Cyclones
cyclone, atmospheric
pressure distribution in which there is a low central pressure relative to the
surrounding pressure. The resulting pressure gradient, combined with the
Coriolis effect, causes air to circulate about the core of lowest pressure in a
counter clockwise direction in the Northern Hemisphere and in a clockwise
direction in the Southern Hemisphere. Near the surface of the earth, the
frictional drag on the air moving over land or water causes it to spiral
gradually inward toward lower pressures. This inward movement of air is
compensated for by rising currents near the center, which are cooled by
expansion when they reach the lower pressures of higher altitudes. The cooling,
in turn, greatly increases the relative humidity of the air, so that
“lows” are generally characterized by cloudiness and high humidity;
they are thus often referred to simply as storms.
Temperate Cyclones
Temperate cyclones are
atmospheric disturbances having low pressure in the centre and increasing
pressure outward. The origin and influence of these cyclones is in the
temperate zone due to which they are known as temperate cyclones. They are more
active in winter in the Northern hemisphere but due to prominence of water
areas in the Southern hemisphere, they are active throughout the year, since,
they travel from West to East they are also termed as Western disturbances.
Life Cycle of a Temperature Cyclone
There are six stages of
life cycle of a temperate cyclone.
First Stage
involves the convergence of two ari masses of contrasting physical properties
and directions. Initially, the air masses (warm and cold) move parallel to each
other and a stationary front is formed. This is called initial stage.
Second Stage
is also called as incipient stage, during which the warm and cold air masses
penetrate into the territories of each other and thus a wave-like front is formed.
Third Stage
is the mature stage when the cyclone is fully developed and isobars become almost circular.
Fourth Stage
warm sector is narrowed in extent due to the advancement of cold front than
warm, as cold front comes nearer to warm front.
Fifth Stage
starts with the occlusion of cyclone when the advancing cold front finally
overtakes the warm front and an occluded front is formed.
Sixth Stage
warm sector completely disappears, occluded front is eliminated and ultimately
cyclone dies out.
Tropical Cyclones
Cyclones developed in the
regions lying between the tropics of Capricorn and Cancer are called tropical
cyclones. All types of cyclones originate because of frontogenesis. Inspite of
the absence of two contrasting air masses in the equatorial region fronts are
formed due to meeting of land and sea winds. This frontal concept of the origin
of tropical cyclone is no longer acceptable because tropical cyclones in no
case are related to fronts.
They are characterized by the following salient
features-:
-Tropical cyclones move at
the speed of about 32 km per hour while hurricanes attain the velocity of 180
km per hour or more.
-On an average, their
diameters range between 80 km and 300 km but some time they become so small
their diameter I restricted to 50 km or even less.
-Like temperate cyclones,
tropical are not characterized temperature variation in their different parts because they do not have different front.
-Generally, direction is
Westerly up to 15° latitude from the equator, pole–ward between 15° to 60°
latitudes, and again Westerly. These cyclones are weaker when they enter
sub-tropical regions.
-Tropical cyclones become
disastrous natural hazards because of their high wind speed of 180 km to 400 km
per hour, high tidal surges-high rainfall intensity, very low atmospheric
pressure.
