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AIR MASS: A large body of air having uniform temperature and moisture.

"ALBERTA CLIPPER": A low pressure system that develops in near Alberta in central Canada and moves rapidly east to southeast through the Great Lakes and northeast United States.

ANTICYCLONE: A high-pressure weather system. Winds rotate clockwise around an anticyclone. It is often associated with fair, sunny weather, but in the winter can produce very cold, even frigid weather.

ANVIL: A fan of ice clouds (cirrus) at the top of a thunderstorm cloud (cumulonimbus).

ARCTIC HIGH: A high pressure system, which originates in northern Canada, or the Arctic. When it moves south toward the U.S., it produces very cold, sometimes frigid weather.

"BACKDOOR" COLD FRONT: A cold front which approaches from the north to northeast instead of from the more common directions of north to west. Affects the northeastern U. S.

BERMUDA HIGH: A high pressure system centered in the western Atlantic Ocean, near Bermuda. Over much of the eastern United States, this usually causes a southerly wind flow of warm and humid air, and in some cases, heat waves during the summer.

CEILING: The height of the lowest layer of clouds which covers more than half of the sky.

CIRRUS: Clouds made up of ice crystals, occurring at a height of between 15,000 and 45,000 feet.

COLD FRONT: Boundary between advancing cold air and warm air which it is replacing. A cold frontal passage is accompanied by showers or thunderstorms, rising air pressure, and a sudden wind shift, generally from south or southwest to north or northwest.

CUMULUS: Clouds made up of detached individual domes, resembling hills, sometimes referred to as "fair-weather" or "puffy" clouds.

A Cumulus Line: a line of cumulus clouds which form initially over keys communities as a result of differential heating between the island chain and surrounding waters. This phenomenon is the Florida Keys version of a sea breeze front which is common along coastal areas. Pending on both atmospheric moisture and instability, the line could contain moderate cumulus, towering cumulus and possibly cumulonimbus.

CUMULONIMBUS: A cumulus cloud with great vertical development, often capped by an "anvil", sometimes called a "thunderhead". These clouds frequently produce thunderstorms and heavy rain.

CYCLONE: A low-pressure weather system, or storm. Winds rotate counter-clockwise around a cyclone. A cyclone generally produces cloudy, rainy weather.

DEGREE-DAY (HEATING/COOLING): Difference between the average daily temperature and 65 degrees. When the average temperature is greater than 65, the degree days are called "cooling" If the average temperature is less than 65 they are referred to as "heating" (HDD).

DEGREE-DAY (GROWING): The difference between the average daily temperature and 50 degrees, if this difference is greater than 0, is called a growing degree day (GDD). When the average temperature is less than 50, there are 0 GDDs for that day.

DEW: Water which condenses out of the air onto objects near the ground.

DEW POINT: The temperature to which the air must be cooled in order to have water vapor condense out of it.

DOWNBURST: A strong localized downdraft of air, usually from a thunderstorm cloud.

DRIZZLE: Precipitation of very small water droplets resembling a fine mist. Droplets of drizzle are much smaller than those of rain.

EQUINOX: The time which marks the beginning of spring or autumn. The day that the equinox occurs has 12 hours of both daylight and darkness.

EL NINO: Change in global weather patterns caused by anomalous warming of the eastern Pacific Ocean by at least 3 degrees. Associated with droughts affecting some areas and incessant storms and rainfall inundating others.

EYE: The relatively calm area in the center of a hurricane. In this area, winds are light and the sky is often only partly covered by clouds.

FOG: A dense cloud in contact with the ground.

FREEZING RAIN/DRIZZLE: Precipitation which falls as a liquid, but freezes on contact with the ground or objects near the ground with temperatures below freezing.

FROST: Small ice crystals which form when water condenses into droplets onto objects near the ground at temperatures below freezing.

FUJITA TORNADO SCALE: A scale ranging from F0 to F5 used to estimate wind speeds of a tornado, based on damage to buildings.

FUNNEL CLOUD: A violently rotating column of air extending downward from the base of a thunderstorm cloud. If and when it touches the ground, it becomes a tornado.

GALE: Wind speeds between 39 and 54 miles per hour.

GUST: A rapid fluctuation in wind speed where the peak wind is at least 10 knots (12 miles per hour) higher than the lowest wind.

HAIL: Precipitation which falls in the form of lumps or balls of ice. Hail can be as large as 5 inches in diameter.

HAZE: Fine particles of dust suspended in the air, which reduce visibility.

HEAT INDEX: The apparent temperature that the body feels due to the combination of abnormally high air temperature and high relative humidity.

HUMIDITY: The amount of water vapor in the air.

HURRICANE: A very intense area of low pressure which forms in the tropics and has a sustained wind speed of at least 74 miles per hour (64 knots).

"INDIAN SUMMER": A period of warm weather during the fall in the northern United States which comes after a period of cool weather, usually the first frost of the season.

INVERSION: A situation where temperature increases as you go higher up in the air. For example, an inversion is needed in order to have freezing rain occur.

JET STREAM: A band of strong winds located between eight and twelve miles up in the atmosphere. The jet stream is responsible for steering areas of low pressure (storms) and high pressure (fair weather makers).

KNOT: One nautical mile per hour, which is equal to 1.15 statute (land) miles per hour.

LIGHTNING: The electrical discharge from a thunderstorm cloud. Lightning is responsible for more deaths in Florida, by far, than any other weather-related cause.

MONSOON: A persistent seasonal wind and rainfall pattern that occurs over parts of Asia and Africa, as well as the southwest United States.

NOR'EASTER: A low pressure system which tracks along the east coast of the United States, usually during fall and winter. These storms can cause strong northeast winds (hence its name), large waves and beach erosion, and if accompanied by snow, a blizzard.

OVERRUNNING: A condition where warm moist air rides up an over cooler air at the surface, usually resulting in precipitation.

OZONE: A form of oxygen having 3 molecules, found in the upper atmosphere. Ozone filters out much of the harmful ultra-violet radiation of the sun.

PRECIPITATION: Liquid or solid water that falls from clouds and reaches the ground.

PRESSURE: The force created on the atmosphere by gravity. It can be thought of as the "weight" of the air.

RAIN: Precipitation which falls as a liquid, whose drops are larger than those of drizzle.

RAIN OR SNOW SHADOW: A downwind side of a mountain range which receives little or no precipitation during a storm. The upwind sides of mountain ranges receive much more precipitation than the downwind sides because the moisture in the air condenses out as the air rises up the range and dries out as it descends down the lee side of the range.

RELATIVE HUMIDITY: The amount of water vapor actually in the air, compared to how much water vapor the air can hold, expressed in percent.

RIDGE: An elongated area of high pressure in the atmosphere.

RIP CURRENT: A narrow 10 to 30 yard channel of water flowing seaward from the beach, resulting from the sand bar becoming breached. Rip currents are responsible for pulling swimmers out to sea and a number of drownings and most lifeguard rescues.

SAFFIR/SIMPSON HURRICANE SCALE: A scale ranging from one to five based on a hurricane's present intensity. Used to estimate the potential damage to property, flooding, storm surge along the coast from a hurricane.

SHOWER: Precipitation that falls intermittently (off and on) and has varying rates of intensity.

SLEET: Precipitation that falls as small ice pellets. Sleet is much smaller than hail, only about 1/8th of an inch in diameter.

SNOW: Precipitation made up of ice crystals in the shape of a hexagon.

SNOW FLURRY: Light showers of snow, with little or no accumulation.

SNOW SQUALL: Intense showers or bands of locally heavy snow which are accompanied by gusty wind. Snow squalls which form downwind of the Great Lakes (lake-effect squalls) can persist for several hours and produce 6 inches of snow or more in 3 to 6 hours.

SQUALL LINE: A line of showers or thunderstorms, often accompanied by strong gusty wind and heavy rain.

STATIONARY FRONT: A boundary between air masses, showing little, if any movement.

STORM SURGE: An abnormal rise in sea level accompanying a tropical cyclone or other intense storm. Computed as the difference between the sea surface level observed with the storm and the level that would have occurred in the absence of the storm.

STORM TIDE: The actual sea level resulting from the combination of actual astronomical tide and the storm surge.

SUBTROPICAL CYCLONE: A non-frontal, yet non-tropical cyclone which forms over the subtropical waters.

SUBTROPICAL DEPRESSION: A subtropical cyclone which has sustained wind speeds of 23 to 38 miles per hour (20 to 33 knots).

SUBTROPICAL STORM: A subtropical cyclone which has sustained wind speeds of at least 39 miles per hour (34 knots). Subtropical storms are not given names.

THUNDER: The sound caused by a lightning discharge as it heats the air, causing it to rapidly expand.

TORNADO: A funnel cloud which reaches the ground.

TROPICAL CYCLONE: An closed area of low pressure which forms in the tropics and possesses organized thunderstorm activity and a definite cyclonic wind circulation.

TROPICAL DEPRESSION: A tropical cyclone which has sustained winds of 23 to 38 miles per hour (20 to 33 knots).

TROPICAL STORM: An intense area of low pressure which forms in the tropics and has sustained winds of at 39 to 73 miles per hour (34 to 63 knots). Once a tropical storm forms, it is given a name.

TROPICAL WAVE: A trough or weak area of low pressure embedded within the tropical easterlies which is generally migratory (fast-moving) and without a defined cyclonic circulation.

TROUGH: An elongated area of low pressure in the atmosphere.

TYPHOON: A hurricane which occurs in the western Pacific Ocean.

VIRGA: Precipitation falling from a cloud that evaporates before reaching the ground.

WARM FRONT: Boundary between advancing warm air and colder air which it is replacing. A warm frontal passage is usually accompanied by low clouds, steady rain, falling air pressure, and a gradual wind shift, generally from east or southeast to south or southwest.

WATERSPOUT: A tornado which occurs over water, albeit usually, but not always, weaker.

WIND SHEAR: A rapid change of wind speed and/or direction with height or distance.

WIND CHILL: The apparent temperature that the body feels due to the combination of air temperature and wind speed.



POP: Probability of measurable precipitation (at least .01 inches) expressed as a percent.

LIKELIHOOD OF PRECIP: Probability of precipitation expressed in non-percent terms.

AREAL EXTENT: To what extent the precipitation covers a land area.


10% slight chance isolated

20% slight chance
30-50% chance scattered
60-70% likely numerous
80-100% see terms below widespread

The following terms are generally used when the probability of precipitation is 80 to 100 percent:

BRIEF (PERIODS OF): Some precipitation for a short duration.

OCCASIONAL: Precipitation "Now and then" over a few hours.

INTERMITTENT: Precipitation occurring at frequent intervals, or "on and off"

FREQUENT (PERIODS OF): Precipitation occurring with very short breaks or let-ups in between.