Hurricane, tropical cyclone glossary
Definitions of terms used to describe tropical cyclones, including
hurricanes
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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.
Hurricane warning: A warning that sustained winds 64 kt (74
mph or 119 kph) or higher associated with a hurricane are
expected in a specified coastal area in 24 hours or less. A
hurricane warning can remain in effect when dangerously high
water or a combination of dangerously high water and
exceptionally high waves continue, even though winds may be
less than hurricane force.
Hurricane watch: An announcement of specific coastal areas
that a hurricane or an incipient hurricane condition poses a
possible threat, generally within 36 hours.
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.
Millibar: A metric measurement of air pressure.
North Atlantic Basin (sometimes called just the "Atlantic
Basin"): The Atlantic Ocean north of the equator, the
Caribbean Sea, and the Gulf of Mexico.
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.
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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 of 20 north degrees latitude) 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.
Tropical storm warning: A warning for tropical storm
conditions including sustained winds within the range of 34 to
63 kt (39 to 73 mph or 63 to 118 kph) that are expected in a
specified coastal area within 24 hours or less.
Tropical storm watch: An announcement that a tropical
storm poses or tropical storm conditions pose a threat to
coastal areas generally within 36 hours. A tropical storm
watch should normally not be issued if the system is forecast
to attain hurricane strength.
Typhoon: A hurricane in the north Pacific west of the International Date Line.
Sometimes the word is used to refer to any tropical cyclone, no matter what its wind
speed. It has been used to refer to tropical cyclones in the South Pacific or Indian
Ocean, but this use is not considered to be correct today.
How tropical and extratropical cyclones differ
Tropical cyclone
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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
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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.