Five things that a hurricane needs Click on image for full size (36 Kb) Lisa Gardiner/Windows to the Universe

A tropical thunderstorm can grow into a massive hurricane under certain conditions. Sometimes several thunderstorms start rotating around a central area of low pressure. This is called a tropical depression. If the depression strengthens so that winds reach at least 39 mph, it is called a tropical storm. And if wind speeds increase to more than 74 mph, it is called a tropical cyclone or hurricane.

Once formed, hurricanes take energy from the warm ocean water to become stronger. A storm will strengthen if there is a supply of warm, moist air to feed it. Warm, moist air is found above warm, tropical ocean waters. While a hurricane is over warm water it will continue to grow. A hurricane dies when it moves away from the tropics. When a hurricane moves into areas with cooler ocean water, it weakens. It will also weaken if it travels over land.

The rotation of the storm is due to the Coriolis Effect, a product of the Earth's rotation. This causes the air being drawn into the central low pressure to curve. The air rises as it rotates. This rising air, which is saturated with water, cools and condenses, forming clouds. Hurricanes do not occur within 300 miles (500 kilometers) of the equator because there is no Coriolis Effect at the equator.