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Tropical Cyclones - What Is a Tropical Cyclone?

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Hurricane Felix

Image of Hurricane Felix from the Moderate Resolution Imaging Spectroradiometer (MODIS) on the NASA Terra Satellite

NASA
A tropical cyclone is composed of a system of thunderstorms that shows a cyclonic rotation around a central core or eye. A tropical cyclone is a generic term for a storm with an organized system of thunderstorms that are not based on a frontal system.

Each individual tropical cyclone differs, but several characteristics are common to most all tropical cyclones including a central low-pressure zone and high wind speeds of at least 34 knots. At this point, the storms are given a pre-determined storm name. Most storms are accompanied by a lot of rain and storm surges near the shore. Often, once the storms make landfall, the tropical cyclone can cause tornadoes.

A tropical cyclone needs warm ocean temperatures in order to form. Temperatures in the ocean need to be at least 82 degrees Fahrenheit in order to form. Heat is drawn up from the oceans creating what is popularly called a 'heat engine'. Tall convective towers of clouds are formed within the storm as warm ocean water evaporates. As the air rises higher it cools and condenses releasing latent heat which causes even more clouds to form and feed the storm.

Rotation and Forward Speed

The rotation of tropical cyclones in the Northern Hemisphere is counter-clockwise due to the Coriolis Effect. The opposite is true in the Southern Hemisphere.

The forward speed of a tropical cyclone can be a factor in determining the amount of damage the storm will cause. If a storm remains over one area for a long period of time, torrential rains, high winds, and flooding can severely impact an area. The average forward speed of a tropical cyclone is dependent on the latitude where the storm is currently. Generally, at less than 30 degrees of latitude, the storms will move at about 20 mph on average. The closer the storm is located the the equator, the slower the movement. Some storms will even stall out over an area for an extended period of time. After about 35 degrees North latitude, the storms start to pick up speed.

A good example of the fast formation of tropical cyclones comes when several storms stack up in the ocean back-to-back. Such an example occurred in 2009 with the formation of Ana, Bill, and Claudette as seen in this satellite image. The storms were very close to one another. Storms can also become entangle with one another in a process known as the Fujiwhara Effect where tropical cyclones can interact with each other.

Naming Tropical Cyclones

In the Atlantic Ocean and Eastern Pacific, tropical cyclones are known as hurricanes. In the Western Pacific Ocean, tropical cyclones are known as typhoons. In the Indian Ocean, a tropical cyclone is called a cyclone. These names are described in the article – Is it a typhoon, a cyclone, or a hurricane?

Specific storm names in each of the ocean basins vary based on conventional naming practices. For instance, in the Atlantic Ocean, storms are given names based on an alphabetical pre-determined list of Atlantic hurricane names. Severe hurricanes names are often retired.

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