Weather Glossary
Apogee
The point in the orbit of an object that is orbiting the Earth where it is furthest from the Earth.
Barometric Pressure
Barometric Pressure is the measure of pressure at a specific point in the
atmosphere. Generally, the more air above an area, the higher the
atmospheric pressure. Barometric pressure changes with local weather
conditions, making it an important weather forecasting tool. Rising pressure
indicates improving weather conditions, while falling pressure indicates
deteriorating weather conditions.
Beaufort Wind Scale
A system of estimating and reporting wind speed devised by British
Rear-Admiral, Sir Francis Beaufort (1774-1857) in 1805.
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Enhanced Fujita Scale
A scale for estimating the strength of tornadoes based on the damage they cause.
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Dew Point
Dew Point is the temperature to which a measurement of air must be cooled at
constant barometric pressure for water vapor to condense into water. The
condensed water is called dew. The dew point is also known as a Saturation
Point.
Evapotranspiration (ET)
The sum of evaporation and plant transpiration from the ground to the atmosphere.
Flash Flood Watch
A Flash Flood Watch is issued to indicate current or developing conditions that are favorable
for producing flash flooding in and close to the watch area.
Flash Flood Warning
A Flash Flood Warning issued to inform the public, emergency management, and
other cooperating agencies that flash flooding is in progress or is imminent.
Heating and Cooling Degrees
Heating and Cooling Degrees are indices that reflect demand for energy to heat or cool houses and
businesses.
Humidity
Humidity, or relative humidity, measures the amount of water vapor in the air compared to the
maximum amount of water vapor the air can hold at a specific temperature.
Perigee
The point in the orbit of an object that is orbiting the Earth where it is closest to Earth.
Severe Thunderstorm Watch
A Severe Thunderstorm Watch is issued when conditions are favorable for the
development of severe thunderstorms in and close to the watch area. A severe
thunderstorm by definition is a thunderstorm that produces 3/4 inch in
diameter or larger hail and/or winds of or exceeding 58 miles an hour. A
watch typically lasts 4 to 8 hours and are normally issued well in advance
of the actual occurrence of severe weather.
Severe Thunderstorm Warning
A Severe Thunderstorm warning is issued when either a severe thunderstorm is
indicated by radar or a spotter reports a thunderstorm producing hail 3/4
inch or larger in diameter and/or winds equal or exceeding 58 miles an hour.
People in the affected area should seek safe shelter immediately. Severe
thunderstorms can produce tornadoes with little or no advance warning.
Warnings are usually issued for a duration of one hour and can be issued
without a Severe Thunderstorm Watch being already in effect.
Skywarn
Skywarn is a nationwide network of volunteer weather spotters who report to and are
trained by the National Weather Service. These spotters report many forms of
significant or severe weather.
Snow Advisory
A Snow Advisory is issued when a low pressure system produces snow that may cause
significant inconveniences, but do not meet warning criteria.
Solar Radiation
The electromagnetic radiation emitted by the sun, measured in Watts per
square Meter (W/m2).
THSW Index
THSW Stands for Temperature, Humidity, Solar Radiation and Wind Speed.
Similar to Heat Index, the THSW Index uses humidity and temperature to
calculate the apparent temperature. Additionally, THSW incorporates the
heating effects of solar radiation and the cooling effects of wind on a
person's perception of temperature.
Tornado Watch
A Tornado Watch is issued when conditions are favorable for the development
of tornadoes in and close to the watch area. The size of the watch area can
vary depending on the weather situation. Tornado Watches are usually issued
for a duration of 4 to 8 hours and normally are issued well in advance of
the actual occurrence of severe weather.
Tornado Warning
A Tornado Warning is issued when a tornado is indicated by radar or sighted
by spotters. People in the affected area should seek safe shelter
immediately. They can be issued without a Tornado Watch being already in
effect. A Tornado Warning is usually issued for a duration of around 30
minutes.
UV Index
The UV index is a measurement of how strong Ultraviolet (UV) radiation from the sun is at a location.
The following table shows the UV Index scale primarily used in daily forecasts.
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Wind Run
Wind run is the total distance that the wind has traveled during a period of time in a
specific direction.
Winter Storm Watch
A Winter Storm Watch is issued when there is a potential for snow or
significant ice accumulations, usually issued at least 24 to 36 hours in
advance. You should be alert to changing weather conditions and
avoid unnecessary travel.
Winter Storm Warning
A Winter Storm Warning is issued when a winter storm is producing or is forecast to
produce heavy snow or significant ice accumulations. Stay indoors during the storm.
If you must go outdoors, several layers of lightweight clothing will keep you warmer
than a single heavy coat. Wearing gloves and a hat will prevent loss of body heat.
Cover your mouth to protect your lungs. Understand The Hazards of wind chill.
Walk carefully on snowy or icy sidewalks.
Winter Weather Advisory
A Winter Weather Advisory is issued when a low pressure system produces a combination
of winter weather (snow, freezing rain, sleet, etc.) that present a hazard, but does
not meet warning criteria.