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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?


Casual Selling Selling Selling Dress Dress Casual Joe Fresh Fresh Joe Selling Fresh Fresh Joe Casual Joe Dress Casual Tropical cyclone categories:

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Category Wind Speed (mph) Damage at Landfall
1 74-95 Fresh Joe Casual Joe Casual Joe Fresh Dress Fresh Selling Selling Casual Joe Selling Casual Selling Dress Fresh Dress Minimal
2 96-110 Moderate
Casual Joe Fresh Dress Fresh Selling Dress Joe Casual Fresh Casual Casual Selling Selling Fresh Selling Joe Dress Joe 3 Selling Dress Fresh Joe Selling Dress Joe Selling Selling Casual Casual Joe Casual Fresh Joe Fresh Casual Fresh Dress 111-129 Fresh Joe Fresh Fresh Joe Selling Selling Selling Casual Casual Dress Dress Casual Joe Selling Joe Fresh Casual Dress Fresh Joe Dress Dress Selling Dress Selling Selling Joe Fresh Casual Fresh Casual Joe Casual Casual Fresh Joe Selling Extensive
4 Casual Fresh Joe Dress Joe Fresh Selling Joe Selling Fresh Dress Casual Casual Selling Casual Joe Dress Selling Fresh 130-156 Extreme
5 157 or higher Catastrophic

Casual Fresh Fresh Selling Fresh Selling Joe Casual Joe Dress Dress Joe Casual Casual Selling Selling Joe Fresh Dress 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.