How tropical cyclones form

Overview

The sea temperature, latitude, wind shear, moisture and upper-air conditions a tropical cyclone needs, and why one cannot form on the equator itself.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is a tropical cyclone, and what is it called in different parts of the world?

   Model answer

   A tropical cyclone is an intense area of very low pressure that forms over warm tropical oceans, in which air spirals inwards and rises violently, producing sustained winds above 118 kilometres per hour and torrential rain. The same phenomenon has different regional names: it is called a hurricane in the North Atlantic, the Caribbean and the north-east Pacific, a typhoon in the north-west Pacific around Japan, the Philippines and China, and a cyclone in the Indian Ocean, the Bay of Bengal, the South Pacific and around northern Australia.

2. 2

   What sea surface temperature is needed for a tropical cyclone to form, and to what depth?

   Model answer

   The sea surface must be at least 26.5 to 27 degrees Celsius, and that warmth must extend down to a depth of about 50 to 60 metres. The temperature threshold matters because only water this warm evaporates fast enough to load the air with the moisture whose latent heat drives the storm. The depth matters because a developing storm churns the ocean and drags up water from below; if only a shallow skin is warm, the storm mixes cold water to the surface and starves itself.

3. 3

   Within what band of latitudes do tropical cyclones form, and why can they not form on the equator itself?

   Model answer

   They form mostly between roughly 5 and 30 degrees north and south of the equator, where the sea surface is at least about 26.5 °C. Poleward of that band the sea is generally too cool. Within about 5 degrees of the equator the Coriolis effect is too weak, because the deflection falls to zero at the equator itself, so converging air simply flows straight into the centre of a low and fills it instead of being turned into a spinning vortex. Without spin the system cannot organise into a cyclone, whatever the sea temperature.

4. 4

   Why must vertical wind shear be low for a tropical cyclone to develop?

   Model answer

   Vertical wind shear is a change in wind speed or direction with height. A tropical cyclone works by stacking a column of warm, moist, rising air vertically above one point, so that the latent heat released aloft sits directly over the low pressure at the surface and keeps deepening it. If strong shear is present, the upper part of the developing storm is blown downwind of the lower part, the warm core is displaced from the surface low and the structure is pulled apart before it can organise. This is why hurricanes rarely form in seasons or regions where shear is strong.

5. 5

   What role does a pre-existing disturbance play in the formation of a tropical cyclone?

   Model answer

   A cyclone does not appear from nothing; it needs an existing area of low-level convergence to gather the moist air together and start it rising. In the Atlantic this trigger is usually an easterly wave, a ripple of low pressure that moves off the coast of West Africa every few days. Elsewhere the trigger may be a disturbance along the Inter-Tropical Convergence Zone. Only a small minority of such disturbances go on to become cyclones, but almost every cyclone begins as one.

6. 6

   Why does a tropical cyclone need divergence in the upper atmosphere?

   Model answer

   Surface pressure is the weight of the whole air column above. If air rising inside the storm reached the top and simply piled up there, the weight of the column would not fall and the surface pressure would stop dropping. Divergence aloft, with air spreading outwards near the tropopause faster than it arrives, exports that air away from the column, so the pressure at the surface keeps falling, more air is drawn in at the base and the storm intensifies. A cyclone therefore needs an efficient outflow at the top as well as a strong inflow at the bottom.

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