A hurricane is a large rotating storm centred around an area of very low pressure, with strong winds blowing at an average speed in excess of 72 miles per hour. The whole storm system may be five to six miles high and 300 to 400 miles wide. It moves forward like an immense spinning top, at speeds of up to 30 m.p.h.

There are various trigger mechanisms required to transform frequent storms into rarer hurricanes. These trigger mechanisms depend on several conditions being 'right' at the same time. The most influential factors are:

i) a source of very warm, moist air - derived from tropical oceans with surface temperatures greater than 26 °C;

ii) sufficient spin or twist from the rotating Earth - this is related to latitude.

As the warm sea heats the air above it, a current of very warm moist air rises quickly, creating a centre of low pressure at the surface. Trade winds rush in towards this low pressure and the inward spiralling winds whirl upwards releasing heat and moisture before descending. The rotation of the Earth causes the rising column to twist, gradually taking on the form of a cylinder whirling around an eye of relatively still air, free from clouds. The rising air cools and produces towering cumulus and cumulonimbus clouds. Further aloft at six miles, the cloud tops are carried outwards to give thick layer clouds due to the outward-spiralling winds leaving the hurricane core.

Great amounts of energy are transferred when warm water is evaporated from tropical seas. This energy is stored within the water vapour contained in moist air. As this air ascends, 90% of the stored energy is released by condensation, giving rise to the towering cumulus clouds and rain. The release of heat energy warms the air locally, causing a further decrease in pressure aloft. Consequently, air rises faster to fill this area of low pressure, and more warm, moist air is drawn off the sea, feeding further energy to the system. Thus, a self-sustaining heat engine is created.

Only as little as 3% of the heat energy may be converted mechanical energy of the circulating winds. This relatively small amount of mechanical energy equates to a power supply of 360 billion kilowatt hours per day - or six months' supply of electrical energy for the whole of the USA!

Hurricanes form between 5° and 30° latitude and initially move westward (owing to easterly winds) and slightly towards the poles. Many hurricanes eventually drift far enough north or south to move into areas dominated by westerly winds (found in the middle latitudes). These winds tend to reverse the direction of the hurricane to an eastward path. As the hurricane moves poleward it picks up speed and may reach between 20 and 30 m.p.h. An average hurricane can travel about 300 to 400 miles a day, or about 3,000 miles before it dies out.

Hurricanes occur between July and October in the Atlantic, eastern Pacific and the western Pacific, north of the equator. South of the equator, off Australia and in the Indian Ocean, they occur between November and March

The name hurricane should only be used for those tropical storms occurring in the Atlantic. In the Pacific they are known as typhoons, in the Indian Ocean as cyclones and in Australia as willy-willies. They are given names beginning with 'A', 'B', etc., in order of occurrence, and the names are alternately male and female.

• hurricane waves - large waves of up to 15 metres high are caused by the strong winds and bring about extensive flooding;
• swells - an increase in ocean level;
• rain - the hurricane picks up about 2 billion tons of moisture per day and releases it as rain.

These phenomena can cause major destruction, especially when the hurricane's path takes it over land. However, a path over land also causes the destruction of the hurricane itself. As it moves over land, its energy source is depleted and friction across the land surface distorts the air flow. This leads to the eye filling with cloud and the hurricane dies.

Other than basic knowledge of general hurricane occurrence, there are no atmospheric conditions that can be measured and combined to predict where a hurricane will develop. Therefore, we can only forecast its path once formed. A network of instruments, men and equipment at the National Hurricane Center in Miami, Florida, search out potential hurricanes in their early stages and track them through their life-cycle until they decay and die. Satellites detect hurricanes in their early stages of development and can help to provide early warning of imminent hurricanes. Reinforced aircraft, fitted with instruments, fly through and over hurricanes, and weather radar can locate storms within 200 miles of the radar station.

America has the most sophisticated hurricane warning system. When there are definite indications that a storm may be growing into a hurricane, the Weather Bureau puts this vast system into action.

A hurricane warning is issued to coastal areas where winds of 74 m.p.h. or greater are definitely expected to occur, or dangerously high water or high waves are predicted. The general public are usually informed via television broadcasts and through a system of flying flags by day and lanterns by night.

Strictly speaking, hurricanes do not occur over the British Isles. However, we are sometimes affected by deep depressions that are the remnants of hurricanes. The most widely publicised such depression occurred on 16 October 1987. Although some gusts were as strong as a hurricane, the average wind speed was only sufficient to classify the storm as a severe gale. Weather forecasters underestimated the strength of the wind because of a lack of weather-reporting ships in the area at the time.