Hurricane Terminology, Hurricane Facts, Saffir-Simpson Hurricane Scale, Hurricane Preparedness

Select the first letter of the word from the list below to jump to an appropriate section of the glossary:
A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

A

Advisory (ADVY):

Highlights special weather conditions that are less serious than a warning. They are for events that may cause significant inconvenience, which, if caution is not exercised, could lead to circumstances that may threaten life and/or property.

Air Mass:

A body of air covering a relatively wide area and exhibiting horizontally uniform properties.

Anticyclone (see also High):

A large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere.
See related term(s): Cyclone, Low Pressure System/Low Pressure Area, Tropical Cyclone

Area of Uncertainty:

The shaded area around the forecast storm track showing the range of area in which the storm might move over a given period of time.
See related term(s): Cone of Uncertainty

Atmospheric Pressure:

The pressure exerted by the earth's atmosphere at any given point, determined by taking the product of the gravitational acceleration at the point and the mass of the unit area column of air above the point.
See related term(s): Sea Level Pressure

Australian Tropical Cyclone Category System:

The Australian Bureau of Meteorology (BOM) uses a scale in which the severity of a tropical cyclone is described in terms of categories ranging from 1 (weakest) to 5 (strongest) related to the zone of maximum wind gusts as shown in this table.
Note: Corresponding sustained winds and central pressure are also provided as a guide. Stronger gusts may be observed over hilltops, in gullies and around structures. (See BOM FAQ, 2)
See related term(s): Beaufort Scale, Dvorak Scale, Saffir-Simpson Scale

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B

Barometer:

An instrument that measures atmospheric pressure.

Barometric Pressure:

The pressure of the atmosphere as indicated by a barometer.

Baroclinic Zone:

A region in which a temperature gradient exists on a constant pressure surface. Baroclinic zones are favored areas for strengthening and weakening systems. Wind shear is characteristic of a baroclinic zone.

Barotropic System:

A weather system in which temperature and pressure surfaces are coincident, i.e., temperature is uniform (no temperature gradient) on a constant pressure surface. Barotropic systems are characterized by a lack of wind shear, and thus are generally unfavorable areas for severe thunderstorm development.

Beaufort Scale:

The Beaufort wind scale is a system used to estimate and report wind speeds when no measuring apparatus is available. Originally used only at sea, the scale has been adapted for use on land, as well.

Beaufort scale		Cyclone category	Average wind speed (knots)	Average wind speed (km/h)	Estimating speed over land	Estimating speed over water
0	Calm		Less than 1	less than 1	Calm, smoke rises vertically	Sea like mirror
1	Light Air		1 - 3	1 - 5	Direction of wind shown by smoke drift, but not by wind vanes	Ripples with the appearance of scales are formed, but without foam crests
2	Light breeze		4 - 6	6 - 11	Wind felt on face; leaves rustle; ordinary wind vane moved by wind	Small wavelets, still short, but more pronounced; crests have a glassy appearance and do not break
3	Gentle breeze		7 - 10	12 - 19	Leaves and small twigs in constant motion; wind extends light flag	Large wavelets; crests begin to break; foam of glassy appearance; perhaps scattered white horses
4	Moderate breeze		11 - 16	20 - 28	Raises dust and loose paper; small branches moved	Small waves, becoming longer; fairly frequent white horses
5	Fresh breeze		17 - 21	29 - 38	Small trees in leaf begin to sway; crested wavelets form on inland waters	Moderate waves, taking a more pronounced long form; many white horses are formed (chance of some spray)
6	Strong breeze		22 - 27	39 - 49	Large branches in motion; whistling heard in telegraph wires; umbrellas used with difficulty	Large waves begin to form; the white foam crests are more extensive everywhere (probably some spray)
7	Near gale		28 - 33	50 - 61	Whole trees in motion; inconvenience felt when walking against the wind	Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind
8	Gale	1	34 - 40	62 - 74	Breaks twigs off trees; generally impedes progress	Moderately high waves of greater length; edges of crests begin to break into the spindrift; the foam is blown in well-marked streaks along the direction of the wind
9	Strong gale	1	41 - 47	75 - 88	Slight structural damage occurs (chimney pots and slates removed)	High waves; dense streaks of foam along the direction of the wind; crests of waves begin to topple, tumble and roll over; spray may affect visibility
10	Storm	2	48 - 55	89 - 102	Seldom experienced inland; trees uprooted; considerable structural damage occurs	Very high waves with long overhanging crests; the resulting foam, in great patches, is blown in dense white streaks along the direction of the wind; on the whole, the surface of the sea takes a white appearance; the tumbling of the sea becomes heavy and shock-like; visibility affected
11	Violent storm	2	56 - 63	103 - 117	Very rarely experienced; accompanied by widespread damage	Exceptionally high waves (small and medium sized ships might be for a time lost to view behind the waves); the sea is completely covered with long white patches of foam lying along the direction of the wind; everywhere the edges of the wave crests are blown into froth; visibility affected
12	Hurricane	3, 4 ,5	64 and over	118 and over	Severe and extensive damage	The air is filled with foam and spray; sea completely white with driving spray; visibility very seriously affected

Australian Government Bureau of Meteorology Website

See related term(s): Australian Tropical Cyclone Category System, Dvorak Scale, Saffir-Simpson Scale

Best Track:

A subjectively-smoothed representation of a tropical cyclone's location and intensity over its lifetime: latitude, longitude, maximum sustained surface winds, and minimum sea-level pressure at 6-hourly intervals. Best tracks may differ from values contained in storm advisories.
See related term(s): Track

Bulletin:

A public release from a weather office issued in the event of the occurrence or forecast occurrence of severe weather. Bulletins emphasize features which are significant for the safety of the public and summarize all warnings in effect.

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C

Cb:

Cumulonimbus cloud.

Cell:

Convection in the form of a single updraft, downdraft, or updraft/downdraft couplet, typically seen as a vertical dome or tower as in a towering cumulus cloud. A typical thunderstorm consists of several cells.

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.
See related term(s): Eye, Eyewall

Central Pacific Hurricane Center (CPHC):

The Central Pacific Hurricane Center (CPHC) issues tropical cyclone warnings, watches, advisories, discussions, and statements for all tropical cyclones in the Central Pacific from 140 Degrees West Longitude to the International Dateline.

Circulation:

General or primary patterns of wind-flow in the atmosphere. Cyclonic circulation is considered positive, and Anticyclonic circulation negative.

Closed Low:

A low pressure area with a distinct center of cyclonic circulation which can be completely encircled by one or more isobars or height contour lines.
See related term(s): Cutoff Low

Coastal/Lakeshore Flooding:

Flooding which occurs when water is driven onto land from an adjacent body of water, often when there are significant storms, such as tropical and extra-tropical cyclones.

Cone of Uncertainty:

One of several forecast products of the National Hurricane Center (NHC) dealing with uncertainty about the exact track of the center of a storm. It is created such that two-thirds of historical forecast errors over the most-recent five-year sample fall within the cone. It should not be construed as a map of the area of impact of the storm. Impacts can be much broader than the area included in the cone of uncertainty.
See related term(s): Area of Uncertainty

Confluence:

A pattern of wind flow in which air flows inward toward an axis oriented parallel to the general direction of flow. It is the opposite of difluence. Confluence is not the same as convergence. Winds often accelerate as they enter a confluent zone, resulting in speed divergence which offsets the (apparent) converging effect of the confluent flow.

CONUS:

Continental United States.

Convection:

In meteorology, this term describes the vertical transport of heat and moisture in the atmosphere, especially by updrafts and downdrafts in an unstable atmosphere.

Convergence:

A contraction of a vector field; the opposite of divergence. Convergence in a horizontal wind field indicates that more air is entering a given area than is leaving at that level. Compare with confluence.
See related term(s): Inter-tropical Convergence Zone (ITCZ)

CPHC:

Central Pacific Hurricane Center.

Cumulonimbus (Cb):

Cloud characterized by strong vertical development in the form of mountains or huge towers topped at least partially by a smooth, flat, often fibrous anvil. Also known colloquially as a "thunderhead."
See related term(s): Convection, Thunderstorm

Cutoff Low:

A closed upper-level low which has become completely displaced (cut off) from basic westerly current, and moves independently of that current. "Cutoff low" and "closed low" often are used interchangeably, however, not all closed lows are completely removed from the influence of the basic westerlies.
See related term(s): Closed Low

Cyclogenesis:

The process that creates a new cyclone or intensifies an existing one.
See related term(s): Deepening, Filling

Cyclone (CYC):

A large-scale circulation of winds around a central region of low atmospheric pressure, counterclockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere.
See related term(s): Anticyclone, High, Tropical Cyclone

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D

Data Collection Platform (DCP):

In hydrologic terms, this an electronic device that connects to a river or rainfall gauge and records data from the gauge and transmits that data through a satellite to a remote computer.

Date Time Group (DTG):

Data that indicates the date and time when the tropical cyclone will be closest to the specified location.

Deepening:

The process by which the central pressure of a system (i.e. cyclone) decreases with time. This results in an increase of the wind speed around a low pressure area, meaning that the storm is intensifying.
See related term(s): Cyclogenesis, Filling

Depression:

A region of low atmospheric pressure that is usually accompanied by low clouds and precipitation.
See related term(s): Tropical Depression

Difluence:

A pattern of wind flow in which air moves outward (in a "fan-out" pattern) away from a central axis that is oriented parallel to the general direction of the flow. It is the opposite of confluence.

Direct Hit:

A close approach of a tropical cyclone to a particular location.
See related term(s): Indirect Hit, Landfall, Strike

Divergence:

The expansion or spreading out of a vector field; usually said of horizontal winds. It is the opposite of convergence.

Dropwindsonde:

A small radio transmitter, that is dropped (with a parachute) from an aircraft, which transmits data on temperature, pressure, relative humidity and wind to the plane.

Dvorak Scale:

Scale of eight full-number and seven x.5 categories used to estimate tropical cyclone intensity based on the analysis of cloud patterns in visible and infrared imagery from geostationary and polar-orbiting satellites. JTWC primarily uses the Dvorak technique.

T-Number	Estimated Intensity (kt)
1.0	25
1.5	25
2.0	30
2.5	35
3.0	45
3.5	55
4.0	65
4.5	77
5.0	90
5.5	102
6.0	115
6.5	127
7.0	140
7.5	155
8.0	170

Joint Typhoon Warning Center Frequently Asked Questions (FAQ)

See related term(s): Australian Tropical Cyclone Category System, Beaufort Scale, Saffir-Simpson Scale

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E

El Niño/Southern Oscillation (ENSO):

The ENSO cycle refers to the coherent and sometimes very strong year-to-year variations in sea- surface temperatures, convective rainfall, surface air pressure, and atmospheric circulation that occur across the equatorial Pacific Ocean. El Niño and La Niña represent opposite extremes in the ENSO cycle.

• El Niño [Warm Phase] refers to the above-average sea-surface temperatures across the east-central equatorial Pacific. During El Niño equatorial easterly trade winds diminish, resulting in an eastward shift of the Pacific warm pool and associated area of tropical convective rainfall. During a strong El Niño the warm pool covers the entire eastern half of the equatorial Pacific.
• La Niña, a.k.a. El Viejo [Cold Phase] refers to the periodic cooling of sea-surface temperatures across the east-central equatorial Pacific. During La Niña easterly trade winds strengthen, colder-than-average sea surface temperatures develop over the eastern equatorial Pacific, confining the Pacific warm pool and equatorial convective rainfall to the extreme western part of the basin.
• ENSO-neutral refers to those periods when neither El Niño nor La Niña is present, often the transition period between El Niño and La Niña events. Ocean temperatures, tropical rainfall patterns, and atmospheric winds over the equatorial Pacific are near the long-term average.

EOC:

Emergency Operations Center.

Eye:

The relatively clear and calm, roughly circular area of comparatively light winds that encompasses the center of a severe tropical cyclone, either completely or partially surrounded by the eyewall cloud.
See related term(s): Center, Eyewall, Moat

Eyewall/Wall Cloud:

An organized band or ring of cumulonimbus clouds surrounding the eye of a tropical cyclone. Eyewall and wall cloud are used synonymously.
See related term(s): Center, Eye, Moat

Explosive Deepening:

A decrease in the minimum sea-level pressure of a tropical cyclone of 2.5 millibars (mb)/hr for at least 12 hours or 5 mb/hr for at least six hours.

Extra-Tropical Cyclone:

A cyclone that 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 processes. Cyclones can become extra-tropical and still retain winds of hurricane or tropical storm force.

F

Fetch:

Length of the section over sea water along which wind blows with almost uniform direction and speed.

Filling:

Process by which the central pressure of cyclones increases. It is the opposite of "deepening".
See related term(s): Cyclogenesis, Deepening

Frontogenesis:

Process of formation or intensification of a front or frontal zone.

Frontolysis:

Process of weakness or dissipation of a front or frontal zone.

Fujita Scale/F Scale:

A scale of tornado intensity in which wind speeds are inferred from an analysis of wind damage. All tornadoes, and most other severe local windstorms, are assigned a single number from this scale according to the most intense damage caused by the storm.

Rating	Wind, Damage
F0 (weak)	40-72 mph, light damage
F1 (weak)	73-112 mph, moderate damage
F2 (strong)	113-157 mph, considerable damage
F3 (strong)	158-206 mph, severe damage
F4 (violent)	207-260 mph, devastating damage
F5 (violent)	260-318 mph (rare), incredible damage

Fujiwhara Effect:

The tendency of two nearby tropical cyclones to rotate cyclonically about each other.

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G

Gale Force Wind:

Sustained winds within the range of 34 to 47 knots, about 39 to 54 miles per hour.

GOES:

Satellites in geosynchronous orbit 22,370 miles above the Equator sending back satellite pictures to earth and relaying Data Collection Platform’s river and rainfall data back to the ground.

Gradient:

Change rate of any element value with distance in any given direction.

H

High (HI):

In meteorology, a region of high pressure; also known as anticyclone.
See related term(s): Cyclone, Low Pressure System/Low Pressure Area, Tropical cyclone

Hurricane (see also Typhoon, and Tropical Cyclone):

A tropical cyclone is an intense weather system of strong thunderstorms, with a well-defined surface circulation, in which the maximum sustained surface wind is 64 knots (74 mph or 119 km/hr) or more. They are called hurricanes in the Northern Hemisphere between the International Dateline and the Greenwich Meridian. The same phenomena are called typhoons in the Pacific North of the Equator and west of the International Dateline. In the Indian Ocean, they are called cyclones or tropical cyclones.

Hurricane Local Statement:

A public release prepared by local National Weather Service offices in or near a threatened area giving specific details for its 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 seven tropical cyclone areas. Atlantic basin, June 1 to November 30; Northeast Pacific basin, eastern area, May 15 to November 30; Central Pacific basin, June 1 to November 30; Northwest Pacific basin, unofficially described as, "any month of the year. Most likely period for activity is summer thru early fall, with peaks August thru October; and the lowest activity from January thru March;" North Indian Ocean basin, Bay of Bengal and Arabian Sea, April to May, and October to December; North Indian Ocean, May to June and October to November with weaker tropical cyclones during the Monsoon Season of July to September. Southwest Indian Ocean basin, unofficially described as, "Peak activity, December through April or early May; with lowest activity June through November;" Southeast Indian Ocean basin/Australian basin, any month, with lowest risk in winter; Australian/Southwest Pacific basin, officially, November to April.

Hurricane Warning:

A warning that sustained winds of 64 knots (74 mph or 119 km/hr) 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 for specific coastal areas that hurricane conditions are possible within 36 hours.

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I

Inches of Mercury:

Unit of atmospheric pressure used in the United States. One "inch of mercury" is equivalent to 33.86 millibars or 25.40 millimeters.

Indirect Hit:

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 have tides of at least 4 feet above normal.
See related term(s): Direct Hit, Landfall, Strike

Instability (INSTBY):

The tendency of air parcels to accelerate when they are displaced from their original position; especially, the tendency to accelerate upward after being lifted. Instability is a prerequisite for severe weather, and the greater the instability, the greater the potential for severe thunderstorms.

International Date Line:

The International Date Line is the imaginary line on the Earth that separates two consecutive calendar days. It can be anywhere on the globe, but is thought of, by convention, as 180° away from the Greenwich Meridian.

International Dateline.
United States Naval Observatory Astronomical Applications Department.

Inter-tropical Convergence Zone (ITCZ):

The region where the northeasterly and southeasterly trade winds converge, forming an often continuous band of clouds or thunderstorms near the equator.
See related term(s): Convergence

Inversion:

Condition of the atmosphere when the temperature of an air layer increases (rather than diminishing) with height.

Isobar:

A line on a map connecting points of equal pressure.

J

Joint Typhoon Warning Center (JTWC):

The U.S. Department of Defense agency responsible for issuing tropical cyclone warnings for the Pacific and Indian Oceans since 1959. JTWC is located at Pearl Harbor, Hawaii.

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K

KM:

Kilometers, also written km.

Knot (Kt):

Unit of speed used in navigation. It is equal to one nautical mile (i.e., the length of one minute of latitude) per hour or about 1.15 statute miles per hour, or 0.5 meters/sec.

L

La Niña

see El Niño

Landfall:

The intersection of the surface center of a tropical cyclone with a coastline.
See related term(s): Direct Hit, Indirect Hit, Strike

Low Pressure System / Low Pressure Area:

An area of a relative pressure minimum that has converging winds and rotates in a counterclockwise direction in the Northern Hemisphere and clockwise in the Southern Hemisphere.

M

M:

Statute mile, or on a weather map, Major Hurricane.

Major Hurricane:

A hurricane which reaches Category 3 (sustained winds greater than 110 mph; 96 knots; 49.2 m/s) on the Saffir/Simpson Hurricane Scale.

MB:

Millibar.

Mean:

The arithmetic average of a set of data (numbers).

Millibar (MB):

A unit of atmospheric pressure equal to 1/1000 bar, or 1000 dynes per square centimeter.

Moat:

A region between the eyewall and an outer rainband of a cyclone, such as a secondary eyewall rainband. A moat has relatively light rainfall.

Robert Houze, University of Washington. A conceptual model shows a hurricane undergoing eyewall replacement, with a moat structure based on observations from the Hurricane Rainband and Intensity Change Experiment. (March 8, 2007) From Stricherz, Vince, "Lessons from Rita: How hurricane intensity change happens," UWeek News, University of Washington, March 8, 2007

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N

National Digital Forecast Database (NDFD):

A resource of the National Weather Service which provides access to gridded forecasts of sensible weather elements (e.g., wind, wave height) as a seamless mosaic of digital forecasts from NWS field offices.

National Oceanic and Atmospheric Administration (NOAA):

A bureau of the of the U.S Government, within the Department of Commerce, that maps the oceans and conserves their living resources; predicts changes to the earth’s environment; provides weather reports and forecasts floods and hurricanes and other natural disasters related to weather.

National Weather Service (NWS):

The agency within the National Oceanic and Atmospheric Administration (NOAA) that is responsible for providing observations, forecasts and warnings of meteorological and hydrological events in the interest of national safety and economy.

Nautical Mile (NM):

A unit of distance used in marine navigation and marine forecasts. It is equal to 1.15 statute miles, 1852 meters, or 1 minute of latitude.

NEXRAD:

Next Generation Radar, a National Weather Service network of about 140 Doppler radars operating throughout the U.S.

O

Outlook:

A longer-term view than a forecast, used to indicate that a hazardous weather or hydrologic event may develop.

P

Post-Tropical Cyclone:

A cyclone that no longer possesses sufficient tropical characteristics to be considered a tropical cyclone. Former tropical cyclones that have become fully extra-tropical and remnant lows are two classes of post-tropical cyclones.

Present Movement:

The best estimate of the movement of the center of a tropical cyclone at a given time and given position. This estimate does not reflect the short-period, small scale oscillations of the cyclone center.

Q

(empty)

R

Radius of Maximum Winds:

The distance from the center of a tropical cyclone outward to the location of the cyclone's maximum winds.
See related term(s): Center, Eyewall

Remnant Low:

A system no longer having the convective organization required of a tropical cyclone.

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S

Saffir-Simpson Scale:

A scale to estimate the possible damage a hurricane's sustained winds and storm surge could do to a coastal area. As the hurricane intensifies or weakens, the scale number is reassessed accordingly.

Category	Central Pressure (millibars, mb)	Wind Speed (mph)	Storm Surge (ft.)	Damage
1	980 or >	74 - 95	4 - 5	Minimal
2	965 - 979	96 - 110	6 - 8	Moderate
3	945 - 964	111 - 130	9 - 12	Extensive
4	920 - 944	131 - 155	13 - 18	Extreme
5	< 920	> 155	> 18	Catastrophic

NOAA National Weather Service Glossary

Sea Level Pressure:

The atmospheric pressure at sea level at a given location.

Sea Surface Temperatures (SST):

The mean temperature of the ocean in the upper few meters.

Severe Tropical Storm:

A tropical cyclone with the maximum sustained winds of 48 knots (24.5 m/s, 89 km/h) to 63 knots (32.6 m/s, 117 km/h) near the center.

Short Term Watches and Warnings:

Detailed information about specific hurricane threats, such as flash floods and tornadoes.

Squall:

Atmospheric phenomenon characterized by a very large variation of wind speed: it begins suddenly, has a duration on the order of minutes, and decreases its speed quickly. It is often accompanied by showers or storms.

Squall Line:

An imaginary moving line, sometimes of considerable extension, along which squall phenomena occur.

Station Pressure:

The absolute air pressure at a given reporting station, before the reading is corrected/normalized to sea-level pressure.

Storm Force Winds:

Average surface wind speed of 48 to 63 knots.

Storm Surge:

An abnormal rise in sea level accompanying a tropical cyclone or other intense storm, and whose height is the difference between the observed level of the sea surface and the level that would have occurred in the absence of the cyclone.

Storm Tide:

The actual level of sea water resulting from the combination of storm surge and the normal tide.

Storm Warning:

Statement about sustained surface winds of 48 knots (55 mph or 88 km/hr) or greater, or wind force 10 or 11 in the Beaufort scale, predicted or already occurring, but not directly associated with a tropical cyclone.

Strike:

For any particular location, a tropical cyclone "strike" occurs if a circle of 125 nautical miles (n mi) in diameter (75 n mi to the right of the storm’s center and 50 n mi to the left) passes over the place.
See related term(s): Direct Hit, Indirect Hit, Landfall

Subtropical Cyclone:

A non-frontal low pressure system that has characteristics of both tropical and extra-tropical cyclones.

Swell:

Any water-waves system that was not locally generated.

Synoptic Track:

Weather reconnaissance mission flown to provide vital meteorological information in data sparse ocean areas as a supplement to existing surface, radar, and satellite data.

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T

Thunderstorm:

A transient, sometimes violent storm of thunder and lightning, often accompanied by rain and sometimes hail.

Tornado:

A severe rotating windstorm of small diameter and great destructive power. Although tornadoes associated with several weather situations occur over land in many parts of the world, they relatively frequently occur within a hurricane’s circulation.

Track:

The path that a storm or weather system follows, or a line drawn to represent a forecast of that movement.

Tropical Cyclone:

Generic term for a warm-core, non-frontal, synoptic-scale cyclone (rotating mass of air) originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation about a well-defined center. The term may be applied throughout the "life" cycle of the phenomenon as it goes from low pressure system/low pressure area to tropical depression (system of clouds and thunderstorms with a defined surface circulation and maximum sustained winds of 33 knots/38 mph, or less) to tropical storm (sustained winds of 34-63 knots/39–73 mph) to severe tropical storm (maximum sustained winds of 48 knots/89 km/h to 63 knots/117 km/h near the center). This includes the stages at which the cyclone would be called a hurricane/typhoon, severe tropical cyclone, or a severe cyclonic storm, with the names depending on where it occurs. The most intense tropical cyclones also are called super cyclones/super typhoons in some places.

Tropical Depression (TD):

Rotary circulation at surface highest constant wind speed 38 miles per hour (33 knots).

Tropical Disturbance:

A discrete tropical weather system of apparently organized convection--generally 100 to 300 nautical miles in diameter--originating in the tropics or subtropics, having a non-frontal migratory character, and maintaining its identity for 24 hours or more. It may or may not be associated with a detectable perturbation of the wind field.

Tropical Prediction Center (TPC):

A National Centers for Environmental Prediction (NCEP) facility located at Florida International University in Miami, Florida, which produces marine offshore and high seas forecasts south of 30N in the Eastern Pacific, Gulf of Mexico and Caribbean.

Tropical Storm (TS):

An organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds of 34-63 knots (39-73 mph).

Tropical Wave:

A trough or cyclonic curvature maximum in the trade-wind easterlies. The wave may reach maximum amplitude in the lower middle troposphere.

Typhoon:

The terms "typhoon" and "hurricane" are regionally distinct names for a strong tropical cyclone.

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U

Unstable Air:

Air that is able to rise easily, and has the potential to produce clouds, rain, and thunderstorms.

Unsettled:

In meteorological use, a colloquial term used to describe a condition in the atmosphere conducive to precipitation.

V

Vortex:

Any rotating wind system.

Vorticity:

Tendency of a fluid (or mass of air) to turn or rotate around an arbitrarily oriented axis.

W

Warning:

A warning is issued when a hazardous weather or hydrologic event is occurring, is imminent, or has a very high probability of occurring. A warning is used for conditions posing a threat to life or property.

Watch:

A watch is issued when the risk of a hazardous weather or hydrologic event has increased significantly, but its occurrence, location, and/or timing is still uncertain. It is intended to provide lead time for those who may need to evacuate or set other safety plans in motion.

Water Spout:

Small, revolving storm over ocean or inland waters. They occasionally move inland and cause some damage, but the winds are less severe than those in tornadoes, which they resemble in appearance.

Wind Shear:

The rate at which wind velocity changes from point to point in a given direction (as, vertically). The shear can be speed shear (where speed changes between the two points), direction shear (where direction changes between the two points) or a combination of the two.

X

(empty)

Y

(empty)

Z

Zone of Disturbed Weather:

A zone in which the pressure is low relative to the surrounding region and there are convective cloud masses which are not organized.

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Hurricane