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Definitions of terms used to describe tropical cyclones, including hurricanes

  • Easterly Wave: A wavelike disturbance in the tropical easterly winds that usually moves from east to west. Such waves can grow into tropical depressions.
  • Extratropical cyclone: A storm that forms outside the tropics, sometimes as a tropical storm or hurricane changes. See table below for differences between extratropical and tropical cyclones.
  • Eye: The low pressure center of a tropical cyclone. Winds are normally calm and sometimes the sky clears.
  • Eye wall: The ring of thunderstorms that surrounds a storm's eye. The heaviest rain, strongest winds and worst turbulence are normally in the eye wall.
  • Hurricane: A tropical cyclone with winds of 74 mph or more. Normally applied to such storms in the Atlantic Basin and the Pacific Ocean east of the International Date Line.
  • Knot: A measure of speed. It is one nautical mile per hour. Never refer to "knots per hour" unless you want to describe acceleration. A nautical mile is one minute of one degree of latitude and is slightly longer than the ordinary, or statute, mile used in the United States. To convert nautical miles to miles or knots to miles per hour, multiply by 1.15. To convert miles to nautical miles or miles per hour to knots, divide by 1.15. Use our calculator to try out a few figures.
  • Millibar: A metric measurement of air pressure.
  • Storm surge: The dome of water that builds up as a hurricane moves over water. As this water comes ashore with the storm, it causes flooding that is usually a hurricane's biggest killer.
  • Tropical cyclone: A low-pressure weather system in which the central core is warmer than the surrounding atmosphere. See the table below for differences between tropical and extratropical cyclones. The term "tropical cyclone" is also used in the Indian Ocean and around the Coral Sea off northeastern Australia to describe storms called "hurricanes" and "typhoons" in other areas.
  • Tropical depression (TD): A tropical cyclone with maximum sustained winds near the surface of less than 39 mph. Tropical Depressions are listed only with a number, not a name.
  • Tropical storm: Tropical cyclone with winds of 39 to 74 mph. In most of the world, a storm is given a name when it reaches tropical storm intensity.
  • Tropical disturbance: Often the earliest stages of a tropical cyclone. Normally an organized area of thunderstorms that forms in the tropics and persists for more than 24 hours. Low pressure might form at the surface, but winds around remain below 30 mph.
  • Tropical wave: A kink or bend in the normally straight flow of surface air in the tropics which forms a low pressure trough, or pressure boundary, and showers and thunderstorms. Can develop into a tropical cyclone.
  • Subtropical cyclone: A low pressure system that develops in subtropical waters (north and south of the of 20 degrees North and South latitude respectively) and initially has non-tropical features (see table below for a list of tropical features) but does have some element of a tropical cyclone's cloud structure (located close to the center rather than away from the center of circulation). Many of these systems are classified as "hybrid" storms.
  • Typhoon: A hurricane in the north Pacific west of the International Date Line.
How tropical and extratropical cyclones differ

Tropical cyclone

  • Forms over a tropical ocean.
  • Center of storm is warmer than the surrounding air.
  • Has no fronts.
  • Strongest winds are near the Earth's surface.

Extratropical cyclone

  • Forms outside the tropics.
  • Center of storm is colder than the surrounding air.
  • Has fronts.
  • Strongest winds in the upper atmosphere.
  • A USA TODAY online weather graphic and text gives more details on these big weather makers.

Source: The USA TODAY Weather Book by Jack Williams, 
USA TODAY.com