-shaped plan.
This is not to say that all buildings must be square. But it is to say
that one must be aware of the implications of design decisions and
take appropriate action to counter negative features.
The best shape to resist high winds is a square.
27. If other shapes are desired, efforts should be made to strengthen the
If longer shapes are used, they must be designed to withstand the forces of
Most houses are rectangular and the best layout is when the length is not
more than three (3) times the width.
28. In case of construction of group of buildings, a
cluster arrangement can be followed in
preference to row type.
29. Lightweight flat roofs are easily blown off in
In order to lessen the effect of the uplifting
forces on the roof, the roof Pitch should not be
less than 22º.
Hip roofs are best
30. General Design Considerations
Avoid a low pitched roof, use a hip roof or a high
pitched gable roof.
Avoid overhanging roofs. If overhangs or canopies
are desired, they should be braced by ties held to the
Avoid openings which cannot be securely closed
during a cyclone
31. The foundation is the part of the house which
transfers the weight of the building to the
ground. It is essential to construct a suitable
foundation for a house as the stability of a
building depends primarily on its foundation.
It is desirable that information about soil type
be obtained and estimates of safe bearing
32. Effect of surge or flooding:-
Invariably a cyclonic storm is accompanied by torrential rain and tidal
surge (in coastal areas) resulting into flooding of the low lying areas.
Flooding causes saturation of soil and thus significantly affects the safe
bearing capacity of the soil.
In flood prone areas, the safe bearing capacity should be taken as half
of that for the dry ground.
Also the likelihood of any scour due to receding tidal surge needs to
be taken into account while deciding on the depth of foundation
Buildings on stilts:-
Where a building is constructed on stilts it is necessary that stilts are
properly braced in both the principal directions.
Knee bracings will be preferable to full diagonal bracing so as not to
obstruct the passage of floating debris during storm surge.
33. 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
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
34. When the Cyclone starts
Listen to the radio about weather warnings
Pass on the information to others. Believe in the official information
Remember that a cyclone alert means that the danger is within 24 hours.
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
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.
35. 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
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
36. 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
Clear debris from your premises immediately.
Report the correct loss to appropriate authorities