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TROPICAL STORMS ( cyclones, hurricanes, typhoon, willy wilies)
Definition: Tropical cyclones are low pressure systems or belts which develop in tropical areas due to
overhead sun and are associated with heavy rainfall and strong winds.
(a) Describe the distribution of tropical storms ( cyclone, hurricanes. Typhoons)/Where do cyclones
form? [3]
Tropical storms originate in oceans like Pacific, Indian and Atlantic
They are formed in the tropical areas (between Tropic of Cancer and Tropic of Capricon)
Most form in the Pacific ocean (42) and few in the Atlantic ocean(9)
They all move towards the west once formed to affect east sides of continents.
NB: Cyclones don’t form along/on the equator since there is no Coriolis force for rotation.
(b) When do cyclones form? [1]
They form in late summer when the ocean water surfaces would have reached 27⁰C. For
example in the southern Hemisphere, (Zimbabwe, Mozambique) summer begins around
October and so late summer is from end of February to end of March, that’s when cyclones
normally form
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(c) Why do cyclones form? (3)
Ocean surface would have reached 27⁰C so warm moist air rises, expands, cools and
cumulonimbus clouds form
There is extreme low pressure created which draws in air towards it
There is Coriolis force in the tropics which allows the spinning/rotation of the cyclone.
(d) Under what conditions do cyclones form? (4)
Large ocean surface which is a source of moisture
Ocean surface temperature at minimum of 27⁰C to allow high evaporation of water
Within the tropics (between Tropic of Cancer and Tropic of Capricon) in late summer to allow
heating of sea water.
Away from the equator because there is no Coriolis force on the equator
Low pressure must be created.
At least 60m depth of sea
NB: There is a close relationship between why and how and conditions needed for cyclones to form.
These are almost the same. Check from (c) and (d) above.
STRUCTURE OF A CYCLONE
LABELLING
C EYE—Is the calm centre with low speed winds.
Reason---There is falling/sinking air
B VORTEX---The surface winds are fastest towards the centre. There are rapid rising air currents
Reason---winds are being drawn to be sucked up by the strong vertical currents to replace the air
that is rising.
HAZARDS CAUSED BY CYCLONES
1. Strong winds
These winds blow off roof tops and houses are destroyed killing people
Trees are uprooted and fall on houses, block roads and destroy telephone wires and electricity
wires causing fires. Telephone boosters are also destroyed.
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2. Heavy Rainfall which causes flooding
This causes flooding of rivers which drowns houses, people and property is destroyed
Roads, bridges and communication lines are destroyed
3. Storm Surges
A storm surge is a high wave caused by friction between strong wind and sea water. In coastal
areas, this can be up to 8 metres causing flooding, buildings destroyed, roads and bridges
destroyed.
4. Landslides
Saturated soil on steep areas loses stability and collapses on top of roads, buildings killing
people and obstructing transport and communication systems
EFFECTS OF CYCLONES
Loss of life
Destruction of houses making people homeless
Destruction of roads and bridges making areas unreachable to provide aid
Destruction of network boosters/terminal
Destruction of windmills and electric poles so cutting off electricity supply
Destruction of crops in fields causing famine
Factories in cities are destroyed making people jobless
Explain the formation of a tropical cyclone. [4]
Formed on oceans /seas
Formed in the tropical areas between 5⁰ and 20⁰N and south of the equator to allow heating by
the overhead sun in summer and presence of Coriolis force that allows rotation
The sea surface temperature should be above 27⁰ Celsius to allow high evaporation
The sea depth should be more than 60metres to allow large water body to heat up
Air above ocean is heated, and evaporates
Warm moist air rises, expands, cools and condenses to form cumulonimbus clouds
This creates low pressure at the centre which sucks in more air
Wind speed increase
Winds go in a circular motion/rotation due to rotation of earth
Release of heat by condensation at high level fuels the cyclone
NB: Provide all conditions and explain how the form a cyclone.
WAYS OF REDUCING EFFECTS OF CYCLONES
Monitoring and early warning (prediction by using geostationary satellites, using weather
forecasting at sea and ground weather stations etc)
Doing emergency drills
Emergence rescue teams
Evacuation and preparation of evacuation routes/relocating people
building and reinforcing buildings
building emergency shelters
providing relief aid ( clean water supply by bowsers, providing medical aid, providing food, tents
etc)
educating people on how to react during a cyclone
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government provide aid for rebuilding
provide fuel supplies/electricity
international aid to rebuild and to provide aid.
NB: CLASSISY ALL THE MEASURES/STRATEGIES ABOVE INTO 3 CLASSES
1. Before the cyclone
2. During the cyclone
3. Immediately after the cyclone
THE STRUCTURE OF A TROPICAL STORM/HURRICANE/CYCLONE/TYPHOON
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(b) The map shows areas where tropical cyclones occur.
(a) Use the information on the map to complete the passage which describes tropical storms
Generally all cyclones form over ......................................... between the Tropic
of ..................................................... to the north of the Equator and the Tropic
of ........................................................ to the south of the Equator. The tracks of cyclones generally move
in a clockwise direction in the............................................... side of the Equator and an anticlockwise
direction south of the ................................................... The cyclones forming in the Atlantic Ocean affect
narrow areas of land on the eastern side of........................................... There is no cyclone that begins at
the ……………………..due to lack of Coriolis force and most of them die outside the Tropic of Cancer and
Tropic of……………………….. due to low temperature. [8]
END
