Weather Glossary
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  • Advisory
    A weather advisory is issued when conditions may cause inconvenience or concern.
  • Anomalous Propagation
    The radar can also be fooled by a phenomenon known as "anomalous propagation". The radar waves bend in regions of strong vertical temperature contrast. When the temperatures warm with increasing height (called an inversion), the beam bends down and can reflect off the ground and back to the radar. On the radar scope, these false echoes can often look like heavy precipitation.
  • Avg Dir
    See Radial Velocity.
  • Barometric Pressure
    The force per unit are exerted by the weight of the atmosphere above.
  • Base Reflectivity
    Flat display of radar reflectivity from a single scan angle (usually from 0.5 to 3.5 degrees). Reflectivity is a measure of how efficient a target is at returning the radar. See also Echo Tops.
  • Beam Blockage
    The blockage of the radar beam by mountain barriers. When this occurs, the radar can’t see precipitation beyond.
  • Ceiling
    Cloud ceiling. The lowest cloud layer which the observer sees as broken or overcast. Thin or partial clouds are not considered a ceiling.
  • Cell
    Storm cell. A mass of air that is the smallest unit of a storm-producing weather system.
  • Clouds
    Go to the Cloud Info Page.
  • Convective Clouds
    Puffy clouds which may extend 60,000 feet or more into the atmosphere resulting from rapidly rising, unstable air. May be accompanied by heavy precipitation (brief or intermittent), strong winds, hail and sometimes tornadoes.
  • CUR
    Current.
  • Cyclone
    See Tropical storm.
  • Depression (D, Depress)
    Area of low pressure. Depressions often bring Precipitation.
  • Dew Point (Dewpt)
    The temperature that the air must reach to hold the maximum amount of moisture. When the temperature cools to the dew point, the air becomes saturated, and the result is fog, dew, or frost.
  • Doppler Radar
    A radar which can measure atmospheric motion (wind) as well as detect precipitation. This ability to monitor winds enables forecasters to recognize rotation associated with tornadic storms.
  • Echo
    The "bounce-back" of radar energy from an object or objects encountered such as airplanes, birds, mountains or most importantly precipitation. In the case of precipitation, the heavier the precipitation, the stronger the echo return.
  • Echo Top
    The highest point where the "bounce-back" of radar energy from precipitation is greater than 18 dBZ. See also Base Reflectivity.
  • El Nino
    Go to the El Nino Info Page.
  • Elev
    Radar elevation. Scan angle of the radar.
  • ET
    Eastern Time.
  • False Echoes
    Non-precipitation objects picked up by the radar beam; for example, birds, buildings, even sects, airplanes and under certain atmospheric situations, the ground itself.
  • FCST
    Forecast.
  • GFS
    Global Forecast System.
  • Ground Clutter
    Nearest the radar site when the radar beam is closest to the ground, the non-precipitation "noise" is the greatest and is referred to as "ground clutter".
  • Gusts
    Sudden increases in wind speed, usually lasting less than 20 seconds.
  • Heat Index
    The temperature it feels like to people and animals. The heat index is a combination of temperature and humidity.
  • Humidity
    See Relative Humidity.
  • Hurricane (Hurr)
    A Tropical storm with inward spiraling winds exceeding 73 miles per hour.
  • Infrared (IR)
    Light waves with wavelengths between visible red light and radio waves.
  • Isobars
    Lines that join areas of equal air pressure.
  • Jetstream
    Fast corridors of air flowing 20,000 to 30,000 feet above the earth. There can be up to three jetstreams over the US at any one time.
  • Knots (KTS)
    Speed in nautical miles per hour. 1 knot is approximately equal to 1.15 miles per hour.
  • Lightning
    Discharge of electrostatic energy accompanied with the emission of light. Lightning passing through the air rapidly heats and expands it, producing thunder. When lighting forms, a negative charge usually collects in the cloud, and a positive charge collects in the ground under the charge. Lightning produced from this configuration is called a negative strike. Positive strikes are formed by the collection of a positive charge in the cloud and a negative charge in the ground.
  • Max Away
    See Radial Velocity.
  • Max Towards
    See Radial Velocity.
  • Mean
    The average or expected value.
  • Mode
    Whether the radar is operating in Clear Air or Precipitation mode. Please revise this definition as it applies to the US Nexrad Base Reflectivity Map.
  • Mosaic Radar
    A composite of radar data reported by multiple radar sites. NOWrad is an example of a mosaic radar product. Mosaics show the "big picture".
  • MPH
    Miles per hour.
  • Negative Strike
    See Lightning.
  • Nexrad
    Next Generation Radar from the National Weather Service. Also called Doppler Radar.
  • Orographic Uplift
    The forced lifting of air over mountain barriers. Often leads to clouds and precipitation or enhancement of precipitation.
  • Positive Strike
    See Lightning.
  • Precipitation
    Water falling from the sky. Water can fall as snow, rain, sleet, freezing rain, and hail, or evaporate before it reaches the ground.
  • Pressure
    The barometric pressure is the force exerted by the weight of the atmosphere above.
  • Radar
    Radio waves bounced off raindrops within clouds. The time at which the echo is received indicates the distance from the antenna, allowing the location of the clouds to be calculated.
  • Radial Velocity
    Motion towards or away from a given location. Used in tropical storms to measure the speeds of the inward spiraling winds characteristic of a Tropical storm or Hurricane.
  • Range
    Maximum distance from the radar antenna at which objects can be detected.
  • Range Effect
    The accuracy of the radar display diminishes with increasing distance from the site.
  • Raw Report Data
    The data provided by the Reporting station in its original format.
  • Reflectivity
    The intensity of the return echo. When precipitation, proportional to the precipitation rate /amount.
  • Relative Humidity (Humid)
    Reported as a percentage, relative humidity is the ratio of the actual moisture content of the air to the potential moisture content.
  • Reporting Station
    Weather station that provided the weather data. Shown on maps as a three-letter code.
  • Res
    Radar resolution. Minimum separation between two objects that allows them to be distinguished as two objects by radar. Objects closer together are merged into one object.
  • Sky Conditions
    Either CLR (clear), OVC (overcast), BKN (broken clouds), or SCT (scattered clouds).
  • Storm Rotation Detected
    Detection of radial velocity in opposite directions on either side of the radar beam, indicating rotating winds. If an area of low pressure contains winds that rotate, it may become a Tropical storm or Hurricane.
  • Stratiform Clouds
    Layered clouds resulting from large scale lifting of air usually associated with low pressure and fronts. Usually signifies stable atmospheric conditions. Precipitation is usually continuous.
  • Strong Thunderstorm
    See Thunderstorm.
  • Sweet Zone
    NEXRAD radar’s most accurate display region from just outside the clutter region to a radius of 120 to 150 km.
  • Temperature
    Degree of the warmth or coolness of the air.
  • Thickness
    Difference in height between two levels of atmospheric pressure.
  • Tilt
    The elevation angle of the radar transmitter/receiver during a scan rotation. NEXRAD utilizes a volume scan approach with up to 14 different tilts (from 0.5 degrees above the horizon to 19.5 degrees) in every update (every 5, 6 or 10 minutes). This provides a nearly complete look at the atmosphere in the region.
  • Thunderstorm
    A local storm with thunder and lightning, usually with gusty winds, heavy rain, and sometimes hail. A thunderstorm is produced by a cumulonimbus cloud, a tall, dense cloud which may look like a tower or mountain and is sometimes topped by a smooth, flat anvil shape. A strong thunderstorm is associated with a passing cold front, and may produce a tornado.
  • Tropical Storm (TS)
    Cyclone. An area of low pressure with inward spiraling winds less than 74 miles per hour and ascending air at the center. May become a Hurricane.
  • UTC
    Universal Time Coordinated. Also known as Greenwich Meridian Time (GMT) or Zulu Time (Z).
  • Valid
    Time at which the weather data was received. In the case of a forecast, time at which the forecast applies.
  • Virga
    Precipitation falling from the clouds but evaporating before reaching the ground.
  • Visibility (Obstructions To)
    Rain, fog, or other atmospheric conditions that may reduce or obstruct visibility.
  • Visibility (Visible)
    The greatest distance that a person with normal eyesight can see and identify an object.
  • Visible
    Light waves with wavelengths visible to the human eye.
  • Warning
    A weather warning is issued when a severe weather condition, such as a tornado or flash flood, is occurring or about to occur.
  • Watch
    A weather watch is issued when there is the possibility of a severe weather condition, such as a tornado or flash flood. Watches are provided well in advance of potential problems.
  • Water Vapor
    Water present as a gas in the atmosphere.
  • Wave
    Large ocean wave. Please revise this definition as it applies to the Storm Center’s Atlantic Analysis map.
  • Weather/Sky Conditions
    The state of cloudiness of the sky. Conditions may be: clear, cloudy, broken clouds, or scattered clouds.
  • Wet Bulb
    The lowest temperature that can be reached by evaporating water into the air. As precipitation falls into dry air, the temperatures fall and the dewpoints rise due to evaporation of moisture from the falling precipitation. The temperature they converge on is the wet-bulb temperature.
  • Wind
    Go to the Wind Info Page.
  • Wind Chill
    The temperature it feels like to people and animals. The wind chill is a combination of temperature and wind.
  • Z
    Zulu Time. Also known as Greenwich Meridian Time (GMT) or Universal Time Coordinated (UTC).
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