What Is a Tropical Cyclone?


Tropical depressions, tropical storms, hurricanes, and typhoons are all examples of tropical cyclones -- organized systems of clouds and thunderstorms that form over warm waters and rotate around a low pressure center.

A Generic Term

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.

To learn more about what tropical cyclones are called depending on their winds blow, read What TCs are called from birth to dissipation.

Tropical cyclones are not only called certain things here in the U.S. depending on how strong they are, they're also known by different names depending on where you are in the world. 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 simply called a cyclone. These names are described in the article – Is it a typhoon, a cyclone, or a hurricane?

These Ingredients are Must-Haves

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.

    Tropical cyclones can form any time these conditions are met, but they are most prone to form from during the warm season months (May to November in the Northern Hemisphere).

    Rotation and Forward Speed

    Like ordinary low pressure systems, 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.

    Storms can also become entangled with one another in a process known as the Fujiwhara Effect where tropical cyclones can interact with each other.

    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.

    Edited by Tiffany Means