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How does a hurricane form?

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Tropical cyclones are like engines that require warm, moist air as fuel. So the first ingredient needed for a tropical cyclone is warm ocean water. That is why tropical cyclones form only in tropical regions where the ocean is at least 80 degrees F for at least the top 50 meters (about 165 feet) below the surface.

The second ingredient for a tropical cyclone is wind. In the case of hurricanes that form in the Atlantic Ocean, the wind blowing westward across the Atlantic from Africa provides the necessary ingredient. As the wind passes over the ocean's surface, water evaporates (turns into water vapor) and rises. As it rises, the water vapor cools, and condenses back into large water droplets, forming large cumulonimbus clouds. These clouds are just the beginning.


Meteorologists have divided the development of a tropical cyclone into four stages: Tropical disturbance, tropical depression, tropical storm, and full-fledged tropical cyclone.

  1. Tropical disturbance

    When the water vapor from the warm ocean condenses to form clouds, it releases its heat to the air. The warmed air rises and is pulled into the column of clouds. Evaporation and condensation continue, building the cloud columns higher and larger. A pattern develops, with the wind circulating around a center (like water going down a drain). As the moving column of air encounters more clouds, it becomes a cluster of thunderstorm clouds, called a tropical disturbance.

  2. Tropical depression

    As the thunderstorm grows higher and larger, the air at the top of the cloud column is cooling and becoming unstable. As the heat energy is released from the cooling water vapor, the air at the top of the clouds becomes warmer, making the air pressure higher and causing winds to move outward away from the high pressure area. This movement and warming causes pressures at the surface to drop. Then air at the surface moves toward the lower pressure area, rises, and creates more thunderstorms. Winds in the storm cloud column spin faster and faster, whipping around in a circular motion. When the winds reach between 25 and 38 mph, the storm is called a tropical depression.

Next is tropical storm.


  1. Tropical storm

    When the wind speeds reach 39 mph, the tropical depression becomes a tropical storm. This is also when the storm gets a name. The winds blow faster and begin twisting and turning around the eye, or calm center, of the storm. Wind direction is counterclockwise (west to east) in the northern hemisphere and clockwise (east to west) in the southern hemisphere. This phenomenon is known as the Coriolis effect.

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  2. Tropical cyclone

    When the wind speeds reach 74 mph, the storm is officially a tropical cyclone. The storm is at least 50,000 feet high and around 125 miles across. The eye is around 5 to 30 miles wide. The trade winds (which blow from east to west) push the tropical cyclone toward the west—toward the Caribbean, the Gulf of Mexico, or the southeastern coast of the U.S. The winds and the low air pressure also cause a huge mound of ocean water to pile up near the eye of the tropical cyclone, which can cause monster storm surges when all this water reaches land.

Tropical cyclones usually weaken when they hit land, because they are no longer being fed by the energy from the warm ocean waters. However, they often move far inland, dumping many inches of rain and causing lots of wind damage before they die out completely.

Next, what are the fives categories tropical cyclones?


Sleeve Short Printed TOGETHERNESS Heart Tee Letter Leisure Neck Round Tropical cyclone categories:

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Category Wind Speed (mph) Damage at Landfall
1 74-95 Sleeve Short Leisure TOGETHERNESS Tee Round Letter Neck Heart Printed Minimal
2 96-110 Moderate
Leisure Tee Letter Sleeve Short Heart Printed Round TOGETHERNESS Neck 3 Heart Round Tee Short Printed TOGETHERNESS Letter Leisure Sleeve Neck 111-129 Tee Short Heart Printed Leisure Round TOGETHERNESS Letter Sleeve Neck Heart TOGETHERNESS Short Tee Round Leisure Letter Printed Neck Sleeve Extensive
4 TOGETHERNESS Leisure Short Neck Sleeve Printed Heart Tee Letter Round 130-156 Extreme
5 157 or higher Catastrophic

Leisure Letter Tee Short Sleeve Neck Round TOGETHERNESS Heart Printed Here is a GOES-12 movie of Hurricane Katrina, which struck the coasts of Louisiana, Mississippi and Alabama on August 29, 2005, as a Category 4. In the movie you can see the storm starting to form in the Atlantic on August 24 and becoming more and more structured as it gains energy over the warm waters of the Gulf of Mexico.