Weather Glossary

ADVECTION The horizontal transport of air or atmospheric properties. Commonly used with temperatures, i.e., "warm air advection", or moisture, i.e., "moisture advection".

ADVISORY  Issued for weather situations that cause significant inconveniences but do not meet warning criteria and, if caution is not exercised, could lead to life-threatening situations.

AIR MASS A large body of air having similar horizontal temperature and moisture characteristics.

COLD FRONT The boundary between a cold air mass that is advancing and a relatively warmer airmass.

CONDENSATION The process of gas changing to liquid. The process by which water vapor changes into water droplets and clouds.

CYCLONE An area of low pressure around which winds blow counterclockwise in the Northern Hemisphere. Also the term used for a hurricane in the Indian Ocean and in the Western Pacific Ocean.

DEW Moisture that has condensed on objects near the ground, whose temperatures have fallen to the dew point temperature.

DEW POINT The temperature to which the air must be cooled for water vapor to condense.

DOPPLER RADAR A type of weather radar that determines whether atmospheric motion is toward or away from the radar. It uses the Doppler effect to measure the velocity of particles suspended in the atmosphere.

DRIZZLE Small, slowly falling water droplets, with diameters between .2 and .5 millimeters.

FOG The visible aggregate of minute water droplets suspended in the atmosphere near the earth's surface. Essentially a cloud whose base is at the earth's surface.

FREE AIR FREEZING LEVEL The general level in the atmosphere where temperatures drop below freezing. "Free Air" is used to make a distinction between the true freezing level in the mixed atmosphere as opposed to localized pockets of subfreezing air which collect in valley bottoms under an inversion during fall and winter nights (and even in summertime in high elevation complex terrain).

FREEZING LEVEL The altitude in the atmosphere where the temperature equals 32F.

FREEZING RAIN Rain which falls as liquid then freezes upon impact, resulting in a coating of ice on exposed objects.

FRONT The transition zone between two distinct air masses. The basic frontal types are cold fronts, warm fronts, occluded fronts, and stationary fronts.

FUNNEL CLOUD A rotating, cone-shaped column of air extending downward from the base of a thunderstorm, but not in contact with the ground. When it reaches the ground it is then called a tornado.

HAIL Precipitation in the form of circular or irregular-shaped lumps of ice.

HALOS Rings or arcs that seem to encircle the sun or moon. They are caused by the refraction of light through the ice crystals in cirrus clouds.

HAZE Fine dry or wet dust or salt particles in the air that reduce visibility.

HIGH The center of an area of high pressure, accompanied by anticyclonic and outward wind flow in the northern hemisphere. Also known as an anticyclone.

HUMIDITY The amount of water vapor in the atmosphere. (See relative humidity).

JET STREAM Strong winds concentrated within a narrow band in the atmosphere. The jet stream often "steers" surface features such as fronts and low pressure systems.

KNOT One nautical mile per hour (1.15 mph)

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NEXRAD NEXt Generation RADar A NWS network of about 160 Doppler radars being installed nationwide.

NOAA WEATHER RADIO (NWR) Continuous, 24 hour-a-day VHF broadcasts of weather observations and forecasts directly from National Weather Service offices. A special tone allows certain receivers to alarm when watches or warnings are issued.

OCCLUDED FRONT A complex frontal system that occurs when a cold front overtakes a warm front. Also known as an occlusion.

RADIOSONDE An instrument attached to a weather balloon that transmits pressure, humidity, temperature, and winds as it ascends.

RAIN Liquid water droplets that fall from the atmosphere, having diameters greater than drizzle.

RAINBOW Optical phenomenon when light is refracted and reflected by moisture in the air into concentric arcs of color.

RELATIVE HUMIDITY The amount of water vapor in the air, compared to the amount the air could hold if it was totally saturated. (Expressed as a percentage).

SEVERE THUNDERSTORM A strong thunderstorm with wind gusts in excess of 58 mph (50 knots) and/or hail with a diameter of 3/4" or more.

SHOWER Precipitation that is intermittent, in space, time, or intensity.

SLEET A type of frozen precipitation, consisting of small transparent ice pellets.

SNOW Frozen precipitation composed of ice particles in complex hexagonal patterns.

SNOW FLURRIES Light snow showers, usually of an intermittent nature with no measurable accumulation.

STATIONARY FRONT A transition zone between air masses, with neither advancing upon the other.

THUNDERSTORM A storm with lightning and thunder, produced by a cumulonimbus cloud, and usually associated with gusty winds, heavy rain, and sometimes hail and tornadoes.

TORNADO A violently rotating column of air below the base of a thunderstorm, and in contact with the ground. A tornado does not require the visible presence of a condensation funnel cloud.

WARM FRONT A boundary between a warm air mass that is replacing a cooler air mass.

WARNING Issued when a particular hazard is "imminent" or already occurring (e.g., tornado warning, flash flood warning).

WATCH Forecast issued in advance to alert the public of the possibility of a particular hazard (e.g., tornado watch, flash flood watch).