Types of Tropical Cyclones
Hurricane Rita and Sea Surface Temperatures, September 21, 2005. Orange and red depict ocean waters that are
82°F or warmer—areas warm enough for hurricanes to form.
A hurricane is a type of tropical cyclone, a generic term for a low pressure system that forms in the tropics or subtropics and is accompanied by thunderstorms organized into rainbands. In the Northern Hemisphere, tropical cyclones have a counterclockwise circulation of winds near the earth’s surface. Tropical cyclones do not have cold or warm fronts attached; those systems are called extratropical cyclones or winter storms.
Tropical cyclones are classified into four types, based on their wind speeds. These are terms commonly used by forecasters and you'll encounter them throughout this module:
- Tropical Depression: Maximum sustained winds of 38 miles per hour (mph) or 33 knots or less (a knot is a nautical mile per hour, abbreviated kt; 1 knot = 1.15 mph)
- Tropical Storm: Maximum sustained winds of 39-73 mph (34-63 kt)
- Hurricane: Maximum sustained winds of 74 mph (64 kt) or higher
- Major Hurricane: Maximum sustained winds of 111 mph (96 kt) or higher
Once a tropical storm forms, NHC gives it a name from the list for the current year. (Click here for a link to the current list of tropical cyclone names.) Each list is reused every six years, although names of hurricanes that have resulted in substantial damage or death are retired. The letters Q, U, X, Y, and Z are not included in the Atlantic list because of the scarcity of names beginning with those letters. If more than 21 named tropical cyclones occur in the Atlantic basin in a season, additional storms are named from the Greek alphabet: Alpha, Beta, Gamma, Delta, and so on.
Each list alternates between male and female names. Names utilized are English, Spanish, or French, reflecting the diversity of communities throughout North America, Central America, and the Caribbean that are impacted by these cyclones.
In the rest of this section, we'll talk about how, when, and where tropical cyclones form and their structure and intensity, which relate to the hazards that accompany these storms.