The following is a glossary of tropical cyclone terms.
A Advisory Official information issued by tropical cyclone warning centers describing all tropical cyclone watches and warnings in effect along with details concerning tropical cyclone locations, intensity and movement, and precautions that should be taken. Advisories are also issued to describe: (a) tropical cyclones prior to issuance of watches and warnings and (b) subtropical cyclones.
B Bar This is used to describe a dark layer of clouds that precedes the approach of a tropical cyclone. It is usually followed by heavy precipitation, and can also been seen in the eyewall of a cyclone. The arrival of the storm is often determined by this bar, as barometric pressure begins to fall and conditions begin to deteriorate following its passage. Best track A subjectively-smoothed representation of a tropical cyclone's location and intensity over its lifetime. The best track contains the cyclone's latitude, longitude, maximum sustained surface winds, and minimum sea-level pressure at 6-hourly intervals. Best track positions and intensities, which are based on a post-storm assessment of all available data, may differ from values contained in storm advisories. They also generally will not reflect the erratic motion implied by connecting individual center fix positions.
C Center Generally speaking, the vertical axis of a tropical cyclone, usually defined by the location of minimum wind or minimum pressure. The cyclone center position can vary with altitude. In advisory products, refers to the center position at the surface.
Center / Vortex Fix The location of the center of a tropical or subtropical cyclone obtained by reconnaissance aircraft penetration, satellite, radar, or synoptic data.
Central Dense Overcast A dense mass of clouds that covers and eyewall or the most tightly curved inner bands of a tropical cyclone.
Central North Pacific Basin The region north of the Equator between 140W and the International Dateline. The Central Pacific Hurricane Center (CPHC) in Honolulu, Hawaii is responsible for tracking tropical cyclones in this region.
Coriolis force The fictitious force that causes the apparent deflection of an object that is moving in a straight line in an inertial reference frame as perceived by an observer in a rotating reference frame. The effect of this force is at its minimum at the equator and increases away from it. In the synoptic scales of the atmosphere, the Coriolis force will cause flow flowing towards a low pressure area to deflect perpendicularly to the pressure gradient driving the flow. This causes the flow to rotate cyclonically about its center, producing geostrophic flow. Tropical cyclones require a minimum distance of from the equator to sustain tropical cyclogenesis.
Cyclone An atmospheric closed circulation rotating counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
D Direct hit A close approach of a tropical cyclone to a particular location. For locations on the left-hand side of a tropical cyclone's track (looking in the direction of motion), a direct hit occurs when the cyclone passes to within a distance equal to the cyclone's radius of maximum wind. For locations on the right-hand side of the track, a direct hit occurs when the cyclone passes to within a distance equal to twice the radius of maximum wind. Compare indirect hit, strike.
E Eastern North Pacific Basin The portion of the North Pacific Ocean east of 140W. The National Hurricane Center in Miami, Florida is responsible for tracking tropical cyclones in this region.
Eye The roughly circular area of comparatively light winds that encompasses the center of a severe tropical cyclone. The eye is either completely or partially surrounded by the eyewall cloud.
Eyewall / Wall Cloud An organized band or ring of cumulonimbus clouds that surround the eye, or light-wind center of a tropical cyclone. Eyewall and wall cloud are used synonymously.
Extratropical A term used in advisories and tropical summaries to indicate that a cyclone has lost its "tropical" characteristics. The term implies both poleward displacement of the cyclone and the conversion of the cyclone's primary energy source from the release of latent heat of condensation to baroclinic (the temperature contrast between warm and cold air masses) processes. Cyclones can become extratropical and still retain winds of hurricane or tropical storm force.
Extratropical cyclone A cyclone of any intensity for which the primary energy source is baroclinic, that is, results from the temperature contrast between warm and cold air masses.
F Fujiwhara effect The tendency of two nearby tropical cyclones to rotate cyclonically about each other.
G Gale Warning A warning of 1-minute or 10-minute sustained surface winds in the range 34 kn (39 mph or 63 km/h) to 47 kn (54 mph or 87 km/h) inclusive, either predicted or occurring and not directly associated with tropical cyclones.
H High Wind Warning A high wind warning is defined as 1-minute average surface winds of 35 kn (40 mph or 64 km/h) or greater lasting for 1 hour or longer, or winds gusting to 50 kn (58 mph or 93 km/h) or greater regardless of duration that are either expected or observed over land.
HURDAT The short name for the Hurricane Database, the database for all tropical cyclones in the Atlantic Ocean, Gulf of Mexico, and Caribbean Sea since 1851, and the Northeast Pacific Ocean since 1949.
Hurricane / Typhoon A tropical cyclone in which the maximum sustained surface wind (using the U.S. 1-minute average) is 64 kn (74 mph or 119 km/h) or more. The term hurricane is used for Northern Hemisphere tropical cyclones east of the International Dateline to the Greenwich Meridian. The term typhoon is used for Pacific tropical cyclones north of the Equator west of the International Dateline.
Hurricane Local Statement A public release prepared by local National Weather Service offices in or near a threatened area giving specific details for its county/parish warning area on (1) weather conditions, (2) evacuation decisions made by local officials, and (3) other precautions necessary to protect life and property.
Hurricane Season The portion of the year having a relatively high incidence of hurricanes. The hurricane season in the Atlantic, Caribbean, and Gulf of Mexico runs from June 1 to November 30. The hurricane season in the Eastern Pacific basin runs from May 15 to November 30. The hurricane season in the Central Pacific basin runs from June 1 to November 30.
Hurricane Warning An announcement that hurricane conditions (sustained winds of 74 mph or higher) are expected somewhere within the specified coastal area. Because hurricane preparedness activities become difficult once winds reach tropical storm force, the hurricane warning is issued 36 hours in advance of the anticipated onset of tropical-storm-force winds.
Hurricane Watch An announcement that hurricane conditions (sustained winds of 74 mph or higher) are possible within the specified coastal area. Because hurricane preparedness activities become difficult once winds reach tropical storm force, the hurricane watch is issued 48 hours in advance of the anticipated onset of tropical-storm-force winds.
Hypercane A hypothetical tropical cyclone that could potentially form over water. Such a storm would produce winds of over . A series of hypercanes may have formed during the asteroid or comet impact that killed the non-avian dinosaurs 66 million years ago. Such a phenomenon could also occur during a supervolcanic eruption, or extreme global warming.
I Indirect Hit Generally refers to locations that do not experience a direct hit from a tropical cyclone, but do experience hurricane-force winds (either sustained or gusts) or tides of at least 4 feet above normal.
Inland Tropical Storm Warning The equivalent of a Tropical Storm Warning for inland counties, put into use after multiple Tornado Warnings were issued for Hurricane Katrina, when tornadoes were not present, but winds were the equivalent of EF0-2 tornadoes. These are issued by local NWS forecast offices, not the NHC.
Inland Hurricane Warning The equivalent of a Hurricane Warning for inland counties, put into use after multiple Tornado Warnings were issued for Hurricane Katrina, when tornadoes were not present, but winds were the equivalent of EF0-2 tornadoes. These are issued by local NWS forecast offices, not the NHC.
Intertropical Convergence Zone A zonally elongated axis of surface wind confluence of northeasterly and southeasterly trade winds in the tropics.
Inundation The flooding of normally dry land, primarily caused by severe weather events along the coasts, estuaries, and adjoining rivers. These storms, which include hurricanes and nor'easters, bring strong winds and heavy rains. The winds drive large waves and storm surge on shore, and heavy rains raise rivers. (A tsunami — a giant wave caused by earthquakes or volcanic eruptions under the sea or landslides into the sea — is another kind of coastal inundation, but should not be confused with storm surge).
Invest A weather system for which a tropical cyclone forecast center (NHC, CPHC, or JTWC) is interested in collecting specialized data sets (e.g., microwave imagery) and/or running model guidance. Once a system has been designated as an invest, data collection and processing is initiated on a number of government and academic web sites, including the Naval Research Laboratory (NRL) and the University of Wisconsin Cooperative Institute for Meteorological Satellite Studies (UW-CIMSS). The designation of a system as an invest does not correspond to any particular likelihood of development of the system into a tropical cyclone; operational products such as Tropical Weather Outlooks or Significant Tropical Weather Advisories should be consulted for this purpose.
K Kelvin wave An eastward moving atmospheric wave that can enhance deep convection and contribute to tropical cyclogenesis, especially over the Pacific Ocean. It moves towards the east at about 10° to 20° longitude per day.
L Landfall The intersection of the surface center of a tropical cyclone with a coastline. Because the strongest winds in a tropical cyclone are not located precisely at the center, it is possible for a cyclone's strongest winds to be experienced over land even if landfall does not occur. Similarly, it is possible for a tropical cyclone to make landfall and have its strongest winds remain over the water. Compare direct hit, indirect hit, and strike.
M Major hurricane A designation used by the National Hurricane Center reserved for hurricanes in the Atlantic or Northeast Pacific basins that achieve Category 3 in the Saffir–Simpson Hurricane Scale. These storms have winds of at least .
Maximum Sustained Surface Wind The standard measure of a tropical cyclone's intensity. When the term is applied to a particular weather system, it refers to the highest one-minute average wind (at an elevation of 10 meters with an unobstructed exposure) associated with that weather system at a particular point in time.
Monsoon A large-scale, seasonally-reversing surface wind circulation in the tropics accompanied by large amplitude seasonal changes in precipitation.
Monsoon trough A surface trough in association with a monsoon circulation. This is depicted by a line on a weather map showing the location of minimum sea level pressure coinciding with the maximum cyclonic turning of the surface winds, with southwesterly or northwesterly flow prevailing equatorward and northeasterly flow prevailing poleward of the typically zonally oriented trough axis.
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