Over very warm areas of ocean, warm, moist air starts rising so fast that it creates a region of intense low pressure beneath it, pulling warmer, moist air in from the sides. The phenomenon can develop into a vast spiraling weather system – a hurricane. Once formed, hurricanes can continue for many days. They tend to drift in the direction of the prevailing trade winds until they strike land. But then, the spiraling winds can reach speeds of 300Km/h, and the whole weather system may be 800Km across. In the middle known as the eye the air can be clear and deceptively still. But the other side of the storm is not far behind.
Huge cumulonimbus clouds release torrential rain and the winds whip up high waves on the surface of the ocean beneath the hurricane. The intense low pressure can temporarily raise sea levels by as much as 8m in what is known as a storm surge, which can cause serious flooding. Once hurricane is traveling over land, its supply of moist air is cut off and the storm eventually subsides.
Storms that arise in the North Atlantic and batter the Caribbean and southeastern USA are called hurricanes. Those in the Pacific that threaten Southeast Asia are typhoons, and those in the Indian Ocean that have caused such extensive flooding and damage in the Indian subcontinent are cyclones.
Tornadoes are also caused by rapidly rising spirals of air but they are on a much smaller scale than hurricanes, pulling air up into a thundercloud. Though affection a smaller area, they can be just as devastating as hurricanes, tearing off roofs and sucking up almost anything in their path, resulting in some surprising objects raining down later.