How do They Form?

There is a broad zone of low pressure in the tropics which stretches either side of the equator. The winds on the north side of this zone blow from the north-east (the North-east Trade Winds) and on the southern side blow from the south-east (South-east Trade Winds).

Within this area of low pressure the air is heated over the warm tropical ocean. This air rises in discrete bubbles, causing thundery showers to form. These showers usually come and go, but from time to time, they group together into large clusters of thunderstorms. This creates a current of very warm, moist, rapidly rising air, leading to the development of a centre of low pressure, or depression, at the surface.

There are six general factors are necessary to make tropical cyclone formation possible, although tropical cyclones may occasionally form despite not meeting all of these conditions:

1. Water temperatures of at least 26.5°C (80°F) down to a depth of at least 50 m (150 feet). Waters of this temperature cause the overlying atmosphere to be unstable enough to sustain convection and thunderstorms.
2. Rapid cooling with height. This allows the release of latent heat, which is the source of energy in a tropical cyclone.
3. High humidity, especially in the lower-to-mid troposphere. When there is lots of moisture in the atmosphere, conditions are more favourable for disturbances to develop.
4. Low wind shear. When wind shear is high, the convection in a cyclone or disturbance will be disrupted, blowing the system apart.
5. Distance from the equator. This allows the Coriolis force to deflect winds blowing towards the low pressure centre, causing a circulation. The approximate distance is 500 km (310 miles) or about 5 degrees from the equator.
6. A pre-existing system of disturbed weather. The system must have some sort of circulation as well as a low pressure centre.

The development of the surface depression causes an increase in the strength of the Trade Winds. This imparts the twisting force necessary to cause rotation of the entire thunderstorm cluster. The spiralling winds accelerate inwards and upwards, releasing heat and moisture as they do so. The Coriolis force caused by the rotation of the Earth helps the spin of this column of rising air.

As the depression strengthens it becomes tropical storm and then a hurricane or typhoon. A mature hurricane or typhoon takes the form of a cylinder of deep thundercloud around a centre that is relatively free from clouds (the eye of the storm). There is a relatively small area of intense horizontal winds at the surface, often well over 100 mph., while air rises strongly above, maintaining the deep cumulonimbus clouds. Further aloft at about six miles, the cloud tops are carried outwards to give thick layer clouds due to the outward-spiralling winds leaving the tropical cyclone core. At the centre of the tropical cyclone, air is subsiding, which makes it dry and often cloud free, and there is little or no wind at the surface.