Hurricanes are caused by the evaporation and condensation of very warm, tropical water, which forces a great volume of moisture into the sky and keeps it rising. Hurricanes begin as clusters of thunderstorms over open water (usually a large area of warm ocean). These storms multiply, eventually soaring thousands of feet above the earth's surface and fanning out over hundreds of miles. A large area of low pressure develops at the surface beneath the storm cluster and the storms begin to rotate around the center of this low. As with any other smaller storm, the hurricane's storm clouds rotate in a counterclockwise direction. This is due to forces in the atmosphere like gravity and air pressure, which overcome calm air's natural tendency to flow clockwise.
Once this rotation begins, the storm is on its way to becoming much more powerful. It quickly strengthens and its winds become faster, eventually reaching 74 miles per hour near the center, which is considered hurricane strength. The storm is not classified as a hurricane until its strongest winds reach this threshold.
Meanwhile, the storm's thunderstorm cluster is arranged into bands of storm clouds which spiral around the center, giving the hurricane it's telltale comma shape that is easily recognized on satellite images.
---David Murphy
