Hurricane Glossary of Terms

[ A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | R | S | T | U | V | W | Z ]

A

ADVANCED WEATHER INTERACTIVE PROCESSING SYSTEM (AWIPS): It is the computerized system that processes NEXRAD and ASOS data received at National Weather Service Forecast Offices.

ADVISORY: A message from the National Hurricane Center in Miami giving warning information with details on tropical cyclone location, intensity, movement and precautions that should be taken. The advisory will contain a resume of all warnings in effect

AERIAL RECONNAISSANCE WEATHER OFFICER (ARWO): The flight meteorologist for weather reconnaissance flights into a tropical cyclone.

ANEMOMETER: An instrument that measures the speed or force of the wind.

ATMOSPHERIC PRESSURE: The pressure exerted by the atmosphere at a given point. Its measurement can be expressed in several ways. One is in millibars. Another is in inches or millimeters of mercury (Hg). Also known as barometric pressure.

AUTOMATED SURFACE OBSERVING SYSTEMS (ASOS): This system is a collection of automated weather instruments that collect data. It performs surface based observations from places that do not have a human observer, or that do not have an observer 24 hours a day.

AUTOMATION OF FIELD OPERATIONS AND SERVICES (AFOS): It is the computer system that links National Weather Service offices together for weather data transmission.

B

BAROMETER: An instrument for determining the pressure of the atmosphere

BEST TRACK: A subjectively smoothed path, versus a precise and very erratic fix-to-fix path, used to represent tropical cyclone movement. It is based on an assessment of all available data.

C

CAPE VERDE ISLANDS: A group of volcanic islands in the eastern Atlantic Ocean off the coast of West Africa. A Cape Verde hurricane originates near here.

CENTER: The vertical axis or core of a tropical cyclone. It is usually determined by cloud vorticity patterns, wind, and/or pressure distributions.

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 NORTH PACIFIC BASIN: The region north of the Equator between 140W and the International Dateline. The Central Pacific Hurricane Center (CPHC) in Honolulu, HI is responsible for tracking tropical cyclones in this region.

CHIEF, AERIAL RECONNAISSANCE COORDINATION, ALL HURRICANES (CARCAH): The 53rd Weather Reconnaissance Squadron's Air Force Reserve civilians who bridge the gap between the Hurricane Specialists at NHC and the flying squadron. Each day, they publish the Tropical Cyclone Plan of the Day.

CLOSEST POINT OF APPROACH: Point where hurricane eye makes closest contact to shore without actually making landfall.

COASTAL FLOOD WARNING: A warning that significant wind-forced flooding is to be expected along low-lying coastal areas if weather patterns develop as forecast.

COASTAL FLOOD WATCH: An announcement that significant wind-forced flooding is to be expected along low-lying coastal areas if weather patterns develop as forecast.

COLD FRONT: The leading edge of an advancing cold air mass that is underrunning and displacing the warmer air in its path. Generally, with the passage of a cold front, the temperature and humidity decrease, the pressure rises, and the wind shifts (usually from the southwest to the northwest in the Northern Hemisphere). Precipitation is generally at and/or behind the front, and with a fast-moving system, a squall line may develop ahead of the front. See occluded front and warm front.

CONVECTION: Atmospheric motions in the vertical direction resulting from surface heating and the subsequent rising of warm air. This lifting mechanism is capable of generating the rising motions necessary for clouds and precipitation to form.

CONVERGENCE: Wind movement that results in a horizontal net inflow of air into a particular region. Convergent winds at lower levels are associated with upward motion. Contrast with divergence.

CYCLONE: An atmospheric m. circulation (low-pressure system) with rotating and converging winds, in which the center has a relative pressure minimum. It usually has a diameter of 2000 to 3000 kilometers. When developing, a cyclone typically consists of a warm front pushing northward and a cold front pushing southward with the center of low pressure (cyclone center) located at the junction of the two fronts. Cyclones in the Northern Hemisphere rotate counter-clockwise while Southern Hemisphere cyclones rotate clockwise.

D

DATA BUOYS: Buoys placed throughout the Gulf of Mexico and along the Atlantic and Pacific coasts of the United States that relay information on air and water temperature, wind speed, air pressure, and wave conditions via radio signals.

DEPRESSION: In meteorology, it is another name for an area of low pressure, a low, or trough. It also applies to a stage of tropical cyclone development and is known as a tropical depression to distinguish it from other synoptic features.

DEEPENING: Used in describing the history of a low-pressure system or an area of cyclonic circulation, it means a decrease in the central pressure of the system. Although it usually describes the action of a pressure system on a constant pressure chart, it also means a surface low is increasing in cyclonic circulation and acquiring more energy. It is the opposite of filling.

DISTURBANCE: This has several applications. It can apply to a low or cyclone that is small in size and influence. It can also apply to an area that is exhibiting signs of cyclonic development. It may also apply to a stage of tropical cyclone development and is known as a tropical disturbance to distinguish it from other synoptic features

DOPPLER RADAR: Weather radar that measures direction and speed of a moving object, such as drops of precipitation, by determining whether atmospheric motion is horizontally toward or away from the radar. Using the Doppler effect, it measures the velocity of particles.

E

EASTERLY WAVE: An inverted, migratory wave-like disturbance or trough in the tropical region that moves from east to west, generally creating only a shift in winds and rain. The low-level convergence and associated convective weather occur on the eastern side of the wave axis. Normally, it moves slower than the atmospheric current in which it is embedded and is considered a weak trough of low pressure. It is often associated with possible tropical cyclone development and is also known as a tropical wave.

EASTERN NORTH PACIFIC BASIN: The region north of the Equator east of 140W. The National Hurricane Center in Miami, FL is responsible for tracking tropical cyclones in this region.

EL NIÑO: A warming of the Pacific Ocean currents along the coasts of Peru and Ecuador near the Equator that is generally associated with dramatic changes or a shift in the weather patterns of the region. A major El Niño event generally occurs every 3 to 7 years and is associated with changes in the weather patterns worldwide including hurricane.

EMERGENCY ALERT SYSTEM (EAS): A system designed to permit government officials to issue up-to-date and continuous emergency information and instructions to the public in case of a threatened or actual emergency. It is replacing the Emergency Broadcast System.

EMERGENCY PUBLIC INFORMATION: Information disseminated primarily, but not unconditionally, at the time of an emergency frequently includes actions, instructions and direct orders.

EMERGENCY PUBLIC SHELTER: Generally a public school or other such structure designated by county or city officials as a place of refuge. A volunteer group such as the American Red Cross or Salvation Army usually manages a shelter.

EMERGENCY OPERATIONS CENTER (EOC): A State, county, or city emergency facility that serves as a central location for the coordination and control of all emergency preparedness and response disaster activities.

EQUATOR: The geographic circle at 0 degrees latitude on the earth's surface. It is equal distance from the North and South Poles and divides the Northern Hemisphere from the Southern.

EVACUATION TIME: The lead-time that a populated coastal area must have to safely relocates all residents of vulnerable areas from an approaching hurricane. This time can also be perceived as the necessary amount of time between the local official evacuation order and the arrival of sustained gale force winds (40 mph) and/or flooding.

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

EXTENT OF EVACUATION: The identification of vulnerable people who must evacuate based on estimated damage and/or homes susceptible to hurricane force winds.

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. It is important to note that cyclones can become extratropical and still retain winds of hurricane or tropical storm force.

EXTRATROPICAL CYCLONE: A cyclone in the middle and high latitudes often being 2000 kilometers in diameter and usually containing a cold front that extends toward the equator for hundreds of kilometers. These cyclones forms outside the tropics, the center of storm is colder than the surrounding air, have fronts and the strongest winds in the upper atmosphere.

EYE: The center of a tropical storm or hurricane characterized by a roughly circular area of light winds and rain-free skies and the lowest pressure. An eye will usually develop when the maximum sustained wind speeds exceed 78 mph. It can range in size from as small as 5 miles to up to 60 miles (20-50 km) but the average size is 20 miles. In general, when the eye begins to shrink in size, the storm is intensifying.

EYE WALL: An organized band of convection surrounding the eye, or center, of a tropical cyclone. It contains cumulonimbus clouds, severest thunderstorms, heaviest precipitation and strongest winds.

F

FEEDER BANDS: In tropical parlance, the lines or bands of thunderstorms that spiral into and around the center of a tropical system. Also known as outer convective bands, a typical hurricane may have three or more of these bands. They occur in advance of the main rain shield and are usually 40 to 80 miles apart. In thunderstorm development, they are the lines or bands of low-level clouds that move or feed into the updraft region of a thunderstorm.

FILLING: Used in describing the history of a low-pressure system or an area of cyclonic circulation, it means an increase in the central pressure of the system. Although it usually describes the action of a pressure system on a constant pressure chart, it also means a surface low is decreasing in cyclonic circulation and losing its characteristics. The opposite of deepening.

FLOODING: A general and temporary condition of 1) partial or complete inundation of normally dry land areas from the overflow of inland or tidal water or rapid accumulation or runoff of surface waters from any source.

FLOOD PLAIN: Any land area susceptible to being inundated by water from any source. Normally the regulatory flood plain is characterized by the 100-year meaning there is a 1% chance of flooding per year. The flood plain is often referred to as flood prone areas.

FLOOD STAGE: The level of a river or stream where overflow onto surrounding areas can occur.

FLOOD WARNING: The expected severity of flooding (minor, moderate or major) as well as where and when the flooding will begin.

FORECAST: A statement of expected future occurrences. Weather forecasting includes the use of objective models based on certain atmospheric parameters, along with the skill and experience of a meteorologist. Also called a prediction.

FORWARD SPEED: The rate of movement (propagation) of the hurricane eye in miles per hour or knots

FRONT: The boundary between two dissimilar air masses.

FUJIWHARA EFFECT: A binary interaction where tropical cyclones within a certain distance (300-750 nautical miles depending on the sizes of the cyclones) of each other begin to rotate about a common midpoint.

G

GALE WARNING: A warning of 1-minute sustained surface winds in the range 39 to 54 mph (34 to 47 knots) inclusive, either predicted or occurring not directly associated with tropical cyclones.

GEOSTATIONARY OPERATIONAL ENVIRONMENTAL SATELLITES (GOES): Family of NWS weather satellites, which orbit 22,300 miles above the earth and maintain a velocity that allows it to remain over a fixed place above the equator. Images are available to forecasters every 30 minutes.

GREENWICH MEAN TIME (GMT): The name of the twenty-four hour time scale that is used throughout the scientific and military communities. Standard Time begins at Greenwich, England, which is the Prime Meridian of Longitude. The globe is divided into twenty-four (24) time zones of 15 degrees of arc, or one hour in time apart. To the east of this meridian, time zones are numbered 1 to 12 and prefixed with a minus (-), while to the west, the time zones are also numbered 1 through 12 but prefixed with a plus (+). Other names for this time measurement are Universal Time Coordinate (UTC) and Zulu (Z).

H

HIGH-PRESSURE SYSTEM: An area of relative pressure maximum that has diverging winds and a rotation opposite to the earth's rotation. This is clockwise the in Northern Hemisphere and counterclockwise in the Southern Hemisphere. Also known as an anticyclone, it is the opposite of an area of low pressure or a cyclone.

HIGH WIND ADVISORY: Announcement issued by the National Weather Service for substained winds exceeding 25 mph (19 knots).

HIGH WIND WATCH/WARNING: A high inland wind watch/warning issued by the National Weather Service when either of the following occurs or are expected to occur in the near term: 1) Sustained surface winds (1-minute average) of 40 mph (35 knots) or greater lasting for 1 hour or longer; or 2) Sustained winds or gusts of 58 mph (50 knots) or greater for any duration.

HUMIDITY: The amount of water vapor in the air.

HURRICANE: A tropical cyclone in the Northern Hemisphere with substained winds of at least 74 mph (64 knots) or greater in the North Atlantic Ocean, Caribbean Sea or Gulf of Mexico. These winds blow in a large spiral around a relatively calm center of extremely low pressure known as the eye. Around the rim of the eye, winds may gust to more than 200 miles per hour. The entire storm, which can be up to 340 (550) in diameter, dominates the ocean surface and lower atmosphere over tens of thousands of square miles. Hurricanes draw their energy from the warm surface water of the tropics (usually above 27 Celsius) and latent heat of condensation, which explains why hurricanes dissipate rapidly once they move over cold water or large land masses.

HURRICANE ADVISORY: Notice, issued by the National Hurricane Center, numbered consecutively for each storm, describing the present and forecasted position and intensity. Advisories are issued at six-hour intervals at midnight, 6 a.m., noon, and 6 p.m., Eastern Daylight Time. Bulletins provide additional information. Each message gives the name, eye position, intensity and forecast movement of the storm.

HURRICANE CLIPS: A structural bracing device used on the installation of roofs which reinforce the joints of a house and give a stronger connection of wood to wood roofing trusses than just nails. In many coastal communities, hurricane clips are enforced as a code restriction for new homes.

HURRICANE EYE: The relatively calm area near the center of the storm. In this area, winds are light and the sky is often partly covered by clouds.

HURRICANE EYE LANDFALL: When the eye, or physical center of the hurricane, reaches the coastline from the hurricane's approach over water.

HURRICANE HUNTERS: The 53rd Weather Reconnaissance Squadron of the U.S. Air Force Reserve, based out of Keesler Air Force Base in Biloxi, Mississippi. As a part of the 403rd Air Wing, the crew flies Lockheed WC-130 aircraft into tropical storms and hurricanes to gather meteorological data for the National Hurricane Center.

HURRICANE LIAISON TEAM: A team of FEMA, NWS, State and local emergency management officials which respond to the National Hurricane Center prior to the landfall of a hurricane in the Atlantic Ocean, Caribbean Sea or Gulf of Mexico. The primary purpose of the Team is to assist in coordinating the latest advisories from the NHC to the Federal, State and local emergency management agencies.

HURRICANE LOCAL STATEMENT: A public release prepared by local National Weather Service Field 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 PATH OR TRACK: Line of movement (propagation) of the eye through an area.

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/TROPICAL STORM PROBABILITIES: The National Weather Service issues hurricane/tropical storm probabilities in public advisories to realistically assess the threat of a hurricane or tropical storm hitting your community. The probabilities are defined as the chance in percent that the center of the storm will pass within approximately 65 miles of 44 selected locations from Brownsville, Texas, to Eastport, Maine.

HURRICANE WARNING: A warning added to a hurricane advisory that sustained winds of 74 mph (64 knots) or higher associated with a hurricane are expected in a specified coastal area within 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. A warning is used to inform the public and marine interests of the storm's location, intensity, and movement. The NHC chooses a distance of approximately 300 miles.

HURRICANE WATCH: An announcement added to a hurricane advisory that hurricane conditions pose a possible threat to a specified coastal area within 36 hours. A watch is used to inform the public and marine interests of the storm's location, intensity, and movement.

I

INCHES OF MERCURY (Hg): The name comes from the use of mercurial barometers that equate the height of a column of mercury with air pressure. One inch of mercury is equivalent to 33.86 millibars or 25.40 millimeters.

INLAND HIGH WIND WARNING FOR HURRICANE FORCE WINDS: Announcement issued for force winds 74 mph (64 knots) or greater within 12 hours.

INLAND HIGH WIND WATCH FOR HURRICANE FORCE WINDS: Announcement issued for hurricane force winds 74 mph (64 knots) or greater within 24 hours.

INSTABILITY: Occurs when a rising air parcel becomes less dense than the surrounding air. Since its temperature will not cool as rapidly as the surrounding environment, it will continue to rise on its own. Contrasts with stable air.

INTERTROPICAL CONVERGENCE ZONE (ITCZ): The axis dividing the southeast trades from the northeast trades, toward which the surface winds tend to converge The easterly trade winds of both hemispheres converge at an area near the equator called the "Intertropical Convergence Zone (ICTZ)", producing a narrow band of clouds and thunderstorms that encircle portions of the globe.

ISOBAR: The line drawn on a weather map connecting points of equal barometric pressure.

J

JET STREAM: Relatively strong winds concentrated within a narrow current in the atmosphere.

K

KNOT: A unit for the measurement of speed in the nautical system. It is the nautical miles per hour.

L

LANDFALL: The term used to describe where the hurricane eye actually passes over land, usually used to describe the continental States rather than islands in the Caribbean.

LATITUDE: The location north or south in reference to the equator, which is designated at zero (0) degrees. Parallel lines that circle the globe both north and south of the equator. The poles are at 90 degrees North and South latitude.

LEEWARD: The side of an object or obstacle, such as a ship's sail, a mountain, or a hill, furthest away from the wind, and therefore, protected from the direct force of the wind. The opposite of windward.

LOCAL ACTION STATEMENT: A release prepared by a National Weather Service Forecast Office in or near a threatened area giving specific details for its area of responsibility.

LONGITUDE: The location east or west in reference to the Prime Meridian, which is designated as zero (0) degrees longitude. The distance between lines of longitude are greater at the equator and smaller at the higher latitudes, intersecting at the earth's North and South Poles. Time zones are correlated to longitude.

LOW: A region of low pressure.

LOW-LEVEL INVEST: An investigative mission for tropical disturbances to: 1) determine the existence or non-existence of a "closed circulation" (winds blowing in a complete circle); 2) supply weather observations in required areas, and; 3) determine the vortex center, if any. These missions are flown at 500 to 1500 feet.

LOW-PRESSURE SYSTEM: An area of a relative pressure minimum that has converging winds and rotates in the same direction as the earth. This is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Also known as an cyclone, it is the opposite of an area of high pressure, or a anticyclone. See closed low, cold low, and cut-off low for further examples.

M

MAXIMUM ENVELOPE OF WATER (MEOW): Describes the predicted areas inundated and amount of storm surge for a particular area during the landfall of a hurricane. Used in the SLOSH Model.

MAXIMUM ENVELOPE OF WIND (MEOW): Describes the predicted areas inundated and amount of wind for a particular area during the landfall of a hurricane. Used in the Inland Wind Model.

MEAN SEA LEVEL: The heights of the sea surface midway between its average high and low water positions.

MILLIBAR (MB): A metric measurement of atmospheric pressure used by the National Weather Service.. Standard surface pressure is 1,013.2 millibars.

N

NATIONAL CENTERS FOR ENVIRONMENTAL PREDICTION (NCEP): As part of the National Weather Service, the centers provide timely, accurate, and continually improving worldwide forecast guidance products. Some of the centers include the Aviation Weather Center, the Climate Prediction Center, the Storm Prediction Center, and the Tropical Prediction Center. Formerly known as NMC.

NATIONAL HURRICANE CENTER (NHC): A Branch of the Tropical Prediction Center under the National Weather Service, it is responsible for tracking and forecasting tropical cyclones over the North Atlantic, Caribbean, Gulf of Mexico, and the Eastern Pacific.

NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION (NOAA): An Administration of the U.S. Department of Commerce, it is the parent organization of the National Weather Service. It promotes global environmental stewardship, emphasizing atmospheric and marine resources.

NATIONAL WEATHER SERVICE (NWS): A primary office of the National Oceanic and Atmospheric Administration, it is responsible for all aspects of observing and forecasting atmospheric conditions and their consequences, including severe weather and flood warnings.

NAUTICAL MILE: A unit of length used in marine navigation that is equal to a minute of arc of a great circle on a sphere. One international nautical mile is equivalent to 1,852 meters or 1.151 statue miles. Refer to a sea mile.

NEXRAD (NEXT GENERATION WEATHER RADAR): A network of advanced Doppler radars implemented in the United States between 1992 and 1996, it detects the location and intensity of precipitation out to a range of 143 miles from the radar site. NEXRAD Doppler radar is highly sensitive and can detect precipitation from very light rain and snow up to the strongest thunderstorms with accuracy and detail. Sometimes, however, the radar's extreme sensitivity will cause ground clutter and other non-precipitation echoes to be displayed in the vicinity of the radar site

NOAA WEATHER RADIO: A 24-hour continuous broadcast of existing and forecasted weather conditions operated and broadcast by the local field offices of the National Weather Service.

NORTH ATLANTIC BASIN (SOMETIMES CALLED THE ATLANTIC BASIN): The Atlantic Ocean north of the equator, the Caribbean Sea, and the Gulf of Mexico.

O

OCCLUDED FRONT: The front formed by a cold front overtaking a warm or stationary front and lifting the warm air above the earth's surface

P

POST-STORM REPORT: A report issued by a local National Weather Service office summarizing the impact of a tropical cyclone on its forecast area. These reports include information on observed winds, pressures, storm surges, rainfall, tornadoes, damage and casualties.

POLAR-ORBITING SATELLITE: A satellite whose orbit passes over both of the earth's between poles. Compare with a geostationary satellite.

PRE-EYE LANDFALL TIME: The time before actual hurricane eye landfall within which evacuation cannot be carried out because of earlier effects, such as the inundation of evacuation routes from the storm surge or rainfall and the arrival of sustained gale force winds. It is composed of the time of arrival of sustained gale-force winds or the time roadway inundation from storm surge/rainfall begins, whichever comes first.

PRELIMINARY REPORT: A report summarizing the life history and effects of an Atlantic or eastern Pacific tropical cyclone. It contains a summary of the cyclone life cycle and pertinent meteorological data, including the post-analysis best track (six-hourly positions and intensities) and other meteorological statistics. It also contains a description of damage and casualties the system produced, as well as information on forecasts and warnings associated with the cyclone. NHC writes a preliminary report on every tropical cyclone in its area of responsibility.

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.

PRESSURE: The force per unit area exerted by the weight of the atmosphere above a point on or above the earth's surface. Also known as atmospheric pressure or barometric pressure.

PROBABILITY OF TROPICAL CYCLONE CONDITIONS: The probability, in percent, that the cyclone center will pass within 50 miles to the right or 75 miles to the left of the listed location within the indicated time period when looking at the coast in the direction of the cyclone's movement.

PUBLIC INFORMATION OFFICER: A person appointed by a County Emergency Operations Center to be responsible for the formulating and coordinating of the dissemination of emergency public information with both the electronic and written media, ensuring that accurate information is being released to the general public.

R

RADAR (RADIO DETECTION AND RANGING): An electronic instrument using ultra high-frequency radio waves to detect distant objects and measure their range by how they scatter or reflect radio energy. Precipitation and clouds are detected by measuring the strength of the electromagnetic signal reflected back. Doppler radar and NEXRAD are examples

RAIN: Precipitation in the form of liquid water droplets greater than 0.5 mm. If widely scattered, the drop size may be smaller. It is reported as "R" in an observation and on the METAR. The intensity of rain is based on rate of fall. "Very light" (R--) means that the scattered drops do not completely wet a surface. "Light" (R-) means it is greater than a trace and up to 0.10 inch an hour. "Moderate" (R) means the rate of fall is between 0.11 to 0.30 inch per hour. "Heavy" (R+) means over 0.30 inch per hour.

RAPID DEEPENING: A decrease in the minimum sea-level pressure of a tropical cyclone of 1.75 mb/hr or 42 mb for 24 hours.

RECONNAISSANCE (RECCO) CODE: An aircraft weather reconnaissance code that has come to refer primarily to in-flight tropical weather observations, but actually signifies any detailed weather observation or investigation from an aircraft in flight.

RELOCATED: A term used in an advisory to indicate that a vector drawn from the preceding advisory position to the latest know position is not necessarily a reasonable representation of the cyclone's movement.

S

SAFFIR-SIMPSON DAMAGE-POTENTIAL SCALE: A scale, developed in the early 1970s by Herbert Saffir, a consulting engineer, and Robert Simpson, then Director of the National Hurricane Center, to measure the intensity of a hurricane from 1 to 5. The scale categorizes potential damage based on barometric pressure, wind speeds, and storm surge. Scale numbers are available to public safety officials when a hurricane is within 72 hours of landfall. Scale assessments are revised regularly as new observations are made. Public safety organizations are kept informed of new estimates of the hurricane's disaster potential. In practice, sustained surface wind speed (1-minute average) is the parameter that determines the category since storm surge is strongly dependent on the slope of the continental shelf.

SATELLITE: Used in reference to the manufactured objects that orbit the earth, either in a geostationary or a polar manner. Some of the information that is gathered by weather satellites, such as GOES9, includes upper air temperatures and humidity, recording the temperatures of cloud tops, land, and ocean, monitoring the movement of clouds to determine upper level wind speeds, tracing the movement of water vapor, monitoring the sun and solar activity, and relaying data from weather instruments around the world.

SATELLITE PICTURES: Pictures taken by a weather satellite, such as GOES-9, that reveal information, such as the flow of water vapor, the movement of frontal systems, and the development of a tropical system. Looping individual pictures aids meteorologists in forecasting. One way a picture can be taken is as a visible shot, which is best during times of visible light (daylight). Another way is as an IR (infrared) shot, which reveals cloud temperatures and can be used day or night.

SEVERE THUNDERSTORM WARNING: Indicates that severe thunderstorms have been sighted or indicated on radar.

SEVERE THUNDERSTORM WATCH: Indicates that conditions are favorable for lightning, damaging winds greater than 58 miles an hour and hail and/or heavy rainfall.

SHELTER PERIOD: The period in which people are forced to evacuate their homes. This time may vary from several hours to a couple of days depending upon the severity of the hurricane.

SHUTTERS: A physical wind barrier that is affixed over the outside of windows and/or doors to protect these vulnerable areas during a tropical storm. These products are classified by the styles of panel, accordion, or rolling and are manufactured from steel, aluminum, plastic or plywood.

SLOSH (SEA, LAKE AND OVERLAND SURGES FROM HURRICANES): A computerized model that is able to estimate the overland tidal surge heights and winds that result from hypothetical hurricanes with selected characteristics in pressure, size, forward speed, track and winds. The resultant tidal surge is then applied to a specific locale's shoreline, incorporating the unique bay and river configurations, water depths, bridges, roads and other physical features. The model estimates open coastline heights as well as surge heights over land, thus predicting the degree of propagation or run-up of the surge into inland areas.

SMALL CRAFT ADVISORY: An advisory issued for marine interests, especially for operators of small boats or other vessels. Conditions include wind speeds between 20 knots (23 mph) and 34 knots (39 mph). Issued up to 12 hours ahead of conditions.

SPECIAL MARINE WARNING: A warning for hazardous weather conditions, usually short and not adequately covered by existing marine warnings. Such conditions include sustained winds or gusts of 35 knots or more for 2 hours or less.

SPIRAL RAINBANDS: Bands of thunderstorms that spiral inward towards the center, where they wrap themselves around the eye.

SQUALL: A sudden increase of wind speed by at least 18 miles per hour (16 knots) and rising to 25 miles per hour (22 knots) or more and lasting for at least one minute.

STANDARD SURFACE PRESSURE: The measurement of one atmosphere of pressure under standard conditions. It is equivalent to 1,013.25 millibars, 29.92 inches of mercury, 760 millimeters of mercury, 14.7 pounds per square inch, or 1.033 grams per square centimeter.

STATE OF EMERGENCY: A declaration made by the Chief Elected Official of a State, County or City government which entails a heightened level of activation and mobilization of staff to protect property and lives.

STATIONARY FRONT: The boundary between two air masses, neither of which is replacing the other.

STATUTE MILE: Commonly known as a ground mile.

STORM: An individual low-pressure disturbance, complete with winds, clouds, and precipitation. Examples include thunderstorms, tornadoes, or even tropical cyclones. The name is associated with destructive or unpleasant weather

STORM SURGE: An abnormal rise in sea level accompanying a hurricane 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 surge is usually estimated by subtracting the normal or astronomic high tide from the observed storm tide. Note: waves on top of the storm surge will create an even greater high-water mark.

STORM TIDE: The actual level of seawater resulting from the astronomic tide combined with the storm surge. If the storm comes ashore during astronomical low tide, the surge will be decreased by the amount of the low tide. If the storm makes landfall during astronomical high tide, the surge will be that much higher.

STORM TRACKS: The path or tracks generally followed by a cyclonic disturbance.

SUBTROPICAL: The region between the tropical and temperate regions, an area between 35 and 40 degrees North and South latitude. This is generally an area of semi-permanent high pressure that exists and is where the Azores and North Pacific Highs may be found.

SUBTROPICAL CYCLONE: A low pressure system that develops over subtropical waters that initially has a non-tropical circulation but in which some elements of tropical cyclone cloud structure are present. Subtropical cyclones can evolve into tropical cyclones.

SUBTROPICAL DEPRESSION: A subtropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) is 38 mph (33 knots) or less.

SUBTROPICAL HIGH: A semi-permanent high-pressure region near 30 degrees latitude.

SUBTROPICAL STORM: A subtropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) is 39 mph (34 knots) or more.

SWATH: The width of the path of the hurricane. Usually this path area is about 125 miles wide with 75 miles to the right of the eye and 50 miles to the left of the eye.

SYNOPTIC SCALE: The size of migratory high and low pressure systems in the lower troposphere that cover a horizontal area of several hundred miles or more such as hurricanes. Contrasts with macroscale, mesoscale, and storms.

SYNOPTIC SURVEILLANCE 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. Synoptic flights better define the upper atmosphere and aid in the prediction of tropical cyclone development and movement.

T

THUNDER: The sound that follows a flash of lightning and is caused by sudden expansion of the air in the path of the electrical discharge.

THUNDERSTORM: A local storm produced by a cumulonimbus cloud, always with lightning and thunder, and usually accompanied by strong gusts of wind, heavy rain, and sometimes hail.

TORNADO: A violently rotating column of air in contact with and extending between a convective cloud and the surface of the earth. It is the most destructive of all storm-scale atmospheric phenomena. They can occur anywhere in the world given the right conditions, especially after the landfall of hurricanes.

TRADE WINDS: The wind system, occupying most of the tropics, which are northeasterly in the Northern Hemisphere and southeasterly in the Southern Hemisphere.

TROPICS/TROPICAL: The region of the earth located between the Tropic of Cancer, at 23.5 degrees North latitude, and the Tropic of Capricorn, at 23.5 degrees South latitude. It encompasses the equatorial region, an area of high temperatures and considerable precipitation during part of the year.

TROPICAL CYCLONE: A general term for all cyclone circulations originating over tropical waters. Its characteristics include a warm-core, non-frontal pressure system of synoptic scale that originates over the tropical or subtropical waters and has a definite organized surface. Used to define wind circulations rotating around an atmosphere which include tropical depressions, tropical storms, and hurricanes. The strongest winds of this cyclone are near the Earth's center.

TROPICAL CYCLONE PLAN OF THE DAY: A coordinated mission plan that tasks operational weather reconnaissance requirements during the next 1100 to 1100 UTC or Zulu day or as required, describes reconnaissance flights committed to satisfy both operational and research requirements, and identifies possible reconnaissance requirements for the succeeding 24-hour period.

TROPICAL DEPRESSION (TD): A tropical cyclone in which the maximum sustained surface winds (1 minute average) are 38 miles per hour (33 knots) or less. Characteristically having one or more closed isobars, it may form slowly from a tropical disturbance or an easterly wave, which has continued to organize. At this point, it gets a cyclone number, starting with "TD01" at the beginning of each storm season.

TROPICAL DISTURBANCE: A discrete system of clouds, showers, and thunderstorms (organized convection) that originate in the tropics. Generally 100 to 300 miles in diameter and originating in the tropics or subtropics, disturbances have a nonfrontal migratory character, and maintain their identity for 24 hours or more. It may or may not be associated with a detectable perturbation of the wind field. An upper level of low pressure causes this to occur. Approximately 100 of these types of events occur annually during hurricane season.

TROPICAL PREDICTION CENTER: A Division of the National Centers for Environmental Prediction, the Center issues watches, warnings, forecasts, and analyses of hazardous weather conditions in the tropics for both domestic and international communities which include the Atlantic, Caribbean, Gulf of Mexico and Eastern Pacific. The National Hurricane Center is one of its Branches.

TROPICAL STORM (TS): A tropical cyclone in which the maximum sustained surface wind speed (1 minute average) is within the range of 39 to 73 mph (34 to 63 knots). At this point, the system is given a name to identify and track it. In the Atlantic/Caribbean/Gulf of Mexico basin, the names start with "A" each season.

TROPICAL STORM WATCH: An announcement issued by the National Hurricane Center for specific areas that a tropical storm or a forecast of tropical storm conditions poses a possible threat to coastal areas generally within 36 hours. A tropical storm watch normally should not be issued if the system is forecast to attain hurricane strength.

TROPICAL STORM WARNING: A warning issued by the National Hurricane Center for tropical storm conditions including possible sustained winds within the range 39 to 73 mph (34 to 63 knots) which are expected in a specified coastal area within 24 hours or less.

TROPICAL WAVE: Another name for an easterly wave, it is an area of relatively low pressure (trough) moving westward through the trade wind easterlies. Generally, it is associated with extensive cloudiness and showers, and may be associated with possible tropical cyclone development.

TYPHOON: A hurricane that occurs in the Pacific Region of the Philippines or the China Sea.

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UPWELLING: The process by which water rises from a lower to a higher depth, usually as a result of divergence and offshore currents. It influences climate by bringing colder, more nutrient-rich water to the surface. This is a vital factor of the El Niño event.

UNIVERSAL TIME COORDINATE (UTC): One of several names for the twenty-four hour time that is used throughout the scientific and military communities. Other names for this time measurement are Greenwich Mean Time (GMT) or Zulu Time (Z). See Greenwich Mean Time for more information.

V

VORTEX: Any circular or rotary flow in the atmosphere that possesses vorticity.

VORTEX FIX: The location of the surface and/or flight level center of a tropical or subtropical cyclone obtained by reconnaissance aircraft penetration.

VORTICITY: The measurement of the rotation of a small air parcel. It has vorticity when the parcel spins as it moves along its path. Although the axis of the rotation can extend in any direction, meteorologists are primarily concerned with the rotational motion about an axis that is perpendicular to the earth's surface. If it does not spin, it is said to have zero vorticity. In the Northern Hemisphere, the vorticity is positive when the parcel has a counterclockwise, or cyclonic, rotation. It is negative when the parcel has clockwise, or anticyclonic, rotation.

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WARM FRONT: The leading edge of an advancing warm air mass that is replacing a retreating relatively colder air mass. Generally, with the passage of a warm front, the temperature and humidity increase, the pressure rises, and although the wind shifts (usually from the southwest to the northwest in the Northern Hemisphere), it is not as pronounced as with a cold frontal passage. Precipitation, in the form of rain, snow, or drizzle, is generally found ahead of the surface front, as well as convective showers and thunderstorms. Fog is common in the cold air ahead of the front. Although clearing usually occurs after passage, some conditions may produced fog in the warm air. See occluded front and cold front.

WARNING: An announcement that is issued when severe weather: 1) has developed; 2) is already occurring and reported; or 3) is detected on radar. Warnings state a particular hazard or imminent danger, such as tornadoes, severe thunderstorms, flash and river floods, hurricanes, etc

WEATHER SURVEILLANCE RADAR (WSR-88D): The newest generation of Doppler radars. These radar units, with help from a set of computers, show very detailed images of precipitation and other phenomena, including air motions within a storm.

WINDWARD: The direction from which the wind is blowing. Also known as the upwind side of an object. It is the opposite of the downwind or leeward side.

Z

ZULU TIME (Z): One of several names for the twenty-four hour time that is used throughout the scientific and military communities. Other names for this time measurement are Universal Time Coordinate (UTC) or Greenwich Mean Time (GMT). See Greenwich Mean Time for more information.