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

A thunderstorm in the tropics can grow into a massive hurricane under certain conditions. Sometimes several tropical thunderstorms are able to organize, 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.

A study found that out of 608 possible storms only 50 developed into tropical storms. 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.

Once they form, hurricanes take energy from the warm ocean water to become stronger. 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 higher latitudes where there is cooler ocean water, it looses its power. There is no longer a supply of warm, moist air near the ocean surface to feed the storm so it shrinks. 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 incoming air must go somewhere so it 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.

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.

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

One in a while, a tropical thunderstorm grows and grows, becoming a giant hurricane. First the storm grows a little bit. It combines with other thunderstorms and they all spin around an area of low pressure. This is called a tropical depression. Then the storm grows some more. Its winds become stronger and it is called a tropical storm. Then, the storm grows even more, its winds become even faster, and it is called a hurricane.

Hurricanes get their energy from the warm ocean water. When a hurricane is over warm water it will grow. A hurricane dies when it moves away from the warm water. When a hurricane moves into areas with cooler ocean water, it weakens. It will also weaken if it travels over land.