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Tropical and Temperate Cyclones / Anti-cyclones
Cyclones are those atmospheric disturbances which have a low pressure at the centre and
high pressure on the surrounding sides so that the inward movement of the air takes place.
They involve closed air circulation around a low pressure centre. The movement of the air
may be both clock-wise and anti-clockwise. It is anti-clockwise in the northern hemisphere
and clockwise in the southern hemisphere. The centre of the cyclone is known as the eye of
the cyclone while as the surrounding area is known as the periphery area. The path along
which the cyclone moves is called the trajectory of the cyclone. Sometimes, when the cyclone
attains a tremendous speed, they are referred to as super-cyclones. Cyclones are associated
with destructive and violent disturbances, such as heavy squalls and torrential rainfall.
Cyclones play an important role in the complex process of heat exchange between various
latitudinal zones. They lift up the moist air from the oceans and take it to the surrounding
landmasses where precipitation occurs.
Tropical Cyclones
As the name indicates, these are those cyclones which originate in the tropical regions
extending from 00 – 300 N and S of the equator, i.e., at very low latitudes. As a result,
abundant water vapour and latent heat are present to fuel these systems. About 80 tropical
cyclones develop around the world every year, with the strongest becoming extremely
powerful storms that, depending on the region, are called hurricanes, typhoons, or cyclones.
The tropical cyclones in the Atlantic Ocean or eastern Pacific Ocean with a wind speed of
greater than 73 mph is called a hurricane. A storm of similar strength in the western Pacific
Ocean is called a typhoon and if it develops in the Indian Ocean it is simply called a cyclone.
Tropical cyclones are generally thermal in origin and their development needs the following
conditions:
a. Quiet air, b. High saturated atmosphere and c. Intense heat.
Such conditions are prevailing in the tropics mainly over the oceans. The trade winds meet
each other near the equator and the meeting zone is known as inter-tropical convergence zone
(ITCZ). At ITCZ, intense heating takes place and the air rushes upwards. In order to fill the
gap, winds from the surrounding regions move towards the low pressure centre. This attains a
circular shape and gives birth to a tropical cyclone. These cyclones are highly unpredictable
as the thermal effect over the ocean and coastal areas changes very rapidly.
Lecture Delivered by: Mr. Manzoor Jahangir
Tropical and Temperate Cyclones / Anti-cyclones
Favourable breeding grounds: Following are the favourable breeding grounds of the
tropical cyclones:
1. South-east Caribbean region (where tropical cyclones are called Hurricanes).
2. Philippines islands, eastern China and Japan (where tropical cyclones are called
Typhoons).
3. Bay of Bengal and Arabian Sea (where tropical cyclones are called Cyclones).
4. South east African coast
5. North-west Australia.
Shape, size and wind velocity: Tropical cyclones have symmetrical elliptical shapes (2:3
ratio of length and breadth) with steep pressure gradients. Size of tropical cyclones varies
considerably. On an average, their diameter ranges from 80 km to 300 km, but sometimes
they become so small that their diameter is restricted to 50 km or even less. They move with
varying velocities. Weak tropical cyclones move at the speed of about 32 km per hour while
hurricanes attain a speed of 180 km per hour.
Structure: The centre of a tropical cyclone (eye) is characterised by a patch of clear sky with
quiet conditions. There are cirrus clouds surrounding the centre and dark nimbus clouds
further away from the centre which causes occasional torrential downpour and thunderstorms.
The right hand back corner of the cyclone gets heavy rainfall while the left hand back corner
has clear weather as the trough passes.
Associated weather: The approach of a tropical cyclone is marked by cirrus clouds and then
dark nimbus clouds cover the sky and heavy downpour begins. The precipitation is evenly
distributed around the centre and continues till the arrival of tail with clear weather. But just
before the tail comes, there is lightning and thunder accompanied by calm, oppressive
weather. The after effect is felt in the form of fall in temperature and rise in pressure.
Temperate Cyclones
Temperate cyclones are active in the mid-latitudinal region, between 350 and 650 latitude in
both the hemispheres. These are also called as extra-tropical or wave cyclones. There are two
theories of the origin of temperate cyclones. These are the polar front theory by Bjerkenes
and thermodynamic theory by Lampert and Shaw. According to polar front theory, the
warm humid air masses from the tropics meet the dry cold air masses from the poles and a
polar front is formed. The cold air pushed the warm air upwards from underneath. Thus a
Lecture Delivered by: Mr. Manzoor Jahangir
Tropical and Temperate Cyclones / Anti-cyclones
void is created due to decrease in pressure. The air from surroundings rushes to occupy this
void and, coupled with the earth’s rotation, a temperate cyclone gets developed. According to
thermodynamic theory, the overcrowding of vertical currents in sub-tropical areas releases
the surplus energy upwards which after meeting the upper cooler air, converts into an eddy.
This eddy tends to settle down in the shape of inverted ‘V’ thus forming a cyclone.
Favourable breeding grounds: Following are the favourable breeding grounds of the
temperate cyclones:
1. USA and Canada
2. Mexican Gulf
3. Baltic Sea
4. Mediterranean basin extending up to Russia and even up to India during winters.
5. The Antarctic frontal zone.
Shape, size and wind velocity: They are asymmetrical and shaped like an inverted ‘V’. They
stretch from 500 km to 600 km and may go up to 2500 km in North America. They have a
height of 8-11 km. Generally the wind speed increases with the advent of the cyclone and
decreases after the cyclone has passed. Wind velocity is low as compared to tropical cyclones
and ranges from 30-40 km.
Structure: The north-western sector is the cold sector while the north-eastern sector is the
warm sector. As one moves eastwards in the northern sector, dark nimbus clouds and
altostratus clouds are followed by cirrostratus higher up with cirrus clouds finally at the storm
front. In the eastern sector, the extent of cloudiness and rainfall is limited. This sector is
generally dominated by cumulonimbus which causes heavy downpour, thunderstorm,
lightening and hailstorm.
Associated weather: The approach of a temperate cyclone is associated with a fall in
temperature, wind shifts, halo around sun and moon and a thin veil of cirrus clouds. A light
drizzle follows which turns in to heavy downpour. These conditions change with the arrival
of the warm front which halts the fall in mercury level and the rising temperatures. Rainfall
stops and clear weather prevails until the cold front arrives which results in to fall in
temperature and rainfall occurs. After this the clear weather is established.
Anti-cyclones
Lecture Delivered by: Mr. Manzoor Jahangir
Tropical and Temperate Cyclones / Anti-cyclones
An anti-cyclone is an area of high pressure, with roughly concentric isobars and clockwise
circulation of winds. It is meteorologically opposite to cyclones. Anticyclones usually cover a
wider area and tend to be more persistent and slow moving. The high pressure in an
anticyclone is a direct result of the presence of cold dense air. On the basis of where this cold
air occurs in the vertical structure of the anticyclone, they are sometimes classified into warm
and cold types.
Cold anticyclone: In cold anticyclones, the cold air is confined to the lowest parts of the
atmosphere. A simple thermal mechanism accounts for the cold air. It is caused by the
cooling of the earth’s surface in winter which chills the adjacent air, causes it to contract and
initiate the subsidence above. This mechanism is prominent in the great seasonal anticyclones
which develop over Siberia and Canada in winter.
Warm anticyclones: These are characterised by relatively warm air in the lower parts of the
troposphere. The excess of pressure seems to arise from the coldness of air in the upper
troposphere and lower stratosphere. Some anticyclones gradually change from cold to warm
type as the lower layers of the atmosphere warm up.
Lecture Delivered by: Mr. Manzoor Jahangir