Off the west coast of Africa, just north of the equator, a thunderstorm forms. It is just a typical towering thunderstorm cloud, but it might grow into something quite different – a hurricane.

Perhaps several other thunderstorms form in the same area. And perhaps all those dark towering thunderstorm clouds begin to rotate around an area of low atmospheric pressure called a tropical depression. Drawing enough energy from the warmth of the tropical ocean water, these circling thunderstorms might grow into a single tropical storm with winds blowing more than 39 miles per hour. If it grows even larger and winds swirl faster than 74 miles per hour, it is called a hurricane.

This happens in many other warm, tropical areas of the world too but only under certain circumstances. These storms are called hurricanes in the Atlantic and are called typhoons and tropical cyclones in other parts of the world. For one to form, there needs to be warm ocean water and moist, humid air in the region. When humid air is flowing upward at a zone of low pressure over warm ocean water, the water is released from the air as creating the clouds of the storm.

As it rises, the air in a hurricane rotates. Air drawn into the center of a hurricane curves to the right in the Northern Hemisphere and toward the left in the Southern Hemisphere due to the Coriolis effect - a result of the Earth's rotation. Near the equator, where there is no Coriolis effect, hurricanes cannot form within 300 miles (500 kilometers) of the equator.

Storms grow if there is a continuous supply of energy from warm ocean water and warm, moist air. Tropical storms can grow into hurricanes, and hurricanes can grow into stronger hurricanes. However, only a small number of storms grow into tropical storms. Even fewer become hurricanes.

Storms weaken when they moves over areas with cooler ocean water. There isn’t nearly as much energy in the water to fuel the storm. Nor is there as much humidity in the air. Hurricanes also weaken when they travel over land.

What does a storm needs to form and grow?

  1. Hurricanes take energy from the warm ocean water to become stronger. While a hurricane is over warm water it will continue to grow.
  2. Because of low pressure at its center, winds flow towards the center of the storm and air is forced upward. High in the atmosphere, winds flow away from the storm, which allows more air from below to rise.
  3. The air that rises needs to be warm and moist so that it forms the clouds of the storm. Warm, moist air is found above warm, tropical ocean waters.
  4. A hurricane also needs the winds outside the storm to be light. These winds steer the storm, but are not strong enough to disrupt it.

As a storm grows, it changes.

As a storm grows, it goes through a series of stages. It starts as a tropical disturbance. Then, with cyclonic circulation and faster wind speeds, it becomes a tropical depression. If the wind keeps getting faster, it becomes a tropical storm and then a hurricane if winds are more than 74 miles per hour (mph).

The classifications are based on the wind speeds in the storm, not the size of the storm. Hurricanes that look small on radar can have very high wind speeds. And large storms can have low wind speeds. Wind speeds in hurricanes are often measured in knots. Compared to a mile per hour, knots are a little bit faster. Thirty miles per hour are about the same as 26 knots.

Tropical DisturbanceThunderstorms with light cyclonic circulation
Tropical DepressionWind speeds between 20 and 34 knots (23-39 mph)
Tropical StormWind speeds between 35 and 64 knots (40-73 mph)
HurricaneWind speed greater than 64 knots (74 mph)