From Flat Calm to Perfect Storm

Hurricanes draw their power from the warm waters of the tropics, and although they can reach category five at sea, with winds exceeding 155mph, it is rare for them to hit land with such destructive force. Last year, Hurricane Dean became only the fourth category five storm to reach land. Gustav has been downgraded twice from category four in recent days, once after it passed Cuba, and yesterday as it approached Louisiana. Winds from the category two storm are still expected to reach 110mph.

Sea surface temperatures have to be at least 26.5C for a hurricane to form. If the winds are calm, the heat can build up to form an intense low pressure weather system, which causes winds to spiral round and suck up water vapor. As the warm vapor rises in the core of the hurricane, it cools and condenses to form rain, dumping energy into the storm and helping it to grow. The most ferocious winds are whipped up in the hurricane's "eye wall", off to the side of the eye, which itself can be completely calm.

Hurricanes typically lose power as they make landfall because their fuel supply is cut off. Even if the land surface is warm, they are unable to draw enough water vapor up to sustain their power.

Some scientists believe global warming is making hurricanes more intense by heating up the world's oceans, although others believe it is hard to rule out natural variation in hurricane cycles.