Weather Glossary

Air MassA large body of air having similar horizontal temperature and moisture characteristics.Alberta ClipperA small, fast-moving low-pressure system that forms in western Canada and travels southeastward into the United States. These storms, which generally bring little precipitation, generally precede an Arctic air mass.AnticyclonicDescribes the movement of air around a high pressure, and rotation about the local vertical opposite the earth's rotation. This is clockwise in the Northern Hemisphere.AnvilA flat, elongated cloud formation at the top of a thunderstorm.Back Door Cold FrontA front that moves east to west in direction rather than the normal west to east movement.Back Building ThunderstormA thunderstorm in which new development takes place on the upwind side (usually the west or southwest side), such that the storm seems to remain stationary or propagate in a backward direction.Bow EchoAn accelerated portion of a squall line of thunderstorms, taking on bow configuration, created by strong downburst winds.CapA layer of relatively warm air aloft (usually several thousand feet above the ground) which suppresses or delays the development of thunderstorms. Air parcels rising into this layer become cooler than the surrounding air, which inhibits their ability to rise further. As such, the cap often prevents or delays thunderstorm development even in the presence of extreme instability.CeilingThe height of the lowest layer of clouds, when the sky is broken or overcast.CellConvection in the form of a single updraft, downdraft, or updraft/downdraft couplet, typically seen as a vertical dome or tower as in a cumulus or towering cumulus cloud. A typical thunderstorm consists of several cells.Cold Air FunnelA funnel cloud or (rarely) a small, relatively weak tornado that can develop from a small shower or thunderstorm when the air aloft is unusually cold (hence the name). They are much less violent than other types of tornadoes.ConvectionThe transfer of heat within a the air by its movement. The term is used specifically to describe vertical transport of heat and moisture, especially by updrafts and downdrafts in an unstable atmosphere.Dew PointThe temperature to which the air must be cooled for water vapor to condense.DownburstA severe localized downdraft from a thunderstorm.DowndraftA column of generally cool air that rapidly sinks to the ground, usually accompanied by precipitation as in a shower or thunderstorm.Dry LineA line that separates very warm, moist air to the east from hot, dry air to the west.El NioA major warming of the equatorial waters in the Pacific Ocean. El Nino events usually occur every 3 to 7 years, and are characterized by shifts in "normal" weather patterns.Flanking LineA line of cumulus connected to and extending outward from the most active portion of a parent cumulonimbus, usually found on the southwest side of the storm. The cloud line has roughly a stair step appearance with the taller clouds adjacent to the parent cumulonimbus. It is most frequently associated with strong or severe thunderstorms.FrontThe transition zone between two distinct air masses. The basic frontal types are cold fronts, warm fronts, occluded fronts, and stationary fronts.GustA brief sudden increase in wind speed. Generally the duration is less than 20 seconds and the fluctuation greater than 10 mph.Gust FrontThe leading edge of the downdraft from a thunderstorm.GustnadoGust front tornado. A small tornado, usually weak and short-lived, that occurs along the gust front of a thunderstorm. Often it is visible only as a debris cloud or dust whirl near the ground.Hook EchoA radar pattern sometimes observed in the southwest quadrant of a tornadic thunderstorm. Appearing like a fishhook turned in toward the east, the hook echo is precipitation aloft around the periphery of a rotating column of air 2-10 miles in diameter.HumidityThe amount of water vapor in the atmosphere. (See relative humidity).Indian SummerAn unseasonably warm period near the middle of autumn, usually following a substantial period of cool weather.Inflow NotchA radar signature characterized by an indentation in the reflectivity pattern on the inflow side of the storm. The indentation often is V-shaped, but this term should not be confused with V-notch. Supercell thunderstorms often exhibit inflow notches, usually in the right quadrant of a classic supercell, but sometimes in the eastern part of an HP storm or in the rear part of a storm (rear inflow notch).InstabilityA state of the atmosphere in which convection takes place spontaneously, leading to cloud formation and precipitation.Jet StreamStrong winds concentrated within a narrow band in the atmosphere. The jet stream often "steers" surface features such as fronts and low pressure systems.Lake EffectThe effect of a lake (usually a large one) in modifying the weather near the shore and down wind. It is often refers to the enhanced rain or snow that falls downwind from the lake. This effect can also result in enhanced snowfall along the east coast of New England in winter.La NiaA cooling of the equatorial waters in the Pacific Ocean.LandspoutA tornado that does not arise from organized storm-scale rotation and therefore is not associated with a wall cloud (visually) or a mesocyclone (on radar). Landspouts typically are observed beneath Cbs or towering cumulus clouds (often as no more than a dust whirl), and essentially are the land-based equivalents of waterspouts.Low-level JetA region of relatively strong winds in the lower part of the atmosphere.MacroburstLarge thunderstorm downbursts with a 2.5 mile diameter or greater outflow of damaging winds lasting 5 to 20 minutes.MesonetA regional network of observing stations (usually surface stations) designed to diagnose mesoscale weather features and their associated processes.Mesoscale Convective ComplexA large mesoscale convective system, generally round or oval-shaped, which normally reaches peak intensity at night. The formal definition includes specific minimum criteria for size, duration, and eccentricity (i.e., "roundness"), based on the cloud shield as seen on infrared satellite photographs.MicroburstA strong localized downdraft less than 2.5 miles in diameter from a thunderstorm. Peak gusts last from 2 to 5 minutes.Multicell Cluster ThunderstormA thunderstorm consisting of two or more cells, of which most or all are often visible at a given time as distinct domes or towers in various stages of development.Nor'easterA low-pressure disturbance forming along the South Atlantic coast and moving northeast along the Middle Atlantic and New England coasts to the Atlantic Provinces of Canada. It usually causes strong northeast winds with rain or snow. Also called a Northeaster or Coastal Storm.OutflowAir that flows outward from a thunderstorm.Pendant EchoRadar signature generally similar to a hook echo, except that the hook shape is not as well defined.Prevailing WesterliesWinds in the middle latitudes (approximately 30 degrees to 60 degrees) that generally blow from west to east.Pulse StormA thunderstorm within which a brief period (pulse) of strong updraft occurs, during and immediately after which the storm produces a short episode of severe weather. These storms generally are not tornado producers, but often produce large hail and/or damaging winds. See overshooting top, cyclic storm.Rain-Free BaseA horizontal, dark cumulonimbus base that has no visible precipitation beneath it. This structure usually marks the location of the thunderstorm updraft. Tornadoes most commonly develop (1) from wall clouds that are attached to the rain-free base, or (2) from the rain-free base itself. This is particularly true when the rain-free base is observed to the south or southwest of the precipitation shaft.

Red Flag Warning{}{}{} A term used by fire-weather forecasters to call attention to limited weather conditions of particular importance that may result in extreme burning conditions. It is issued when it is an on-going event or the fire weather forecaster has a high degree of confidence that Red Flag criteria will occur within 24 hours of issuance. Red Flag criteria occurs whenever a geographical area has been in a dry spell for a week or two, or for a shorter period , if before spring green-up or after fall color, and the National Fire Danger Rating System (NFDRS) is high to extreme and the following forecast weather parameters are forecasted to be met:{}{}{} 1) a sustained wind average 15 mph or greater{}{}{} 2) relative humidity less than or equal to 25 percent and{}{}{} 3) a temperature of greater than 75 degrees F.{}{}{} In some states, dry lightning and unstable air are criteria. A Fire Weather Watch may be issued prior to the Red Flag Warning.

ReflectivityRadar term referring to the ability of a radar target to return energy; used to estimate precipitation intensity and rainfall rates.Relative HumidityThe amount of water vapor in the air, compared to the amount the air could hold if it was totally saturated. (Expressed as a percentage).Return FlowSouth winds on the back (west) side of an eastward-moving surface high pressure system. Return flow over the central and eastern United States typically results in a return of moist air from the Gulf of Mexico (or the Atlantic Ocean).Roll CloudA relatively rare, low-level horizontal, tube-shaped accessory cloud completely detached from the cumulonimbus base. When present, it is located along the gust front and most frequently observed on the leading edge of a line of thunderstorms. The roll cloud will appear to be slowly "rolling" about its horizontal axis. Roll clouds are not and do not produce tornadoes.Shelf CloudLong, wedge-shaped clouds associated with the gust front. Shelf clouds indicate the downdraft or outflow of a thunderstorm.Shear (Wind Shear)Variation in wind speed and/or direction over a short distance. Shear usually refers to vertical wind shear, i.e., the change in wind with height, but the term also is used in Doppler radar to describe changes in radial velocity over short horizontal distances.Squall LineA non-frontal band or line of thunderstorms.Stationary FrontA transition zone between air masses, with neither advancing upon the other.Straight-Line WindsThunderstorm winds most often found with the gust front. They originate from downdrafts and can cause damage which occurs in a "straight line", as opposed to tornadic wind damage which has circular characteristics.Subtropical JetThe branch of the jet stream that is found in the lower latitudes.SupercellA highly organized thunderstorm with a rotating updraft, known as a mesocyclone. It poses an inordinately high threat to life and property. Often produces large hail, strong winds, and tornadoes.Sustained WindsThe wind speed obtained by averaging the observed values over a one minute period.Tilted StormA thunderstorm or cloud tower which is not purely vertical but instead exhibits a slanted or tilted character. It is a sign of vertical wind shear, a favorable condition for severe storm development.Trade WindsPersistent low-level tropical winds that blow from the subtropical high pressure centers towards the equatorial low.TroughAn elongated area of low pressure at the surface or aloft.UpdraftA small-scale current of rising air. This is often associated with cumulus and cumulonimbus clouds.Upper Level SystemA general term for any large-scale or mesoscale disturbance capable of producing upward motion (lift) in the middle or upper parts of the atmosphere.VirgaPrecipitation falling from the base of a cloud and evaporating before it reaches the ground.Wall CloudAn isolated lowering of a cloud that is attached to the rain-free base of a thunderstorm, generally to the rear of the visible precipitation area. Wall clouds indicate the updraft of or the inflow to a thunderstorm.WedgeA large tornado with a condensation funnel that is at least as wide (horizontally) at the ground as it is tall (vertically) from the ground to cloud base.Wind AloftThe wind speeds and wind directions at various levels in the atmosphere above the area of surface.Zonal FlowLarge-scale atmospheric flow in which the east-west component (i.e., latitudinal) is dominant.