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Summary IEB Grade 12 Geography-Tropical cyclones
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Geography notes on Grade 12 Focus on Geography Book Notes are on Tropical Cyclones
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Tropical cyclonesNames of tropical cyclones
The first letter of the name tells us how many cyclones have occurred that year
Cyclone Eline = 5th one that year
Hurricane Harvey = 8th one that year
Areas where tropical cyclones form
o Tropical cyclones occur between 30° North and 30° South
o Do not occur between the equator and 5° North and South because they depend on
the Coriolis force
o Tropical cyclones are called different names around the world
o Tropical cyclones usually occur on the east side of continents.
o Tropical cyclones move from a region near the equator towards the coast of a country.
o When they reach land they turn eastwards away from the coast and out to sea.
, Factors necessary for the formation of tropical cyclones
Hot moist air
o Tropical storms develop at the ITCZ where hot, moist air from the tropical easterlies
converges.
o They can only develop over the sea where the temperature of the water is over 28 °C.
o This results in a lot of evaporation and very hot, humid, unstable air.
o This is why tropical cyclones occur at the hottest time of the year, in late summer and
early autumn, when the sea has had time to warm up throughout summer.
o In the southern hemisphere they occur mainly from February to April.
Intense low pressure
o The hot air starts rising and forms an intense low pressure cell on the surface.
o The tropical jet stream in the upper air causes an upper-air low pressure cell and this
intensifies the low pressure cell on the surface.
o Air is sucked into the low pressure cell.
Coriolis force
o Between 5° N and 5° S, there is no Coriolis force so winds are not deflected into the low
pressure cell.
o But if the low pressure cell develops outside 5° N and 5° S, Coriolis force will cause the
winds to spiral towards the low pressure cell.
o This intensifies the low pressure cell to such an extent that a calm, cloud-free funnel,
called the eye of the cyclone, develops.
o Winds blow into the low in a clockwise
direction in the southern hemisphere and in
an anticlockwise direction in the northern
hemisphere.
o The pressure gradient is steep, resulting in
winds of up to 300 km/h.
How does the tropical cyclone continue
to get energy?
o The large scale condensation releases latent
heat into the atmosphere, which provides the
energy needed to sustain the cyclone.
o It makes the air more unstable and contributes to more air rising.
o This causes large cumulonimbus clouds to form, and torrential rain to fall.
o The air pressure is so low in the eye that some air is sucked down, forming calm,
cloudless conditions where the air is warmer due to adiabatic heating.
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