Page 120 - SST Class 07

are formed which bring sudden and heavy rainfall. winds blowing anti-
This sort of rainfall is generally heavy and is clockwise in the
usually marked by thunder. Northern Hemisphere
and clockwise in the
Relief Rainfall : Relief rainfall occurs when
Southern Hemisphere.
moisture-filled air strikes high relief, such as
mountains, and is forced to rise. As wind rises up Cyclones are roughly
circular or oval in form. Cyclone
the slope of
Warm, moist Air cools and
air is forced condenses, mountain, it cools Tropical cyclones develop over oceans in latitudes
to rise over forming
high areas clouds and condenses. 8° to 12° north and south, in the Torrid Zone.
Thus, rain occurs These are violent while spiraling upwards from
on the windward the ocean surface to great height and moving
side of the across the ocean generally from east to west. They
Rain
shadow mountain whereas cause severe damage to life and property. Their
on the leeward chief characteristics are—very low pressure at the
Sea
Air descends, side remains dry. centre of cyclonic circulation, torrential rains,
warms and
It rains becomes Western Ghats of strong winds and severe weather. The gulf of
drier
India experience Mexico Caribbean Sea and the east and west coast
Relief rainfall
this type of rainfall. of India are the regions which experience
cyclones frequently.
Condensation Front
Cyclonic Rainfall : forms clouds
Sometimes, when Info Bits
Info Bits
warm, moist air and
cold air meet in an Cyclones are known by different names. In China,
area, the warm air they are known as typhoons, in the West Indies,
being lighter is (in the Caribbean Sea) as hurricane, in the Guinea
forced over the cold Lands of West Africa as tornadoes and in North-
Warm air forced to It rains heavily
air. This results into rise over cold air along the front West Australia as willy-willies. The Tornado which
cyclonic storm. It is Cyclonic rainfall often occurs in the Mississippi basin may attain the
called cyclonic rainfall and sometimes, frontal velocity of upto 300 km per hour.
rainfall . This type of rainfall generally occurs for
several days at a time. Western part of our country Case Study : Nargis Cyclone
experience such type of rainfall. The Nargis cyclone formed into a tropical cyclone
on 27 April, 2008 in the central Bay of Bengal off
Quiz Time the coast of India. It started to move north-west,

Match the following : but then it started to move north-east where it was
met by strong high altitude winds blowing about
1. Rainfall u u Life giving gas 165 km per hour. On 2 May, 2008, it attained its
2. Humidity u u Envelope of air peak. It was marked as Category 4 cyclone when it
touched its peak wind speed of 215 km per hour,
3. Atmosphere u u Change of water vapour
4. Oxygen u u The uppermost layer

5. Condensation u u Rain gauge
6. Exosphere u u Hygrometer

CYCLONES
A cyclone is a small low pressure system with Nargis Cyclone An aerial view of destruction
caused by Nargis
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