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Hurricanes

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Hurricanes are deadlier than tornadoes, but anyone who has experienced a tornado will need some convincing. It is largely a question of scale.

So how are they different? They differ in the way they are formed and in the areas which they affect, but the chief difference is one of size.

Hurricanes are intense tropical storms, normally affecting the islands of the Caribbean, the Gulf coast of America and its eastern seaboard. They are formed over the western mid-Atlantic, normally 5 to 20 degrees N, and they generally move towards the NW or WNW before making landfall. Technically they are closed low pressure cells, which, according to the Beaufort Wind Scale, develop winds up to Force 12, or more than 75 mph (121 kph) - sometimes much more.

These winds spiral round in an anti-clockwise direction as they are drawn into the hurricane's centre (the eye), before rising. It is the speed of the wind which causes most damage, but there is the added effect of the associated low air pressure which can allow the sea's surface to rise, sometimes to unusual levels.

Around the eye will grow enormous towers of cumulo-nimbus clouds, fed by the rising air, from which very heavy rain and perhaps hail will fall, accompanied by thunder and lightning. The eye, of perfectly still air, can be anything up to 30 miles (50 kms) across, and the whole hurricane may have a diameter of 460 miles (800 kms).

Hurricanes can move at 10-15 mph (16-24 kph), but rarely last longer than three or four days. An average hurricane season can last between late July and early October. So during this period the effects of a hurricane - violent winds, heavy rain and hail, electrical storms and a rise in sea level - produce a deadly combination, and any affected coastal area is likely to face some level of devastation. Once over land hurricanes begin to dissipate, so their effects are restricted almost entirely to coastal regions.

Similar storms also occur in the Pacific Ocean, where they are called cyclones and, in Australia, willy-willies.

Tornadoes, by contrast, are local in occurrence and affect inland areas. They form, during spring and early summer, along squall lines between cool northerly air and warm, damp air from the tropics, and occur most often in the centre and midwest of the USA. However they can also form over the UK and the western plains of Europe.

Local intense ground heating causes a sudden uprush of air in a vortex which appears to pull heavy, dark, funnel-shaped

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