Tropical cyclones
Names 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.
Names 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.
