But the cloud tops rise higher than about 6,000 feet. At that height in the tropics, there is usually a layer of warm, dry air that acts like an invisible ceiling or lid. Once in a while, something happens in the upper air that destroys this lid. Scientists do not know how this happens at all. But when it its the first step in the birth of a hurricane. With the lid the warm, moist air rises higher and higher. Heat energy, leased as the water vapor in the air condenses. As it condenses the drives the upper drafts to heights of 50,000 to 60,000 feet.
The cumuli become towering thunderheads. From outside the storm area, air moves in over the sea surface to replace the air soaring upwards in the thunderheads. The air begins swirling around the storm centre, for the same reason that the air swirls around a tornado centre. As this air swirls in over the sea surface, it soaks up more and more water vapor. At the storm centre, this new supply of water vapor gets pulled into the thunderhead updrafts, releasing still more energy as the water vapour condenses.
This makes the updrafts raise faster, pulling in even larger amounts of air and water vapour from the storms edges. And as the updrafts speed up, air swirls faster and faster around the storm centre. The hurricanes wind speed ranges from 75 miles to 200 miles per hour. The winds of a forming hurricane tend to pull away from the centre as the wind speed increases. When the winds move fast enough, the hole develops. This hole is the mark of a full-fledge hurricane. Hurricanes rotate in a counter-clockwise direction around an eye in the Northern Hemisphere and clockwise direction in the Southern Hemisphere.
The center of the storm or eye is the calmest part. It has only light winds and fair weather. Within the eye, all is calm and peaceful. But in the cloud wall surrounding the eye, things are very different although hurricane winds do not blow as fast as tornado winds a hurricane is far more destructive. Thats because tornado winds cover only a small area, usually less than a mile across- hurricanes winds may cover an area 60 miles wide out from the centre of the eye.
Another reason is tornadoes rarely last as long as an hour, or travel more than 100 miles. However, hurricane may rage for a week or more. In that time, it may travel tens of thousands of miles over the sea and land. At sea, hurricane winds whip up giant waves up to 20 feet high. Such waves can tear freighters and other ocean-going ships in half. Over land, hurricane winds can uproot trees, blow down telephone lines and power lines, and tear chimneys off rooftops. The air is filled with deadly flying fragments of brick, wood, and glass.