CYCLONES
Cyclones
● Cyclones are the centres of low pressure
surrounded by closed isobars having
increasing pressure outward and closed air
circulation from outside towards the central
low pressure. They are also called ‘depressions’.
From the locational point of view cyclones are classified
into two types:
1. Tropical Cyclones
2. Extra Tropical Cyclones
NOTE: The winds in Cyclones move ANTICLOCKWISE in the
Northern Hemisphere and CLOCKWISE in the Southern
Hemisphere.
Tropical Cyclones
● Cyclones developed in the regions lying between Tropic of
Cancer and Tropic of Capricorn are called Tropical cyclones.
They are not regular & uniform like Temperate cyclones.
● These cyclones derive their energy from the latent heat of
condensation. They become violent in the deep ocean and weak
while moving over land areas (which is also called LANDFALL of
the cyclone). That’s why they affect only coastal states of the
Continents.
Favourable conditions for Tropical Cyclones
● Convection of Warm moist air temperature of
which is more than 27C
● Presence of Coriolis Force
● Absence of Wind Shear
● Pre-existing Mechanical Instability
● Divergence of convecting air in the upper
surface
Climate Change & Tropical Cyclones
● Studies suggest that warming climate could bring
more destructive cyclones as there would be extra
heat in the oceans and atmosphere, although such
systems could also become less frequent.
● Frequency of Tropical Cyclones is increasing and
according to Council on Energy, Environment & Water
India has recorded 12 fold surge in last 50 Years.
Climate Change & Tropical Cyclones
● Rising sea levels could boost storm surges from
cyclones, making them even more deadly and
destructive.
● Tropical Cyclone precipitation rates are projected to
increase due to enhanced atmospheric moisture
associated with anthropogenic global warming.
Why Arab Sea is emerging as a new Cyclone Hotspot?
● The Arabian Sea experiences cooling twice a year. Once during June to
September (summer monsoon season) due to a ʻupwellingʼ which brings
colder, deeper subsurface waters to the surface. The mixing cools the surface.
The second time is between November and February when the northern
Arabian Sea cools due to 'winter convection' when the cold atmosphere and
air cools the ocean surface. This increases the density of the surface water
which sinks and is replaced by colder waters that rise up.
Destruction caused by Cyclones
Strong Winds/Squall
Cyclones are known to cause severe damage to infrastructure through high
speed winds. Very strong winds which accompany a cyclonic storm damages
installations, dwellings, communications systems, trees etc., resulting in loss
of life and property. Gusts are short but rapid bursts in wind speed are the
main cause for damage. Squalls on the other hand, are longer periods of
increased wind speed and are generally associated with the bands of
thunderstorms that make up the spiral bands around the cyclone.
Destruction caused by Cyclones
Storm Surge
A Storm surge can be defined as an abnormal rise of sea level near the coast
caused by a severe tropical cyclone; as a result of which sea water
inundates low lying areas of coastal regions drowning human beings and
livestock, causes eroding beaches and embankments, destroys vegetation
and leads to reduction of soil fertility.
Destruction caused by Cyclones
Torrential rains and inland flooding
Torrential rainfall (more than 30 cm/hour) associated with cyclones is another major
cause of damages. Unabated rain gives rise to unprecedented floods. Rain water on
top of the storm surge may add to the fury of the storm. Rain is a serious problem for
the people which become shelter less due to cyclone. Heavy rainfall from a cyclone
is usually spread over wide area and cause large scale soil erosion and weakening of
embankments.
Why Arab Sea is emerging as a new Cyclone Hotspot?
● After 1990, Global warming phenomena became more intense and
resultantly, upwelling and winter convection have been inadequate
at removing the excess heat input by ever-increasing carbon dioxide
levels. The warming of the Arabian Sea has been accelerated, and
with it the conditions necessary for cyclones.
RECENT CYCLONIC PHENOMENON IN INDIAN
OCEAN
● Cyclone Karim lies over southern parts of the Indian Ocean and is
currently a category two hurricane with a wind speed of 112 kmph.
● Cyclone Asani remains a Severe Cyclonic Storm over the Bay of Bengal
with wind speeds of 100-110 kmph gusting to 120 kmph.
RECENT CYCLONIC PHENOMENON IN INDIAN
OCEAN
RECENT CYCLONIC PHENOMENON IN INDIAN
OCEAN
● The instigator of this pair of cyclones was the Madden-Julian Oscillation
(MJO), a disturbance near the tropics that moves east around the globe every
30 to 50 days. Westerly winds over the equator line on the Indian Ocean have
been strong past several days, likely due to MJO. Such strong westerly winds
over the equator line can sometimes induce cyclonic circulations and lead to
the simultaneous formation of cyclones over the Northern hemisphere and
Southern hemisphere, called Twin Cyclones.
POLAR VORTEX
The polar vortex is a large area of
low pressure and cold air
surrounding both of the Earthʼs
poles. It always exists near the
poles, but weakens in summer and
strengthens in winter. The term
"vortex" refers to the
counter-clockwise flow of air that
helps keep the colder air near the
Poles.
