GENERAL WEATHER DEFINITIONS
ANTICYCLONE: Motion in a clockwise manner in the northern hemisphere and motion in an anticlockwise manner in the southern hemisphere.
ATMOSPHERE: The gaseous portion of a planet; the planet’s envelope of air; one of the traditional subdivisions of the earth’s physical environment.
COLD ADVECTION: Transport of cold air into a region by horizontal winds.
CYCLONE: An atmospheric closed circulation rotating counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
HURRICANE: A severe tropical cyclone having winds in excess of 64 knots.
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.
ISENTROPIC SURFACE: A two-dimensional surface containing points of equal potential temperature.
ISOBAR: A line connecting points of equal pressure.
ISODROSOTHERM: A line connecting points of equal dew point temperature.
ISOHYET: A line connecting points of equal precipitation amounts.
ISOPLETH: General term for a line connecting points of equal value of some quantity. Isobars, isotherms, etc. all are examples of isopleths.
ISOTACH: A line connecting points of equal wind speed.
ISOTHERM: A line connecting points of equal temperature.
JET STREAM: Relatively strong winds concentrated in a narrow stream in the atmosphere, normally referring to horizontal, high-altitude winds.
METEOROLOGY: The study of the atmosphere and atmospheric phenomena.
RIDGE: An elongated area of relatively high atmospheric pressure; the opposite of trough.
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.
TROPICAL DEPRESSION: A tropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) is 33 kt (38 mph or 62 km/hr) or less.
TROPICAL STORM: Organized thunderstorms with cyclonic wind speed between 35-64 knots.
TROPICAL WAVE: A trough or cyclonic curvature maximum in the trade-wind easterlies. The wave may reach maximum amplitude in the lower middle troposphere.
TROUGH: An elongated area of relatively low atmospheric pressure, usually not associated with a closed circulation, and thus used to distinguish from a closed low. The opposite of ridge.
WARM ADVECTION: Transport of warm air into an area by horizontal winds.
WIND SHEAR: Variation in wind speed (speed shear) and/or direction (directional shear) over a short distance. Shear usually refers to vertical wind shear, i.e., the change in wind with height,
CLOUDS AND RAINFALL DEFINITIONS
BRIEF SHOWERS: Occurring for less than 20 minutes.
CLOUDY: Cloudy most of the time, with relatively high probability of rainfall.
COASTAL AREAS: Less than 5 miles (8km) from the sea and below 500ft.
FAIR AND SUNNY: Little clouds for most of the time with relatively low probability of rainfall.
FINE: Few clouds, no rain expected.
INLAND AREAS: More than five miles (8km) from the sea.
ISOLATED SHOWERS: Occurring over less than 10% of the area.
HILLY AREAS: All areas above 500ft.
LOW LYING AREAS: Areas below 500ft.
MOST PLACES: Occurring over 80%__95% of the area.
OCCASIONAL: Occurring at least three times.
OVERCAST: Cloudy all the time, with relatively high probability of rainfall.
PARTLY CLOUDY: Varying at times between fair and mainly cloudy with relatively low probability of rainfall.
PROLONG: Occurring for more than 2 hours.
RAIN: Continuous rainfall from mainly stratified cloud (sheet clouds), with intensity from light to moderate or moderate to heavy
SCATTERED SHOWERS: Occurring over 30%__60% of the area.
SHOWERS: Convective rainfall, (not continuous) lasting from several minutes to one or two hours with intensity from light to moderate or moderate to heavy.
SQUALLS: Light or moderate showers, lasting less than half an hour, preceded by strong gusty winds.
THUNDERY SHOWERS (THUNDERSTORMS): Moderate to heavy showers, accompanied by thunder and lightning.
WIDESPREAD: Occurring over 40%__90% of the area.
WARNING TERMS AND MEANING
Bulletin: Issued in the event of the occurrence or forecast occurrence of a Tropical Storm or Hurricane that poses a possible threat.
Flash Flood Watch: Advises that flash flooding is possible and urges residents to take precautionary measures.
Flash Flood Warning: Indicates that flooding has been reported or will occur shortly. Residents are advised to be on the look-out for fast rising waters and to take precautionary action. Motorists and pedestrians are directed not to attempt to cross fords, flooded streets or other flooded areas.
Hurricane Warning: Indicates that the following dangerous effects of a hurricane are expected in 36 hours or less:
(1) Dangerously high water or a combination of dangerously high water and exceptionally high waves, even though wind speeds may be less than hurricane force;
(2) Average winds of 64 knots (118 km/h) or higher;
Hurricane Watch: Indicates that Hurricane conditions pose a possible threat within 48 hours.
Severe Weather Alert: Indicates that weather conditions are expected to deteriorate soon. This can be considered the first level of warning.
Thunderstorm Warning: Issued when severe thunderstorm (frequent lightning and/or thunder and strong gusty winds) activity has been reported or will occur shortly.
Tropical Storm Watch: Indicates that Tropical Storm conditions pose a possible threat within 48 hours.
Tropical Storm Warning: Indicates that Tropical Storm conditions, including possible sustained wind speeds of 34-63 knots or 63-117 km/h, are expected within 36 hours or less.