TOPIC: -

TROPICAL CYCLONES.

PAPER NAME: - ADVANCED CLIMATOLOGY

SUBJECT: - GEOGRAPHY
SEMESTER: - M.A –II
PAPER CODE: - (GEO-201)
UNIVERSITY DEPARTMENT OF GEOGRAPHY,
DR. SHYMA PRASAD MUKHERJEE UNIVERSITY, RANCHI.
INTRODUCTION: -

We all agree to the fact that USA is the super power of the world, but the super power fades
when tropical cyclones strike it occasionally. This shows the power of tropical cyclones.
Tropical cyclones represent a circulatory motion of air towards a low pressure center. These
cyclones are known by various names on the basis of characteristics, intensity and distribution.
The origin and development of tropical cyclones requires presence of some favorable conditions.
They are associated with violent winds and heavy rainfall and represent destructive weather
phenomena with great loss of life and property. In this module, focus is on tropical cyclones –
their genesis and development, distribution and associated weather conditions.

TROPICAL CYCLONES: -
Tropical weather conditions prevail well beyond the Tropics of Cancer and Capricorn due to
shifting of overhead position of sun. About half of the earth’s surface and three-fourth of the
world’s population is concentrated in latitudinal zone between 300 N to 300 S. The low pressure
centre-ward circulatory motion and associated weather disturbances of this region are known as
tropical cyclones. Generally, they have circular isobars, high velocity of winds and anticlockwise
circulation in northern hemisphere. They generally originate on the western side of oceans within
the tropics. Most of the tropical cyclones originate in the shifting doldrums region. Therefore, in
Northern Hemisphere they are most frequent in August-October and in Southern Hemisphere
during March-April. Tropical cyclones are not regular, predictive and uniform like temperate
cyclones. There is a great variety of shape, size, wind velocity and weather conditions. These
cyclones are known by various names on the basis of characteristics, intensity and distribution.
 Fig. 1: Typical Isobars in a Tropical Cyclone Development

CLASSIFICATION OF TROPICAL CYCLONES: -

The tropical cyclones are so diverse in size, characteristics and weather conditions that no two
cyclones are similar and therefore, it is tough to categories them in certain comprehensive
classes. However, the tropical cyclones are normally classified into the following four types.

TROPICAL DISTURBANCE: -
These are migratory wave-like cyclones which move east to west due to easterly trade winds.
Thus, they are also popular as easterly waves. About 80 per cent of these disturbances occur
between 50 and 200 N latitudes on the western side of the oceans. It has been noted that these
disturbances originate near the boundary of trade winds with doldrums(ITCZ). They move
westward at slow pace and are always accompanied by clouds mainly cumulus and
cumulonimbus and moderate to heavy rainfall. A good weather before the disturbance is the
general characteristic and it is followed by bad weather. Initially weather is fine and scattered
cumulus clouds appear. It is followed by occasional rainfall, and finally by moderate to heavy
rainfall from heavy cumulus and cumulonimbus clouds occurs. Occasionally, due to intense
instability they may emerge as hurricanes.
TROPICAL DEPRESSIONS: -
These are small size low pressure centres encircled by more than one closed isobars. The wind
velocities are highly variable, but on an average remain about 40-60 km per hour. They occur
most frequently in inter tropical convergence zone (ITCZ) and are rare in the zone of trade
winds. They move in different directions. In summer season these disturbances influence the
weather conditions of India and Australia. These depressions normally fail to attain the size of a
storm and die out as weak disturbances.

TROPICAL STORMS: -
These are low pressure centres encircled by closely placed isobars and have wind velocities in
the range of 63 to 118 km per hour. They are common in the Bay of Bengal, Arabian Sea,
Caribbean Sea and in the vicinity of Philippines, especially during summer season. They are
associated with heavy rainfall and storm surges in coastal areas. They frequently develop into
more destructive type of tropical cyclones.

Table 1: Classification of Tropical Cyclones

Name or Winds Characteristics

Designation Velocity
Tropical Variable Definite area of surface low pressure, originate near the
Disturbance and low boundary between trade winds and doldrums, life span one to
two weeks and travel about 60 to 70 longitude per day,
cumulonimbus clouds, as easterly waves move east to west.
Tropical Up to 63 kmph Originate in ITCZ, more than one closed isobars, gale force,
Depression organising circulation, light to moderate rain.
Tropical 63-118 kmph Closed isobars, definite circular organisation, heavy rain and
Storm storm surge, assigned a name.
Hurricane/ >119 kmph Circular closed isobars, steep pressure gradient, heavy rain,
Cyclone/Typho storm surges, tornadoes in right front quadrant, well established
on/Willy eye, eye wall and spiral bands of vertical clouds
willies/Taifu
HURRICANES OR TYPHOONS: -

Tropical cyclones are warm vortex circulatory wind systems of tropical origin with closed
circular isobars. They have sustained maximum winds of at least 119 kmph and torrential rains.
They are known by different names in different parts of the world. Hurricane represents most
powerful and destructive tropical cyclone. This term is used for tropical cyclones of the
Caribbean Sea and Gulf of Mexico region. The pressure gradient is very steep and at centre
isobaric value may be 950 mb and pressure variation between centre and fringe may be 50 to 60
mb (Figure 2). Therefore, wind velocities are very high, with a minimum of 119 km per hour.
The tropical cyclones of hurricane level force are called typhoons in the western North Pacific
Ocean. They are known as willy willies in Australia, cyclones in Indian ocean, baguio in
Philippines and taifu in Japan. Indian Meteorological Department (IMD) uses the term ‘Severe
Cyclone Storm’ when the wind velocity exceeds 63 kmph and when above 119 kmph, it is called
‘Severe Cyclone Storm with a core of Hurricane Winds’. Occasionally, when tropical cyclones
have wind velocity above 200 kmph the term ‘Super Cyclone’ is used, for instance, the Super
Cyclone of Odisha, October 29, 1999.

Fig. 2: Hurricane
 ORIGIN OF TROPICAL CYCLONES: -

The exact mechanism of origin and development of a tropical cyclone is still not well
understood, but scholars have identified some conditions which are essentially associated with
their formation. The formation of tropical cyclones depends upon fulfilment of the following
requirements.
(i) CONTINUOUS SUPPLY OF LARGE AMOUNT OF WARM AND MOIST AIR: It is a well observed
fact that tropical cyclones originate only over large tropical ocean surface where temperature is
270C or above. The cold ocean currents lower the surface temperatures below this desired level
on the eastern sides of the tropical oceans. Therefore, tropical cyclones originate in the western
side of tropical oceans. As this condition is not fulfilled in the South Atlantic Ocean they fail
to grow. It is noteworthy that this high temperature condition should prevail not just on surface
but upto a depth of 60-70 metres deep from sea surface. Otherwise, convection process beneath
cyclone will drag cooler water to surface and heat supply which is essential for cyclone will
break. The power of a tropical cyclone depends on latent heat of condensation and thus,
indirectly on supply of warm and moist air. Therefore, tropical cyclones are most frequent over
warmer tropical oceans and especially during warmer parts of the year.

(ii) STRONG CORIOLIS FORCE: Although the temperature conditions are favorable at
equator but tropical cyclones are absent. This is due to absence of coriolis force at equator
which is a prerequisite for circulatory motion of air towards a low pressure centre,
anticlockwise in northern hemisphere and clockwise in southern hemisphere. It is only at 50
latitudes that the minimum required level of coriolis force for deflection of winds prevails.
Therefore, tropical cyclones are mostly concentrated in the belt of 50 to 300 latitudes. Though
the coriolis force increases from equator (zero) to pole but the number of tropical cyclones
does not follow this trend because the surface temperature of the oceans starts decreasing.
Therefore, the tropical cyclones originate and develop most frequently between 100 and
200latitudes. Greater coriolis force is still more in the areas of increasing latitudes but the
decline in the sea surface temperature does not allow the tropical cyclone to develop. It is also
clear that factors do not operate in isolation rather together they determine origin and
development of tropical cyclones.
 (iii) UPPER LEVELAIR DIVERGENCE: The upper troposphere (8 to 15 km) just
above the surface disturbance must have well developed divergence or anticyclone
circulation to pump out the ascending air currents and to maintain continuous supply
from below. This sustains convergence at surface and lifting mechanism in the
cyclone which are its lifeline.

(iv) MINIMAL VERTICAL WIND SHEAR: Wind shear represents the differences
between wind speeds at different altitudes. The ideal condition for tropical cyclone
formation is of minimal vertical wind shear between lower and upper troposphere. In
case, different wind directions and speeds operate vertically over an area, the latent
heat carried aloft would be swept away and a circulatory core ascending area i.e.
cyclone will fail to develop. For instance, in summer season over large parts of South
Asia including India, cyclone formation is least active in July and August when
surface monsoon winds and upper air easterly jet streams prevail simultaneously.
Likewise, the subtropical jet streams limit the extent of tropical cyclones towards
temperate areas.

(v) EXISTENCE OF MILD TROPICAL DISTURBANCES: As mentioned in the

types of tropical cyclones that the weak tropical disturbances such as easterly waves
may occasionally develop into a large tropical cyclone. This happens when abundant
warm and moist air results into the formation of intense column of latent heat induced
instability. In addition to mild tropical disturbances, tropical cyclones develop around
small atmospheric vortices in the ITCZ.

(vi)

STRUCTURE OF TROPICAL CYCLONE: -

From the discussion so far, it is clear that tropical cyclones are circulatory motion towards low
pressure centres. Winds circulation in a cyclone is anticlockwise in northern hemisphere and
clockwise in southern hemisphere. Structure of a typical tropical cyclone of hurricane level
shows six distinct regions. These six regions from core to periphery are – (i) the eye of the
cyclone, (ii) the eye wall, (iii) the spiral bands, (iv) the annular zone, (v) the outer convective
band, and finally (vi) the trade winds cumulus.
The most important structural characteristic of tropical cyclone is the central part known as the
eye of the cyclone. It is small central area of calm winds, clear skies and elliptical to circular
shape. The eye wall is basically wall of vertical clouds i.e. cumulonimbus clouds. The eye wall
region has the strongest winds and heaviest precipitation (Figure 3). The spiral shaped bands in
satellite images give galaxy type appearance to cyclones (Figure 4). These rotating bands are
associated with heavy precipitation, thunder and lightning. In the outer region of cyclone,
subsidence tendency results into the formation of the annular zone. Here cloudiness is less
extensive and high temperature and low humidity conditions prevail. This zone is surrounded by
the outer convective band. Trade wind cumulus clouds constitute the outermost fringe of cyclone
around which normal atmospheric conditions prevail.

Fig. 3: Structure of Tropical Cyclone

 Fig.4: Spiral Bands of Clouds and Eye of Cyclone

CHIEF CHARACTERISTICS OF TROPICAL CYCLONES: -

The chief characteristics of tropical cyclones are:

(i) Tropical cyclones mostly develop in tropical oceans between 50 and 200 latitudes,
except the South Atlantic and the eastern South Pacific Ocean.
(ii) A great variety of tropical cyclones can be identified on the basis of shape, size and
associated weather conditions. For instance, the average diameter of tropical cyclone
is in the range of 80 to 300 km, but some have diameter of only 50 or even less km
and others such as hurricanes in the range of 500-600 km;
(iii) The isobars are normally circular and closely placed, which means pressure gradient
is steep and consequently the wind velocities are high. In case of hurricanes wind
velocities of 120 to 240 km per hour are common and destructive;
(iv) The pressure gradient in hurricanes may be upto 50-60 mb (between eye and
periphery). As per international standards, a hurricane must have at least 119 kmph
sustained wind speed (averaged over a one-minute interval) and rotary wind
circulation.
 (v) They move in fairly well frequented tracks, initially they move westward under the
influence of prevailing trade winds upto 150 latitude, after that they turn poleward
(150 to 250) and finally turn towards east;
(vi) They are very strong over the oceans and travel over great distances but they become
weak as they start moving overland and cold water. This is due to break in the
continuous supply of warm and moist air;
(vii) Almost circular centre is the most characteristic feature of tropical cyclone which is
called its eye. On an average, it has a diameter around 8 to 50 km. This well-known
feature is associated with clear sky, calm and descending air, witnesses no
precipitation.
(viii) Inward flow of warm and moist air approaches the central part towards the ground
and it turns upward and ascends in a circulatory ring of vertical cumulonimbus
clouds. This doughnut shaped wall of intense convective process surrounding the core
is known as the eye wall. It is associated with strongest winds and heaviest rainfall.
(ix) At the top of the cyclone the air flow is outward, carrying the ascending air away
from centre, thereby providing room for more inward (near surface) and upward flow.
(x) The eye wall of the cyclone is surrounded by the curved bands of clouds that are
spread in a spiral manner.
(xi) The strong winds of cyclones blow anticlockwise in northern hemisphere, they spiral
inward and increase in velocity towards centre, as the eye arrives over an area winds
decrease or cease temporarily for some time but again intensify to the previous levels.
The wind direction becomes reverse to the previous phase of the cyclone.
(xii) These cyclones are energised by the release of latent heat of condensation and fusion
(sublimation) associated with cumulus and cumulonimbus clouds. The release of
latent heat warms the upper air and provides buoyancy for its ascending motion and it
reduces the pressure near the surface. This promotes a more rapid inflow of air from
the base. To get this heat engine started, large amount of warm and moist air is
required which is provided by tropical oceans with surface temperature 270C or
above,
(xiii) Tropical cyclones are natural disasters and result in loss of life and property due to
storm surge, destructive winds and heavy rainfall, and finally
(xiv) Tropical cyclones decay or diminish in intensity due to their movement: – (a) over
cool or cold ocean waters, (b) move over landmasses and gets weakened by surface
friction, or (c) interact with cool air or cold air mass which makes large scale flow
aloft difficult due to higher density. Therefore, the main cause for the dissipation of
tropical cyclones is discontinuation of supply of large warm and moist air.

DISTRIBUTION OF TROPICAL CYCLONES: -

Tropical cyclones are confined to tropical areas. The origin and development frequency of
tropical cyclones in the latitudinal range of 30-100 is about 22 per cent, in 100-200 about 65 per
cent and beyond this, it is about 13 per cent (Figure 5). At equator and near equator they are
absent due to absence of Coriolis force and occurrence frequency is at peak in the central parts of
tropics, and it again decreases due to absence of warm water surface with 270C temperature. The
six major source regions of tropical cyclones are:
(i) North Atlantic (western tropical part): Mainly over Caribbean Sea and Gulf of
Mexico and maximum frequency is during August to October;
(ii) Indian Ocean: Bay of Bengal and Arabian Sea, with two maxima one in May and
another in October-November;
(iii) South Indian Ocean: The area extending from Madagascar and Reunion islands upto
900 E longitude and Timor sea in north-western Australia and mainly during January
to March;
(iv) North Pacific Ocean (eastern tropical part): Over western coastal areas of Mexico
and Central America upto California coast and maximum occurrence is during
August to October;
(v) North Pacific Ocean (western tropical part): This region has maximum occurrence
of cyclones in the world with maximum frequency in August and September and
includes mainly, Philippines, China sea and areas around Japan;
(vi) South Pacific Ocean (western tropical part): east coast of Australia, in and around
Samoa and Fiji Islands and around the Coral Sea region and majority occur during
January to March.
Fig. 5: Distribution of Tropical Cyclones
DESTRUCTION BY TROPICAL CYCLONES: -

Tropical cyclones are one of the most destructive disasters, and they are put under
meteorological disaster category. They result in great loss of life and property mainly in coastal
areas. The damage caused by tropical cyclones is mainly associated with three actions – storm
surge, destructive winds and inland freshwater (rain water) flooding (Figure 6A and 6B). The
tropical cyclones are characterised by storm surge, destructive high velocity winds and
exceptionally high rainfall causing flooding. Wind speed of upto 200 kemp and above, rainfall of
30 to 50 cm per day for several consecutive days and storm surge of 5 to 10 m high are not
uncommon. Storm surges are abnormal rise of seawater in coastal areas due to tropical cyclone.
It represents the height of water above the normal tide level and it is greatly amplified by local
factors such as – shallow coastal water, estuarine or funnel shape of coastal region, fall in
pressure over sea surface, tides, winds, the angle and speed with which the cyclone approaches
the coast. Great storm surges are hazardous to shipping, drowning of human beings and animals
along with destruction of vegetation and quantitative and qualitative loss of soil cover. High
velocity winds in tropical cyclones damage installations, dwellings, natural vegetation and crops
and communication systems. The third significant damaging process is flooding associated with
torrential rainfall during cyclones. On the basis of the analysis of past storms, the Saffir-Simpson
scale has been constructed to rank the hurricanes or tropical cyclones according to intensities in
five classes.

MANAGEMENT OF TROPICAL CYCLONES: -

The mitigation strategies of tropical cyclonic disasters focus on integrated use of structural and
non-structural measures. The structural measures include – construction of cyclone shelters,
cyclone resistant buildings, sea dykes and embankments, canals, drains and surface water tanks
safe transport and communication network. Non-structural measures include cyclone warning
and forecasting, disaster risk management and capacity building by involving all stakeholders.
The advance warning is the most effective way to reduce disastrous effects of tropical cyclones.
There are many techniques to generate data to detect and track tropical cyclones. Doppler
weather radar, aircraft reconnaissance and latest and the most advanced is meteorological
satellites to generate information to develop forecasts and to issue warnings.
CONCLUSIONS: -
Tropical cyclones represent circulatory motion of winds towards a low pressure Centre. These
cyclones are known by various names on the basis of characteristics, intensity and distribution.
At macro level on the basis of intensity, they can be classified into two categories – (i) Weak
cyclones – tropical disturbances and tropical depression; (ii) Strong and furious cyclones –
tropical storms, hurricanes or typhoons and tornadoes. The exact mechanism of origin and
development of a tropical cyclone is still not well understood but scholars have identified some
conditions which are essentially associated with their formation. These are – (i) There should be
continuous supply of large amount of warm and moist air – which means large tropical ocean
surface where temperature is 270C or above; (ii) Strong coriolis force is a prerequisite for
circulatory motion of air towards a low pressure centre, (iii) Upper level outflow or divergence;
(iv) Minimal vertical wind shear; and (v) the existence of mild tropical disturbances.
The most characteristic feature of a tropical cyclone is eye of the cyclone and circular isobars.
These cyclones are energised by the latent heat of condensation. The tropical cyclones of
hurricane level force are called typhoons in the western North Pacific Ocean. They are known as
willy willies in Australia, cyclones in Indian ocean, baguio in Philippines and taifu in Japan. The
occurrence frequency of tropical cyclones in the latitudinal range of 30-100 is about 22 per cent,
in 100-200 about 65 per cent and beyond this, it is about 13 per cent. They are absent at and near
equator. There are six major source areas of tropical cyclones which are mainly concentrated on
the western side of tropical oceans. The tropical oceans of South Atlantic and eastern South
Pacific are the exceptions, where they do not occur.
Tropical cyclones are meteorological disasters and damage caused by them can be divided into
three process associated with them namely, storm surge, destructive winds and flooding due to
torrential rainfall. Integrated use of structural and non-structural measures is required to
minimize the loss of life and property caused by tropical cyclones.