Hurricanes are tropical cyclones. West of the International Date Line, the storms are known as typhoons. They are called cyclones in Australia and India. Heat causes the condensation of ocean waters, creating water vapor, a hurricane's primary energy source.
Hurricanes form over warm subtropical and tropical ocean waters. They are low-pressure systems without fronts that have cyclonic winds circling a clear "eye." The center of the storm is calm, warm and has very low pressure.
Atlantic hurricanes form off the coast of Africa, and east Pacific hurricanes form in the eastern North Pacific basin. Both move westward, which is why more Atlantic hurricanes tend to hit land more than Pacific ones.
The direction that a hurricane spins is affected by the Earth's rotation. Hurricanes that form south of the equator spin in a clockwise direction. Those in the north rotate counterclockwise.
Hurricanes act like giant engines that take heat and convert it to wind energy. These storms pull vast amounts of moisture from the oceans to fuel them.
Once the winds in the rotating, rising bands of clouds reach 39 mph, the tropical disturbance becomes a named tropical storm. At a wind speed of 74 mph, the storm is a Category 1 hurricane. The strongest hurricanes have winds in excess of 155 mph and are classified as Category 5.
Hurricanes tend to weaken when they move on land because they have lost their energy source. The warm ocean water is no longer feeding the storm. Most hurricanes still have enough energy and moisture left to cause wind damage and large amounts of rain.