Absolute Humidity - The mass of water vapor in a given volume of air. It represents the density of water vapor in the air.
Acid Rain - Cloud or rain droplets combine with gaseous pollutants, such as oxides of sulfur and nitrogen, to make falling rain or snow acidic.
Advection - Transport of an atmospheric property by the wind.
Advection Fog - Occurs when warm, moist air moves over a cold surface and the air cools to below its dew point.
Air mass Thunderstorm - A thunderstorm not associated with a front. Air mass thunderstorms typically are associated with warm, humid air in the summer months; they develop during the afternoon in response to afternoon heating by the sun, and dissipate rather quickly after sunset.
Air Mass - A large body of air that has similar horizontal temperature and moisture properties.
Air Pressure - The pressure exerted by the weight of air above a given point, usually expressed in millibars (mb) or inches of mercury (in. Hg).
Altitude - A measure of height of an object, usually with reference to the earth's surface, or sea level.
Altocumulus - A middle cloud, usually white or gray. Often occurs in layers of patches with wavy, rounded masses or rolls.
Anemometer - An instrument designed to measure wind speed.
Anticyclonic Rotation - Rotation in the opposite sense as the Earth's rotation, i.e., clockwise in the Northern Hemisphere as would be seen from above. The opposite of cyclonic rotation.
Atmosphere - A gaseous covering to a planet that is bound by gravity. Planets have very different atmospheres and each has very different properties.B
Backing Winds - Winds that shift in a counterclockwise direction with time at a given location (e.g. from northerly to westerly). The opposite of veering winds.
Barometer - An instrument that measures atmospheric pressure.
Beach Erosion - The movement of beach materials by wave action, currents and tides, or wind.
Blizzard - A weather condition characterized by low temperatures and strong winds (greater than 35mph) bearing a great amount of either falling or blowing snow.
Blizzard Warning - Snow, strong winds and low temperatures will combine to produce a blinding snow, deep drifts and life threatening wind chill. This means temperatures below 20 degrees and winds above 35mph.
Blowing Snow - wind-driven snow that reduces visibility near the ground. Blowing snow can be falling snow or snow that has already accumulated but is picked up and blown by strong winds.
Bow Echo - A radar echo that is linear but bent outward in a bow shape. Damaging straight-line winds often occur near the "crest" or center of a bow echo. Areas of circulation also can develop at either end of a bow echo, which sometimes can lead to tornado formation - especially in the left (usually northern) end, where the circulation shows cyclonic rotation.C
Ceiling - The height of the lowest layer of clouds when the weather reports describe the sky as broken or overcast.
Ceilometer - An instrument that automatically records cloud height.
Celsius Scale - A temperature scale where zero is assigned to the temperature where water freezes and 100 to the temperature where water boils (at sea level).
Cirrocumulus - A high cloud that appears as a white patch of cloud without shadows. It consists of very small elements in the form of grains or ripples.
Cirrostratus - A high cloud appearing as a whitish veil that may totally cover the sky. Often produces halo phenomena.
Cirrus - A high cloud composed of ice crystals in the form of thin, white, featherlike clouds in patches, filaments, or narrow bands.
Climate - The accumulation of daily and seasonal weather events over a long period of time.
Cloudburst - Any sudden and heavy rain shower.
Cold Advection - Transport of cold air into a region by horizontal winds.
Cold Front - A transition zone where a cold air mass advances and replaces a warm air mass.
Condensation - The process by which water vapor becomes a liquid.
Convection - Warm air rising or cold air sinking or both.
Convergence - Convergence in a horizontal wind field indicates that more air is entering a given area than is leaving at that level. To compensate for the resulting "excess," vertical motion may result: upward forcing if convergence is at low levels, or downward forcing (subsidence) if convergence is at high levels. Upward forcing from low-level convergence increases the potential for thunderstorm development.
Cumulus - A cloud in the form of individual, detached domes or towers that are usually dense and well defined.
Cyclogenesis - Development or intensification of a low-pressure center (cyclone).D
Desert - Area which averages less than 10" of precipitation per year. Temperature ranges from very hot in the summer to below freezing at night during the winter.
Dew - Water that has condensed onto objects near the ground when their temperatures have fallen below the dew point of the surface air.
Dew Point - The temperature to which the air must be cooled to condense. For example, if the air temperature was 65 degrees and the dew point was 65 degrees the humidity would be 100% and the air would be totally saturated. The larger the spread of temperature and dew point, the drier the air. This spread is called the dew point depression.
Diurnal – Daily occurrence; related to actions which are completed in the course of a calendar day, and which typically recur every calendar day (e.g., diurnal temperature rises during the day, and diurnal falls at night).
Divergence – Spreading out of air molecules away from each other and a certain location. More air is leaving than entering the location. The opposite of convergence, divergence at upper levels of the atmosphere enhances upward motion, and hence the potential for thunderstorm.
Doppler Radar - A radar that determines the velocity of falling precipitation either toward or away from the radar unit.
Downburst - A severe localized downward gust of air that can be experienced beneath a severe thunderstorm.
Downslope winds- typically warm and dry, occur in many parts of the world where mountains stand in the path of strong air currents. Dry air decending in elevation warms to a higher temperature than air at the surface. In the European Alps they are known as the foehn. The foehn has other names in other places: zonda in Argentina, halny wiatr in Poland, koembang in Java, and Santa Ana in California. In the Rocky Mountains, where warm, dry downslope winds can melt a foot of snow in less than a hour, they are called the chinook--after Native Americans of the Pacific northwest, where the winds originate.
Drifting Snow - An uneven distribution of snowfall caused by strong surface winds.
Drought - A period of abnormally dry weather sufficiently long enough to cause serious effects on agriculture and other activities in the affected area.
Dry Line - A boundary separating moist and dry air masses, and an important factor in severe weather frequency in the Great Plains. It typically lies north-south across the central and southern high Plains states during the spring and early summer, where it separates moist air from the Gulf of Mexico (to the east) and dry desert air from the southwestern states (to the west).
Dust Devil - A small atmospheric vortex not associated with a thunderstorm, which is made visible by a rotating cloud of dust or debris (dust whirl). Dust devils form in response to surface heating during fair, hot weather; they are most frequent in arid or semi-arid regions.E
El Nino - An extensive ocean warning that begins along the coast of Peru and Equador.
Evaporation Fog - Fog produced when sufficient water vapor is added to the air by evaporation.
Eye - A region in the center of a hurricane (tropical storm) where the winds are light and skies are clear to partly cloudy.
Eye Wall - A wall of dense thunderstorms that surrounds the eye of a hurricane.F
Fahrenheit Scale - A temperature scale where 32 is assigned to the temperature where water freezes and 212 to the temperature where water boils (at sea level).
Flash Flood - A flood which is caused by heavy or excessive rainfall in a short period of time, generally less than 6 hours.
Flood - The filling with water of a normally dry area of land caused by an increased water level in a stream, river or drainage ditch or by the ponding of rainwater.
Flood Warning - Flooding has been reported or is imminent. Take the necessary precaution if you are in a flood prone area.
Flood Watch - Flooding is possible within the watch area. Sometimes this is called a Flash Flood Watch or an Urban and Small Stream Advisory to indicate the possibility of rapidly rising water and flooding or high water on streets, underpasses and around storm drains.
Fog - A cloud with its base at the Earth's surface.
Freeze - A condition occurring over a widespread area when the surface air temperature remains below freezing for a sufficient time to damage certain agricultural crops.
Freezing Rain/Drizzle - Occurs when super cooled rain or drizzle freezes upon contact with surfaces such as the ground, trees, power lines, etc.
Front - The transition zone between two different air masses.
Frost - A covering of ice produced by water vapor freezing on exposed surfaces when the air temperature falls below the frost point.
Frostbite - The partial freezing of exposed parts of the body, causing injury to the skin and sometimes to deeper tissues.
Frost/Freeze Warning - Below freezing temperatures are expected during the growing season and may cause significant damage to plants and crops.
Fujita Scale (or F Scale) - A scale of wind damage from tornadoes in which wind speeds are inferred from an analysis of wind damage:
F0 (weak): 40- 72 mph, light damage.
F1 (weak): 73-112 mph, moderate damage.
F2 (strong): 113-157 mph, considerable damage.
F3 (strong): 158-206 mph, severe damage.
F4 (violent): 207-260 mph, devastating damage.
F5 (violent): 261-318 mph, (rare) incredible damage.