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CYCLONES AS NATURAL HAZARDS
By
Prof. A. Balasubramanian
Centre for Advanced Studies in Earth Science,
University of Mysore
Mysore

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Introduction:
Earth's Natural hazards include
earthquakes, volcanic eruptions, floods,
droughts, desertification, tsunamis and landslides.
In addition to these, Cyclones have also created
severe damage to the life and properties directly
or indirectly.
The occurrence of unpredictable violent storms
brings devastating effects to the coastal regions
and on islands located in their paths.

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High winds, heavy storms and intensive rainfalls
are all the powerful collective mechanisms of a
cyclone causing the damage.
In this episode, the following aspects of cyclone
are discussed:
1. Origin of Cyclones
2. Types of cyclonic storms and their physical
characteristics
3. Distribution of Cyclones
4. Environmental impacts of cyclones
5. Cyclone disaster Management.

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1. Origin of Cyclones
A Cyclone is defined as any large system of
winds that rotates about a centre of low
atmospheric pressure with a speed over 100
kmph.
This swirling action happens in a
counterclockwise direction north of the Equator
and in a clockwise direction to the south.

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As the sun warms the oceans, evaporation and
conduction transfer heat to the atmosphere.
This happens so rapidly that the air and water
temperatures rarely differ by more than a degree
F.
The water vapor generated by such evaporation is
the driving force for a tropical storm. As the vapor
condenses into clouds and precipitation, it releases
enormous amounts of heat into the cyclone.
This has indicated a major point that cyclones can
not develop when the ocean temperature is below
24 deg. C.

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The diameter of the ring of swirling winds may
range from 500 to 1500 km.
It is a highly dynamic movement and the direction
of its transgression is not precisely predictable.
The barometric pressure will be low at the centre
of this mass and that acts as a chimney through
which air rises, expands, cools dynamically and
produces precipitation after condensation.

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Anticyclones have a flow opposite to that of
cyclones.
-i.e., an outward-spiraling motion, with the winds
rotating clockwise in the Northern Hemisphere
and counterclockwise in the Southern
Hemisphere.
They are usually not as strong as the cyclonic
storms and bring no rainfall.
Both are regions of relatively low and high
pressure, respectively. They occur over several

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parts of the globe in a variety of sizes ranging
from larger to smaller dimensions.
Cyclones of a somewhat different character occur
closer to the Equator, forming in latitudes 10 to 15
N and S over the oceans.
They are an essential part of the mechanism by
which the excess heat received from the Sun in
the equatorial belt is conveyed toward higher
latitudes. These higher latitudes radiate more heat
to space than they receive from the Sun.

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Strong horizontal temperature gradients are
responsible for the formation of cyclones. These
are known as the tropical cyclones.
Those cyclones that form outside the equatorial
belt are known as extra-tropical cyclones.
The name of cyclone differs from region to
region.
The tropical cyclones are called
as hurricanes in the Atlantic and Caribbean,

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as typhoons in the western Pacific , Phillipines
and China , and as willy-willies off the coasts of
Australia.
These storms are of smaller in dimension than the
extratropical cyclones, ranging from 100 to 500
kilometres in diameter.
Based on the their velocities,
wind speeds are classified into various kinds.
According to the famous Beaufort wind scale,

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A Gentle breeze having a wind velocity of 12-19
kmph can move leaves and small twigs
A Strong breeze having a wind velocity of 39-
49kmph can make Large branches sway and make
it difficult to use umbrellas
A Moderate galehaving a wind velocity of 50-
61kmph, will make a person difficult to walk
against the wind.
A Whole gale having a wind velocity of 89-
102kmph, will uproot the trees
A Storm having a wind velocity of 103-
117kmph, will create a widespread damage

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A Hurricane having a wind velocity of more than
117kmph will lead to a violent destruction.
Tropical cyclones are accompanied by winds of
extreme violence. Storms usually occur with
cyclones. It is defined as a circular strom with
rotating wind speeds in excess of 32 metres per
second. The life span of a tropical cyclone is on
an average, six to nine days until it enters the
land. No tropical cyclone follows the same track.

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Falling atmospheric pressure generally indicates
the arrival of a bad weather. But cyclones do not
bring any such condition.
It is necessary to understand the role of Coriolis
force during cyclones.
It prevents winds from the North and South poles
and the equator from moving directly north or
south. Winds that blow toward the equator seem
to curve toward the west. Winds that move away
from the equator seem to curve to the east. The
Coriolis force also influences the direction of

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ocean currents. This has a profound effect on the
movements of cyclonic storms.
The Coriolis force is proportional to both the
latitude and the angular velocity (rotational speed)
of the Earth.
A tropical cyclone is likely to occur whenever
several of the following prerequisites occur
simultaneously:
(1) latitude sufficiently high (5-6) for the
Coriolis force to be appreciable;

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(2) a warm-water surface of at least 27 deg. C,
of sufficient area to supply the overlying air with
large amounts of vapour
(3) pronounced instability in the air column or
relatively low pressure at the surface and
(4) little or no vertical wind shear.
Due to strengthening of convection, the
centripetal wind flow gains speed.
Very soon, the angular velocity component of the
Coriolis force becomes sufficient to impart a

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definite cyclonic curvature to the air flow,
resulting in a cyclone The input of warm, very
moist air continues.
Large-scale condensation of moisture occurs
during the ascent, and enormous amounts of
previously accumulated latent energy are released.
This energy results in stronger winds, which in
turn lead to the intake and uplift of larger amounts
of humid air, with a further release of energy.
The evolution from a tropical depression to a
violent tropical cyclone takes four to eight days.

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Sometimes, strong vertical wind shear (jet
streams) would impede convection and prevent
the development of the cyclone.
2. Types of cyclonic storms and their physical
characteristics
Cyclones may be either warm-core or cold-core
types.
Warm-core cyclones are warmer at the centre than
near the edges. They are fairly shallow and
become weaker in the upper atmosphere. They
often occur over especially warm land areas.

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Cold-core cyclones are coldest near the centre and
warmer near the edges.
These cyclones may be very deep, and are more
intense several thousand feet in the air than they
are at the surface of the earth.
All cyclones have two characteristics as
one is the atmospheric pressure which is lowest at
the centre, and the other one is the inward
spiraling of winds.
The cyclonic storms may bring winds upto 290
kilometres an hour alongwith terrific rains,

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violent thunder, and lightning. They measure 320
to 480 kilometres across.
Hurricanes:
A Hurricane is a powerful, whirling storm that
measures 320 to 480 kilometres in diameter. The
winds near the centre of a hurricane blow at
speeds of 120 kilometres per hour or more. Many
hurricanes have caused widespread death and
destruction.

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The strongest winds and heaviest rain of a
hurricane occur within its wall clouds.

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Hurricanes occur in the North Atlantic and North
Pacific oceans from June to November--most of
them in September. On an average, about six to
eight hurricanes form in these regions.
Tornadoes:
A Tornado is a powerful, twisting windstorm.
These are the most violent winds that occur on the
earth. They whirl around the centre at more than
320 kilometres per hour speed.

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Most of them measure several hundred metres in
diameter. It becomes a rotating funnel cloud that
extends downward from a mass of dark clouds.
Some of them do not reach the earth.
Others may strike the surface of the earth,
withdraw into the dark clouds above, and then dip
down and strike the earth again. It happens in the
United States, and in parts of Australia. Most
tornadoes last less than an hour.

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Some of them last several hours and measure up
to about 2.5 kilometres in diameter.
Statistics show that about 700 tornadoes have
been reported annually in the United States since
the mid-1950's.
Tornadoes can uproot large trees, overturn
railway carriages, and carry cars hundreds of
metres away.

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Typhoons:
A Typhoon is a violent, low-pressure tropical
storm that occurs in the western Pacific Ocean.
They are similar to hurricanes, beginning near the
equator and move westward. They advance
slowly, usually at about 15 to 25 kilometres per
hour speed.
The circular winds around the centre are very
strong, often reaching speeds of 240 kilometres
per hour. The diameter of a typhoon can be as
large as 480 kilometres.

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Typhoons are associated with heavy rains and
powerful winds causing severe land and property
damage and loss of life. The destructive rush of
seawater, called a storm surge, often accompanies
a typhoon as it moves onto coastal lands.
One of the Cyclonic storm surges which occurred
in the east coast of India during 1977, was
reported to have been 5.7 m high, 80 km long, 16
km wide with a speed of 190kmph.

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3. Distribution of Cyclones
Cyclonic winds move across nearly all regions of
the Earth except the equatorial belt. Most of them
are generally associated with heavy rain or
snowfall.
Cyclones occur chiefly in the midlatitude zones of
both the hemispheres.

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In the Southern Hemisphere, where most of the
terrestrial surface is covered by the oceans, they
are distributed in a relatively uniform manner
through various longitudes.
As trade winds, certain tracks are favoured by
these wind systems also.
The principal tracks lie over the oceans. The
ocean surface being smoother than that of the
land, offers less resistance to the strong winds to
move around.

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Extratropical cyclones
The extratropical cyclones range from nearly
1,000 to 4,000 kilometres. Whereas, the tropical
cyclones measure only about 100 to 500
kilometres in diameter.
They are more violent than those occurring in the
midlatitudes and can cause considerable damage.
These are transient cyclones. They are the most
abundant phenomena and exert influence on the
broadest scale, affecting the largest percentage of
the Earth's surface.

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It has an impact on the day-to-day weather
changes.
The life cycle of these events is typically several
days, during which the cyclone may travel from
several hundred to a few thousand kilometres.
In its path and wake occur dramatic weather
changes.

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4. Environmental impacts of cyclones
Cyclones have a destructive power. They attack
coastal regions with sever storms and rainfall.
Activated by the rainstorms, the can easily
remove huts, small shelters, trees and plants, boats
and other vehicles.
Every storm changes the distribution pattern of
surface water bodies, groundwater systems, affect
the growth of freshwater crops, disrupt the
normal cycle of life and occupation.

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It has been noticed that due to these, several
villages have been washed away, and sealed off
by sandcasting.
Flooding, storm surges, sandcasting, erosion and
deposition of silt are the impacts of these
processes.
A storm surge can rise several feet above the
normal ground and cause floods. It is very
destructive if it occurs at high tide level.

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When a hurricane moves over land, strong winds
and heavy rain hit the area for several hours.
It weakens as it moves over the land, but the
heavy rain continues even after the winds
decrease.
The winds may exert a pressure of more than 400
kilograms per square metre on tall structures and
can flatten weak buildings at first impact.
Tropical cyclones can cause immense damage,
both directly (by wind, pressure, and rain) and
indirectly (mainly through storm surges and
floods).

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Roofs and windows are damaged by the suction
produced by strong winds on the downwind side.
All loose objects are lifted by the wind. It also
causes injuries and deaths by toppling structures
and hurling loose and torn off objects with
enormous force.
Torrential rain may erode the soil, causing
landslides in mountainous regions and making the
streams and reservoirs overflow.

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These may, inturn, cause floods. The indirect
damages of these are mostly due to storm surges.
During these periods, the sea level is raised by up
to three to four metres for a period that may last
several hours. Notable and extreme tides have
been recorded on the Gulf Coast due to
hurricanes. Pounding waves are an additional
cause of damage to coastal installations and
structures.

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One of the impacts of this is the coastal erosion.
Marshes, estuaries and lakes are affected in the
same way. Severe flooding is a combined process
of the cyclones in coastal zones.
5. Cyclone disaster Management.
The Cyclone Intensity to damage potential has
been proposed by the National Hurricane Centre
of the United States. This is based on three storm
intensity classifications as, minimal, major and
extreme cyclonic events. :

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a) Minimal cyclone – 120 to 160 kmph with a
maximum central pressure of 983-996 millibars
b) Major cyclone – 161 to 220 kmph with a
maximum central pressure of 949-982 millibars
c) Extreme cyclone – 221 and more kmph with a
maximum central pressure of 948 or less
millibars
Most of the tropical cyclones are a seasonal
phenomenon with their frequencies varying from
an average of one per year in some places to as
many as 20 per years.

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It occurs in the north Pacific any time of the year.
Warning and tracking of tropical cyclones are
essential tasks of the people. Cyclone warning can
help people to vacate the zones of danger and
prepare for advance relief measures.
Attempts to forecast tropical cyclones date back to
the nineteenth century.
The basic criteria to indicate a new cyclone is
a) Occurrence of a sub-normal pressure in the low
latitudes and
b) above normal pressure in high latitudes

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c) disturbance of atmospheric winds,
d) erratic tides, and
e) swelling of seas.
Because of the small diurnal pressure changes
common in the low latitudes, it is difficult to
single out changes due to approaching cyclones.
A local drop in pressure of more than three to four
millibars within a day (or over a distance of 500
kilometres or less) may be considered as a danger
sign.

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The growth of distinct curved isobars moving
toward the land is a much more definite indication
of an approaching cyclone. The roughly circular
shape of the isobars can reveal the position of the
cyclone's centre.
Humidity is another indicator of these incidents. It
is normally high and is a notable and useful
indicator. Cyclone and pressure Monitoring ships
in the oceans and radar units provide warning of
any storm within the range. Earth observation
satellites transmit live images of any part of the
globe and its cloud systems.

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Methods to protect the environment against
tornadoes are a permanent requirement in some
places.
Weather monitoring stations obtain information
from local observers, radar stations, and the pilots
of aircraft when weather conditions appear to be
dangerous.
The receiving stations issue warning messages to
the people in the area of the tornado.
People have built mobile homes as a precaution.

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Global telecommunication systems and global
data communication systems will certainly help
people to understand the occurrences of these
events. Every country having a long coastal belt
has to establish a cyclone warning network and
research stations.
Wind tunnel research is a major activity to
propose cyclone resistant houses and structures. It
is very difficult to protect the vegetation without
constructing sea walls and shelter belts.

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Ports and harbours need frequent maintenance
operations and hence they need to evolve certain
precautionary measures. The origin of a cyclone
may be spotted out using a satellite image atleast
two days in advance. This is one of the major
advantages of organising precautionary measures
and relief related activities.
A cyclone will have immediate and long-term
impacts. Continuous and frequent incidences may
cause a severe decline in the economy of a region
and its country.

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Whenever a natural disaster occurs, it will
certainly cause an economic crisis, starvation,
epidemic, migration, landlessness, homelessness,
orphanage to some children and massive deaths.
Disruption of road networks, railway lines and
power supply are notable impacts. Homeless
people need to be provided with appropriate
shelters, food, clothing and financial support to
sustain and recoup their life.

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Shortage of fuel and food need immediate
attention. Control of disease vectors and medical
treatments are to be planned in advance.
Cyclones disrupt agriculture mainly and destroy
the crops. Coastal zone classification, building
norms, indoor and out door safety measures and
disaster preparedness are the major factors to be
considered.
Awareness programmes should be carried out to
enlighten the issues in all offices, villages forums
and in all schools and colleges.

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Education is a primary mechanism of disaster
management.
Disasters are recurring features over the globe.
There is no place on earth which is free from
these types of natural calamities. It is very
essential to protect the life mainly from these
dangers.
Strong will and sound scientific predictions can
help us to save the souls.