500 mb
Pressure surface (geopotential height) in the troposphere equivalent to about 18,000 feet above sea level. Level of the atmosphere at which half the mass of the atmosphere lies above and half below, as measured in pressure units. This area is important for understanding surface weather, upper air storms tend to be steered in the direction of the winds at this level and are highly correlated with surface weather.
Depletion of snow and ice by melting and evaporation.
Absolutely Stable Air
An atmospheric condition that exists when the environmental lapse rate is less than the moist adiabatic lapse rate.
Absolutely Unstable Air
An atmospheric condition that exists when the environmental lapse rate is greater than the dry adiabatic lapse rate.
The process in which incident radiant energy is retained by a substance by conversion to some other form of energy.
The part of a valley or canyon wall against which a dam is constructed. Right and left abutments are those on respective sides of an observer looking downstream.
Abutment Seeping
Reservoir water that moves through seams or pores in the natural abutment material and exits as seepage.
Generally, data related to a specific WSR-88D unit. These data may consist of meteorological or hydrological parameters or of geographic boundaries, political boundaries, system configuration, telephone numbers (auto dial), or other like data. Such data may be generated at either a centralized location or locally at the WSR-88D unit.
A line on a thermodynamic chart relating the pressure and temperature of a substance (such as air) that is undergoing a transformation in which no heat is exchanged with its environment.
Changes in temperature caused by the expansion (cooling) or compression (warming) of a body of air as it rises or descends in the atmosphere, with no exchange of heat with the surrounding air.
Adiabatic Lapse Rate
The rate of decrease of temperature experienced by a parcel of air when it is lifted in the atmosphere under the restriction that it cannot exchange heat with its environment. For parcels that remain unsaturated during lifting, the (dry adiabatic) lapse rate is 9.8°C per kilometer.
Adiabatic Process
A process which occurs with no exchange of heat between a system and its environment.
Advanced Baseline Imager (ABI)
A moderate resolution imaging system on the GOES and Himawari family of satellites.
Air Force Base
In hydrologic terms, the tail race of a hydroelectric power plant at the outlet of the turbines. The term may be applied to a short stretch of stream or conduit, or to a pond or reservoir.
Air Transportable Mobile Unit
A modularized transportable unit containing communications and observational equipment necessary to support a meteorologist preparing on-site forecasts at a wildfire or other incident.
Airborne Snow Survey Program
In hydrologic terms, Center (NOHRSC) program that makes airborne snow water equivalent and soil moisture measurements over large areas of the country that are subject to severe and chronic snowmelt flooding.
Reflectivity; the fraction of radiation striking a surface that is reflected by that surface.
Alberta Clipper
A fast moving low pressure system that moves southeast out of Canadian Province of Alberta (southwest Canada) through the Plains, Midwest, and Great Lakes region usually during the winter. This low pressure area is usually accompanied by light snow, strong winds, and colder temperatures. Another variation of the same system is called a "Saskatchewan Screamer".
Of the surrounding area or environment.
A diverging branch of a river which re-enters the main stream.
Alphanumeric Backup Replacement System
Aneroid Barometer
An instrument for measuring atmospheric pressure in which a needle, attached to the top of an evacuated box, is deflected as changes in atmospheric pressure cause the top of the box to bend in or out.
Annual Exceedance Probability (AEP)
The probability that a stream reach will have a flow of a certain magnitude in any given year.
At or below
Aviation Support Branch
Atmospheric Boundary Layer
Same as Boundary Layer - in general, a layer of air adjacent to a bounding surface. Specifically, the term most often refers to the planetary boundary layer, which is the layer within which the effects of friction are significant. For the earth, this layer is considered to be roughly the lowest one or two kilometers of the atmosphere. It is within this layer that temperatures are most strongly affected by daytime insolation and nighttime radiational cooling, and winds are affected by friction with the earth's surface. The effects of friction die out gradually with height, so the "top" of this layer cannot be defined exactly.
Aurora Borealis
Also known as the northern lights; the luminous, radiant emission from the upper atmosphere over middle and high latitudes, and centred around the earth's magnetic poles. These silent fireworks are often seen on clear winter nights in a variety of shapes and colors.
Automated Surface Observing System
The ASOS program is a joint effort of the National Weather Service (NWS), the Federal Aviation Administration (FAA), and the Department of Defense (DOD). Completed in the mid-1990s, the ASOS systems serve as the nation's primary surface weather observing network. ASOS is designed to support weather forecast activities and aviation operations and, at the same time, support the needs of the meteorological, hydrological, and climatological research communities.
Abbreviation used on long-term climate outlooks issued by CPC to indicate areas that are likely to be below normal for a given parameter (temperature, precipitation, etc.).
Back Door Cold Front
A cold front moving south or southwest along the Atlantic seaboard and Great Lakes; these are especially common during the spring months.
Back-building Thunderstorm
A 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.
Back-sheared Anvil
[Slang], a thunderstorm anvil which spreads upwind, against the flow aloft. A back-sheared anvil often implies a very strong updraft and a high severe weather potential.
A fire started to stop an advancing fire by creating a burned area in its path.
In hydrologic terms, the backing up of water through a conduit or channel in the direction opposite to normal flow.
(abbrev. BCKG)- A counterclockwise shift in wind direction (for example, south winds shifting to the east).
Backing Winds
Winds which shift in a counterclockwise direction with time at a given location (e.g. from southerly to southeasterly), or change direction in a counterclockwise sense with height (e.g. westerly at the surface but becoming more southerly aloft). The opposite of veering winds.

In storm spotting, a backing wind usually refers to the turning of a south or southwest surface wind with time to a more east or southeasterly direction. Backing of the surface wind can increase the potential for tornado development by increasing the directional shear at low levels.
The portion of power scattered back in the incident direction.
In hydrologic terms, a rod reading taken on a point of known elevation, a benchmark or a turning point. Backsights are added to the known elevation to arrive at the height of the instrument. With a known height of the instrument, the telescope can be used to determine the elevation of other points in the vicinity.
Backwater Curve
In hydrologic terms, the longitudinal profile of the surface of a liquid in a non-uniform flow in an open channel, when the water surface is not parallel to the invert owing to the depth of water having been increased by the interposition of an obstruction such as a dam or weir. The term is sometimes used in a generic sense to denote all water surface profiles; or for profiles where the water is flowing at depths greater than the critical.
Backwater Effect
In hydrologic terms, the effect which a dam or other obstruction has in raising the surface of the water upstream from it.
Backwater Flooding
Hydrologic terms, upstream flooding caused by downstream conditions such as channel restriction and/or high flow in a downstream confluence stream.
Binary Angular Measure
A filter whose frequencies are between given upper and lower cutoff values, while substantially attenuating all frequencies outside these values (this band).
The frequency range between the lowest and highest frequencies that are passed through a component, circuit, or system with acceptable attenuation.
In hydrologic terms, the margins of a channel. Banks are called right or left as viewed facing in the direction of the flow.
Bank Storage
In hydrologic terms, water absorbed and stored in the void in the soil cover in the bed and banks of a stream, lake, or reservoir, and returned in whole or in part as the level of water body surface falls.
The water level, or stage, at which a stream, river or lake is at the top of its banks and any further rise would result in water moving into the flood plain.
Bankfull Stage
An established gage height at a given location along a river or stream, above which a rise in water surface will cause the river or stream to overflow the lowest natural stream bank somewhere in the corresponding reach. The term "lowest bank" is however, not intended to apply to an unusually low place or a break in the natural bank through which the water inundates a small area. Bankfull stage is not necessarily the same as flood stage.
Banner Cloud
A cloud plume often observed to extend downwind behind isolated mountain peaks, even on otherwise cloud-free days.
Bay Area Public Service Unit. Public Service section of the San Francisco Bay Area Weather Service Forecast Office.
An obstacle formed at the shallow entrance to the mouth of a river or bay.
Barber Pole
[Slang], a thunderstorm updraft with a visual appearance including cloud striations that are curved in a manner similar to the stripes of a barber pole. The structure typically is most pronounced on the leading edge of the updraft, while drier air from the rear flank downdraft often erodes the clouds on the trailing side of the updraft.
Baroclinic leaf shield
A cloud pattern on satellite images - frequently noted in advance of formation of a low pressure center.
Baroclinic Zone
A region in which a temperature gradient exists on a constant pressure surface. Baroclinic zones are favored areas for strengthening and weakening systems; barotropic systems, on the other hand, do not exhibit significant changes in intensity. Also, wind shear is characteristic of a baroclinic zone.
A measure of the state of stratification in a fluid in which surfaces of constant pressure (isobaric) intersect surfaces of constant density (isosteric).
An analog record of pressure produced by a barograph
A barometer that records its observations continuously.
An instrument that measures atmospheric pressure.
Barometric Pressure
The pressure of the atmosphere as indicated by a barometer.
Barotropic System
A weather system in which temperature and pressure surfaces are coincident, i.e., temperature is uniform (no temperature gradient) on a constant pressure surface. Barotropic systems are characterized by a lack of wind shear, and thus are generally unfavorable areas for severe thunderstorm development. See baroclinic zone.

Usually, in operational meteorology, references to barotropic systems refer to equivalent barotropic systems - systems in which temperature gradients exist, but are parallel to height gradients on a constant pressure surface. In such systems, height contours and isotherms are parallel everywhere, and winds do not change direction with height.

As a rule, a true equivalent barotropic system can never be achieved in the real atmosphere. While some systems (such as closed lows or cutoff lows) may reach a state that is close to equivalent barotropic, the term barotropic system usually is used in a relative sense to describe systems that are really only close to being equivalent barotropic, i.e., isotherms and height contours are nearly parallel everywhere and directional wind shear is weak.
The state of a fluid in which surfaces of constant density (or temperature) are coincident with surfaces of constant pressure; it is the state of zero baroclinity.
In hydrologic terms, any artificial obstruction placed in water to increase water level or divert it. Usually the idea is to control peak flow for later release.
Barrier Jet
A jet-like wind current that forms when a stably-stratified low-level airflow approaches a mountain barrier and turns to the left to blow parallel to the longitudinal axis of the barrier.
Bartel's Rotation Number
The serial number assigned to 27-day rotation periods of solar and geophysical parameters. Rotation 1 in this sequence was assigned arbitrarily by Bartel to begin in January 1833.
Those digital fields of reflectivity, mean radial velocity, and spectrum width data in spherical coordinates provided at the finest resolution available from the radar.
Base Flood
In hydrologic terms, the national standard for floodplain management is the base, or one percent chance flood. This flood has at least one chance in 100 of occurring in any given year. It is also called a 100 year flood.
Those products that present some representation of the base data. This representation may not necessarily be either in full resolution or depict the full area of coverage. Base products can be used to generate a graphic display or further processing.
Base Reflectivity
Base Reflectivity is the default image. Taken from the lowest (½° elevation) slice, it is the primary image used to "see what's out there". There are two versions of Base Reflectivity image; the short range version which extends out to 124 nautical miles (143 statute miles/230 kilometers) and the long range version which extends out to 248 nautical miles (285 statute miles/460 kilometers). This image is available upon completion of the ½° elevation scan during each volume scan
Base Station
In hydrologic terms, a computer which accepts radio signals from ALERT gaging sites, decodes the data, places the data in a database, and makes the data available to other users.
Base Width
In hydrologic terms, the time duration of a unit hydrograph.
In hydrologic terms, streamflow which results from precipitation that infiltrates into the soil and eventually moves through the soil to the stream channel. This is also referred to as ground water flow, or dry-weather flow.
An area having a common outlet for its surface runoff. Also called a "Drainage Basin."
Basin Boundary
The topographic dividing line around the perimeter of a basin, beyond which overland flow (i.e.; runoff) drains away into another basin.
Basin Lag
In hydrologic terms, the time it takes from the centroid of rainfall for the hydrograph to peak.
Basin Recharge
In hydrologic terms, rainfall that adds to the residual moisture of the basin in order to help recharge the water deficit. i.e; water absorbed into the soil that does not take the form of direct runoff.
The science of measuring depths of the oceans, lakes, seas, etc.
Backing- A counterclockwise shift in wind direction (for example, south winds shifting to the east).
Blowing Dust
Beach Erosion
The movement of beach materials by some combination of high waves, currents and tides, or wind.
The measure of variation of hydrometeor density throughout the radar sampling volume. If there is no variation in density, the beam is considered to be filled.
Angular width of antenna pattern. Usually that width where the power density is one-half that of the axis beam. (Half-Power or 3 dB point)
Bear's Cage
[Slang], a region of storm-scale rotation, in a thunderstorm, which is wrapped in heavy precipitation. This area often coincides with a radar hook echo and/or mesocyclone, especially one associated with an HP storm. The term reflects the danger involved in observing such an area visually, which must be done at close range in low visibility.
Beaufort Scale
The Beaufort wind scale is a system used to estimate and report wind speeds when no measuring apparatus is available. It was invented in the early 19th Century by Admiral Sir Francis Beaufort of the British Navy as a way to interpret winds from conditions at sea. Since that time, the scale has been modernized for effects on land.

Beaver('s) Tail
[Slang], a particular type of inflow band with a relatively broad, flat appearance suggestive of a beaver's tail. It is attached to a supercell's general updraft and is oriented roughly parallel to the pseudo-warm front, i.e., usually east to west or southeast to northwest. As with any inflow band, cloud elements move toward the updraft, i.e., toward the west or northwest. Its size and shape change as the strength of the inflow changes. See also inflow stinger.
Bed Load
In hydrologic terms, sand, silt, gravel, or soil and rock detritus carried by a stream on or immediately above its bed. The particles of this material have a density or grain size such as to preclude movement far above or for a long distance out of contact with the stream bed under natural conditions of flow.
Beginning of Freezup
In hydrologic terms, date on which ice forming a stable winter ice cover is first observed on the water surface
Beginning of the Breakup
In hydrologic terms, date of definite breaking, movement, or melting of ice cover or significant rise of water level.
(Abbrev. BM) - In hydrologic terms, a permanent point whose known elevation is tied to a national network. These points are created to serve as a point of reference. Benchmarks have generally been established by the USGS, but may have been established by other Federal or local agencies. Benchmarks can be found on USGS maps.
Bergeron Process
The process by which ice crystals in a cloud grow at the expense of supercooled liquid water droplets.
Bergy Bit
A piece of ice which has broken away from an iceberg, extending 1-5 meters above the sea surface and 100-300 square meters in area. Can also be the remains of a melting iceberg.
Bermuda High
A semi-permanent, subtropical area of high pressure in the North Atlantic Ocean off the East Coast of North America that migrates east and west with varying central pressure. Depending on the season, it has different names. When it is displaced westward, during the Northern Hemispheric summer and fall, the center is located in the western North Atlantic, near Bermuda. In the winter and early spring, it is primarily centered near the Azores in the eastern part of the North Atlantic. Also known as Azores High.
Best Track
A subjectively-smoothed representation of a tropical cyclone's location and intensity over its lifetime. The best track contains the cyclone's latitude, longitude, maximum sustained surface winds, and minimum sea-level pressure at 6-hourly intervals. Best track positions and intensities, which are based on a post-storm assessment of all available data, may differ from values contained in storm advisories. They also generally will not reflect the erratic motion implied by connecting individual center fix positions.
A systematic difference between an estimate of and the true value of a parameter.
Billow Cloud
A cloud consisting of broad parallel bands oriented perpendicular to the wind.
Radar sample volume.
Breaks in Overcast
Abbreviation for Boundary Layer; a layer of air adjacent to a bounding surface. Specifically, the term most often refers to the planetary boundary layer, which is the layer within which the effects of friction are significant. For the earth, this layer is considered to be roughly the lowest one or two kilometers of the atmosphere. It is within this layer that temperatures are most strongly affected by daytime insolation and nighttime radiational cooling, and winds are affected by friction with the earth's surface. The effects of friction die out gradually with height, so the "top" of this layer cannot be defined exactly.
Black Ice
1. Slang reference to patchy ice on roadways or other transportation surfaces that cannot easily be seen.

2. In hydrologic terms, transparent ice formed in rivers and lakes.
A hypothetical "body" that absorbs all of the electromagnetic radiation striking it - it does not reflect or transmit any of the incident radiation. A blackbody not only absorbs all wavelengths, but emits at all wavelengths with the maximum possible intensity for any given temperature.
Blackbody Radiation
The electromagnetic radiation emitted by an ideal blackbody adhering to the radiation laws; it is the theoretical maximum amount of electromagnetic radiation of all wavelengths that can be emitted by a body at a given temperature.
(abbrev. BLZD)- A blizzard means that the following conditions are expected to prevail for a period of 3 hours or longer:
Blizzard Warning
Issued for winter storms with sustained or frequent winds of 35 mph or higher with considerable falling and/or blowing snow that frequently reduces visibility to 1/4 of a mile or less. These conditions are expected to prevail for a minimum of 3 hours.
Blocked Flow
Flow approaching a mountain barrier that is too weak or too stable to be carried over the barrier.
A descriptor used to amplify observed weather phenomena whenever the phenomena are raised to a height of 6 feet or more above the ground
Blowing Dust or Sand
Strong winds over dry ground, that has little or no vegetation, can lift particles of dust or sand into the air. These airborne particles can reduce visibility, cause respiratory problems, and have an abrasive affect on machinery. A concentration reducing the visibility to ¼ mile or less often poses hazards for travelers.
Blowing Snow
Blowing snow is wind-driven snow that reduces surface visibility. Blowing snow can be falling snow or snow that has already accumulated but is picked up and blown by strong winds. Blowing snow is usually accompanied by drifting snow.
Blowing Snow Advisory
Issued when wind driven snow reduces surface visibility, possibly, hampering traveling. Blowing snow may be falling snow, or snow that has already accumulated but is picked up and blown by strong winds.
Blue Watch or Blue Box
[Slang], a severe thunderstorm watch.
Same as Breezy; 15 to 25 mph winds.
Blizzard- A blizzard means that the following conditions are expected to prevail for a period of 3 hours or longer:
Blowing Sand
Popular expression of a rapid intensification of a cyclone (low pressure) with surface pressure expected to fall by at least 24 millibars in 24 hour.
A regional downslope wind whose source is so cold that it is experienced as a cold wind, despite compression warming as it descends the lee slope of a mountain range.
Border Ice
In hydrologic terms, an ice sheet in the form of a long border attached to the bank or shore.; shore ice.
Boundary Layer
In general, a layer of air adjacent to a bounding surface. Specifically, the term most often refers to the planetary boundary layer, which is the layer within which the effects of friction are significant. For the earth, this layer is considered to be roughly the lowest one or two kilometers of the atmosphere. It is within this layer that temperatures are most strongly affected by daytime insolation and nighttime radiational cooling, and winds are affected by friction with the earth's surface. The effects of friction die out gradually with increasing height, so the "top" of this layer cannot be defined exactly.

There is a thin layer immediately above the earth's surface known as the surface boundary layer (or simply the surface layer). This layer is only a portion of the planetary boundary layer, and represents the layer within which friction effects are more or less constant throughout (as opposed to decreasing with height, as they do above it). The surface boundary layer is roughly 10 meters thick (from the surface up to 10 m above the ground), but again the exact depth is indeterminate. Like friction, the effects of insolation and radiational cooling are strongest within this layer.
Bounded Weak Echo Region (BWER)
(Also known as a vault.) Radar signature within a thunderstorm characterized by a local minimum in radar reflectivity at low levels which extends upward into, and is surrounded by, higher reflectivities aloft. This feature is associated with a strong updraft and is almost always found in the inflow region of a thunderstorm. It cannot be seen visually.
Base of Overcast
Bow Echo
A radar echo which 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 exhibits cyclonic rotation.
Bowen Ratio
For any moist surface, the ratio of heat energy used for sensible heating (conduction and convection) to the heat energy used for latent heating (evaporation of water or sublimation of snow). The Bowen ratio ranges from about 0.1 for the ocean surface to more than 2.0 for deserts; negative values are also possible. It is named for Ira S. Bowen (1898-1978), an American astrophysicist.
Box Model
A computer model used to calculate air pollution concentrations. A box model is based on the assumption that pollutants are emitted into a box through which they are immediately and uniformly dispersed. The sides and bottom of the box are defined by the sidewalls and floor of the valley being studied.
Brackish Ice
In hydrologic terms, ice formed from brackish water.
Braided Stream
In hydrologic terms, characterized by successive division and rejoining of streamflow with accompanying islands. A braided stream is composed of anabranches.
Brash Ice
In hydrologic terms, accumulation of floating ice made up of fragments not more than 2 meters across; the wreckage of other forms of ice.
In hydrologic terms, the failed opening in a dam.
Waves that break, displaying white water. Depends on wave steepness and bottom bathymetry.
In hydrologic terms, the time when a river whose surface has been frozen from bank to bank for a significant portion of its length begins to change to an open water flow condition. Breakup is signaled by the breaking of the ice and often associated with ice jams and flooding.
Breakup Date
In hydrologic terms, date on which a body of water is first observed to be entirely clear of ice and remains clear thereafter.
Breakup Jam
In hydrologic terms, an ice jam that occurs as a result of the accumulation of broken ice pieces.
Breakup Period
In hydrologic terms, the period of disintegration of an ice cover.
15 to 25 mph winds
Bright Band
A distinct feature observed by a radar that denotes the freezing level of the atmosphere. The term originates from a horizontal band of enhanced reflectivity that can result when a radar antenna scans vertically through precipitation. The freezing level in a cloud contains ice particles that are coated with liquid water. These particles reflect significantly more radiation (appearing to the radar as large raindrops) than the portions of the cloud above and below the freezing layer. The bright band can affect the ability of the NEXRAD algorithms to produce accurate rainfall estimates at far ranges because the algorithm may interpret reflectivity from the bright band as an overestimate of precipitation reaching the surface.
Bright Band
The enhanced radar echo of snow as it melts to rain.
Bright Surge on the Disk (BSD)
In solar-terrestrial terms, a bright gaseous stream (surge) emanating from the chromosphere.
Bright Surge on the Limb (BSL)
In solar-terrestrial terms, a large gaseous stream (surge) that moves outward more than 0.15 solar radius above the limb.
A basic visual sensation describing the amount of light that appears to emanate from an object, or more precisely, the luminance of an object
15 to 25 mph winds
Brisk Wind Advisory
A Small Craft Advisory issued by the National Weather Service for ice-covered waters.
(Bulk Richardson Number) A non-dimensional number relating vertical stability and vertical shear (generally, stability divided by shear). High values indicate unstable and/or weakly-sheared environments; low values indicate weak instability and/or strong vertical shear. Generally, values in the range of around 50 to 100 suggest environmental conditions favorable for supercell development.
A method of signaling in which multiple signals share the bandwidth of the transmission by the subdivision of the bandwidth into channels based on frequency.
Brocken Specter
An optical phenomenon sometimes occurring at high altitudes when the image of an observer placed between the sun and a cloud is projected on the cloud as a greatly magnified shadow. The shadow's head is surrounded by rings of color, called a glory.
Broken Level
A layer of the atmosphere with 5/8 to 7/8 sky cover (cloud cover).
Blowing Snow
Bubble High
A mesoscale area of high pressure, typically associated with cooler air from the rainy downdraft area of a thunderstorm or a complex of thunderstorms. A gust front or outflow boundary separates a bubble high from the surrounding air.
Bubbler Gage
In hydrologic terms, a water stage recording device that is capable of attaching to a LARC for data automation purposes.
A software tool used by forecasters to examine the vertical profile and other aspects of the atmosphere.
Bulk Richardson Number
A non-dimensional (i.e., no units) number relating vertical stability to vertical shear (generally, stability divided by shear). High values indicate unstable and/or weakly-sheared environments; low values indicate weak instability and/or strong vertical shear. Generally, values in the range of around 50 to 100 suggest environmental conditions favorable for supercell development.
The tendency of a body to float or to rise when submerged in a fluid; the power of a fluid to exert an upward force on a body placed in it.
In solar-terrestrial terms, a transient enhancement of the solar radio emission, usually associated with an active region or flare.
Slang for an inaccurate forecast, especially one where significant weather (e.g., heavy snowfall) is predicted but does not occur.
Buttress Dam
Buttress dams are comprised of reinforced masonry or stonework built against concrete. They are usually in the form of flat decks or multiple arches. They require about 60 percent less concrete than gravity dams, but the increased form work and reinforcement steel required usually offset the savings in concrete. Many were built in the 1930's when the ratio of labor cost to materials was comparatively low. However, this type of construction is not competitive with other types of dams when labor costs are high.
Abbreviation for Bounded Weak Echo Region; a radar signature within a thunderstorm characterized by a local minimum in radar reflectivity at low levels which extends upward into, and is surrounded by, higher reflectivities aloft. This feature is associated with a strong updraft and is almost always found in the inflow region of a thunderstorm. It cannot be seen visually.
In hydrologic terms, the process of using historical data to estimate parameters in a hydrologic forecast technique such as SACSMA, routings, and unit hydrographs.
Carbon Dioxide
CO2; a colorless and odorless gas which is the fourth most abundant constituent of dry air.
Cumulonimbus cloud, characterized by strong vertical development in the form of mountains or huge towers topped at least partially by a smooth, flat, often fibrous anvil. Also known colloquially as a "thunderhead."
Cumulonimbus Mamma
Computing Development Branch (NCEP)
Centimeter Burst
A solar radio burst in the centimeter wavelength range.
(CFCs) - Manufactured substances used as coolants and computer-chip cleaners. When these products break down they destroy stratospheric ozone, creating the Antarctic Ozone Hole in the Southern Hemisphere spring (Northern Hemisphere autumn). While no longer in use, their long lifetime will lead to a very slow removal from the atmosphere.
Clear Air Turbulence
(CAT) - In aviation, sudden severe turbulence occurring in cloudless regions that causes violent buffeting of aircraft.
Climate Diagnostics Bulletin
(CDB) - The monthly CPC Bulletin reports on the previous months' status of the ocean-atmosphere climate system and provides various seasonal ENSO-related outlooks. It is issued by the fifteenth of the month.
Closed Basin
A basin draining to some depression or pond within its area, from which water is lost only by evaporation or percolation. A basin without a surface outlet for precipitation falling precipitation.
Closed Basin Lake Flooding
Flooding that occurs on lakes with either no outlet or a relatively small one. Seasonal increases in rainfall cause the lake level to rise faster than it can drain. The water may stay at flood stage for weeks, months, or years.
Combined Seas
Generally referred to as SEAS. Used to describe the combination or interaction of wind waves and swells in which the separate components are not distinguished. This includes the case when swell is negligible or is not considered in describing sea state. Specifically, Seas2 = S2+W2 where S is the height of all swell components and W is the height of the wind wave components. When used, SEAS should be considered as being the same as the significant wave height.
Conditionally Unstable Air
An atmospheric condition that exists when the environmental lapse rate is less than the dry adiabatic lapse rate but greater than the moist adiabatic lapse rate.
Convective Boundary Layer
The unstable boundary layer that forms at the surface and grows upward through the day as the ground is heated by the sun and convective currents transfer heat upwards into the atmosphere.
Convective Inhibition
(CIN or B-) - A numerical measure of the strength of "capping," typically used to assess thunderstorm potential. Specifically, it represents the cumulative effect of atmospheric layers the are warmer than the parcel moving vertically along the adiabat. Low level parcel ascent is often inhibited by such stable layers near the surface. If natural processes fail to destabilize the lower levels, an input of energy from forced lift (a front, an upper level shortwave, etc.) will be required to move the negatively buoyant air parcels to the point where they will rise freely. Since CIN is proportional to the amount of kinetic energy that a parcel loses to buoyancy while it is colder than the surrounding environment, it contributes to the downward momentum.
Cooperative Observer
An individual (or institution) who takes precipitation and temperature observations-and in some cases other observations such as river stage, soil temperature, and evaporation-at or near their home, or place of business. Many observers transmit their reports by touch-tone telephone to an NWS computer, and nearly all observers mail monthly reports to the National Climatic Data Center to be archived and published.
Critical Rainfall Probability
(Abbrev. CRP) - In hydrologic terms, the Probability that the actual precipitation during a rainfall event has exceeded or will exceed the flash flood guidance value.
Cubic Feet per Second
(Abbrev. CFP) - In hydrologic terms, a unit expressing rates of discharge. One cubic foot per second is equal to the discharge through a rectangular cross section, 1 foot wide by 1 foot deep, flowing at an average velocity of 1 foot per second. It is also approximately 7.48 gallons per second.
Cumulus Buildups
Clouds which develop vertically due to unstable air. Characterized by their cauliflower-like or tower-like appearance of moderately large size
In hydrologic terms, the Dam Break Forecasting Model.
Nondimensional "unit" of radar reflectivity which represents a logarithmic power ratio (in decibels, or dB) with respect to radar reflectivity factor, Z.
Debris Cloud
A rotating "cloud" of dust or debris, near or on the ground, often appearing beneath a condensation funnel and surrounding the base of a tornado. This term is similar to dust whirl, although the latter typically refers to a circulation which contains dust but not necessarily any debris. A dust plume, on the other hand, does not rotate. Note that a debris cloud appearing beneath a thunderstorm will confirm the presence of a tornado, even in the absence of a condensation funnel.
Detention Basins
Structures built upstream from populated areas to prevent runoff and/or debris flows from causing property damage and loss of life. They are normally dry, but are designed to attenuate storm flows or detain mud/debris during and immediately after a runoff event. They have no spillway gates or valves and do not store water on a long-term basis. Typical detention times for storm flows are on the order of 24 to 72 hours, but may be as long as 5 to 10 days. Basins designed for detention of mud and rock debris are periodically excavated to maintain their storage capacity.
A process which occurs with the addition or loss of heat. The opposite of adiabatic. Meteorological examples include air parcels warming due to the absorption of infrared radiation or release of latent heat.
Diablo Wind
Similar to Santa Ana winds in southern California. These winds occur below canyons in the East Bay hills (Diablo range) and in extreme cases can exceed 60 mph. They develop due to high pressure over Nevada and lower pressure along the central California coast.
Discharge Table
In hydrologic terms,

1. A table showing the relation between two mutually dependant quantities or variable over a given range of magnitude.

2. A table showing the relation between the gage height and the discharge of a stream or conduit at a given gaging station. Also called a Rating Table.
Distribution (Hydro)Graph
In hydrologic terms, a unit hydrograph of direct runoff modified to show the proportions of the volume of runoff that occur during successive equal units of time.
Dobson Unit
Unit used to measure the abundance of ozone in the atmosphere. One Dobson unit is the equivalent of 2.69/ x 1016 molecules of ozone/cm2.
A strong downdraft current of air from a cumulonimbus cloud, often associated with intense thunderstorms. Downdrafts may produce damaging winds at the surface.
Drainage Basin
In hydrologic terms, a part of the surface of the earth that is occupied by a drainage system, which consists of a surface stream or a body of impounded surface water together with all tributary surface streams and bodies of impounded surface water.
Drop-size Distribution
The distribution of rain drops or cloud droplets of specified sizes.
Dry Adiabat
A line of constant potential temperature on a thermodynamic chart.
Dry Adiabatic Lapse Rate
The rate at which the temperature of a parcel of dry air decreases as the parcel is lifted in the atmosphere. The dry adiabatic lapse rate (abbreviated DALR) is 5.5°F per 1000 ft or 9.8°C per km.
Dry Line Bulge
A bulge in the dry line, representing the area where dry air is advancing most strongly at lower levels. Severe weather potential is increased near and ahead of a dry line bulge.
Dry Microburst
A microburst with little or no precipitation reaching the ground; most common in semi-arid regions. They may or may not produce lightning. Dry microbursts may develop in an otherwise fair-weather pattern; visible signs may include a cumulus cloud or small Cb with a high base and high-level virga, or perhaps only an orphan anvil from a dying rain shower. At the ground, the only visible sign might be a dust plume or a ring of blowing dust beneath a local area of virga.
1. An adiabatic process in a hypothetical atmosphere in which no moisture is present. 2. An adiabatic process in which no condensation of its water vapor occurs and no liquid water is present.
E-19a, Abridged Report on River Gage Sta
In hydrologic terms, an abridged version of an E-19, an E-19a updates the E-19 as additional information, or changes occur at the station during the intervening five year period. An E-19a is to be completed anytime a significant change occurs at a forecast point. An E-19a is also used to take the place of an E-19 in documenting any gage history, or information of any non-forecast point (i.e; data point).
Ebb Current
The movement of a tidal current away form the coast or down an estuary.
Emergency Broadcast System
In hydrologic terms, fill material, usually earth or rock, placed with sloping sides and usually with length greater than height. All dams are types of embankments
A collection of numerical model results that show slightly different possible outcomes.
Ensemble Forecast
Multiple predictions from an ensemble of slightly different initial conditions and/or various versions of models. The objectives are to improve the accuracy of the forecast through averaging the various forecasts, which eliminates non-predictable components, and to provide reliable information on forecast uncertainties from the diversity amongst ensemble members. Forecasters use this tool to measure the likelihood of a forecast.
Ensemble Hydrologic Forecasting
In hydrologic terms, a process whereby a continuous hydrologic model is successively executed several times for the same forecast period by use of varied data input scenarios, or a perturbation of a key variable state for each model run. A common method employed to obtain a varied data input scenario is to use the historical meteorological record, with the assumption that several years of observed data covering the time period beginning on the current date and extending through the forecast period comprises a reasonable estimate of the possible range of future conditions.
Reference to a set of computer models run under the concept of Ensemble Forecasting: multiple predictions from an ensemble of models with slightly different initial conditions used as input and/or slightly different versions of models. The objectives are to improve the accuracy of the forecast through averaging the various forecasts, which eliminates non-predictable components, and to provide reliable information on forecast uncertainties from the diversity amongst ensemble members. Forecasters use this tool to measure the likelihood of a forecast.
Equilibrium Drawdown
In hydrologic terms, the ultimate, constant drawdown for a steady rate of pumped discharge.
Equilibrium Level
(EL) - On a sounding, the level above the level of free convection (LFC) at which the temperature of a rising air parcel again equals the temperature of the environment. The height of the EL is the height at which thunderstorm updrafts no longer accelerate upward. Thus, to a close approximation, it represents the height of expected (or ongoing) thunderstorm tops.
Equilibrium Surface Discharge
In hydrologic terms, the steady rate of surface discharge which results from a long-continued, steady rate of net rainfall, with discharge rate equal to net rainfall rate
Equilibrium Time
In hydrologic terms, the time when flow conditions become substantially equal to those corresponding to equilibrium discharge or equilibrium drawdown.
Eruptive Prominence on Limb (EPL)
In solar-terrestrial terms, a solar prominence that becomes activa- ted and is seen to ascend from the sun.
(Great Lakes Freeze-Up/Break-Up Outlook) - A National Weather Service product to keep mariners informed of the projected freeze-up date or break-up date of ice on the Great Lakes.
Feeder Bands
Lines or bands of low-level clouds that move (feed) into the updraft region of a thunderstorm, usually from the east through south (i.e., parallel to the inflow). Same as inflow bands. This term also is used in tropical meteorology to describe spiral-shaped bands of convection surrounding, and moving toward, the center of a tropical cyclone.
In solar-terrestrial terms, a linear pattern in the H-alpha chromosphere of the sun, as seen through an H-alpha filter, occurring near strong sunspots and plage or in filament channels.
Any source of heat, natural or man made, capable of igniting wildland fuels; flaming or glowing fuel particles that can be carried naturally by wind, convection currents, or gravity into unburned fuels.
Flash Flood Table
In hydrologic terms, a table of pre-computed forecast crest stage values for small streams for a variety of antecedent moisture conditions and rain amounts. Soil moisture conditions are often represented by flash flood guidance values. In lieu of crest stages, categorical representations of flooding, e.g., minor, moderate, etc. may be used on the tables.
In hydrologic terms, a length of timber, concrete, or steel placed on the crest of a spillway to raise the retention water level but which may be quickly removed in the event of a flood by a tripping device, or by deliberately designed failure of the flashboard or its supports
Flood Problems
In hydrologic terms, problems and damages that occur during a flood as a result of human development and actions. Flood problems are a result from:
1) Inappropriate development in the floodplain (e.g., building too low, too close to the channel, or blocking flood flows);
2) Development in the watershed that increases flood flows and creates a larger floodplain, or;
3) A combination of the previous two.
A rainbow that has a white band that appears in fog, and is fringed with red on the outside and blue on the inside.
In hydrologic terms, the water behind (upstream) of the dam.
In hydrologic terms, the vertical distance between the normal maximum level of the water surface in a channel, resrvoir, tank, canal, etc., and the top of the sides of a levee, dam, etc., which is provided so that waves and other movements of the liquid will not overtop the confining structure
Human tissue damage caused by exposure to intense cold.
A specific aerial spray dispersion model. The acronym comes from the names of the sponsor and developers (Forest Service, Cramer, Barry, Grim).
Global Ensemble Forecast System (GEFS)
A weather forecast model made up of 21 separate forecasts, or ensemble members. The National Centers for Environmental Prediction (NCEP) started the GEFS to address the nature of uncertainty in weather observations, which are used to initialize weather forecast models.
Global Forecast System
(GFS)- One of the operational forecast models run at NCEP. The GFS is run four times daily, with forecast output out to 384 hours.
Global Temperature Change
The net result of four primary factors including the greenhouse effect, changes in incoming solar radiation, altered patterns of ocean circulations, and changes in continental position, topography and/or vegetation. Three feedback mechanisms which affect global temperature change include cloud height and amount, snow and ice distribution, and atmospheric water vapor levels.
Global Warming
An overall increase in world temperatures which may be caused by additional heat being trapped by greenhouse gases.
A hypothetical "body" that absorbs some constant fraction of all electromagnetic radiation incident upon it.
Great Lakes Faxback
Dissemination systems housed at Weather Forecast Office (WFO) Cleveland by which Great Lakes customers request and receive hard copies of selected marine products.
Great Lakes Freeze-Up/Break-Up Outlook
(FBO) - A National Weather Service product to keep mariners informed of the projected freeze-up date or break-up date of ice on the Great Lakes.
Great Lakes Weather Broadcast
(LAWEB) - A National Weather Service product containing an observation summary prepared to provide Great Lakes mariners with a listing of weather observations along or on the Lakes.
(Gridded Binary Format) - A format used for meteorological data. Typically used in the past for computer generated model data but will be used increasingly in the future for forecaster generated data.
Ground Blizzard Warning
When blizzard conditions are solely caused by blowing and drifting snow.
Headwater Basin
In hydrologic terms, a basin at the headwaters of a river. All discharge of the river at this point is developed within the basin.
Height Above the Nearest Drainage (HAND)
A relative elevation methodology used in inundation mapping, which determines the height of every point on a land surface above the nearest stream reach to which it drains.
High Resolution Ensemble Forecast (HREF)
An ensemble of products from several different models running at ~3 km horizontal grid spacing.
High Water Probability
The probability (%) that a stream will reach its high water flow within the forecast period.
Hydraulic Permeability
In hydrologic terms, the flow of water through a unit cross-sectional area of soil normal to the direction of flow when the hydraulic gradient is unity.
Hydrologic Budget
In hydrologic terms, an accounting of the inflow to, outflow from, and storage in, a hydrologic unit, such as a drainage basin, aquifer, soil zone, lake, reservoir, or irrigation project.
Hydrologic Ensemble Forecast System (HEFS)
A probabilistic forecast tool with the goals to provide hydrologic forecasts including an analysis of “probable outcomes” and to minimize biases in the atmospheric models and in the hydrologic models.
Ice Boom
In hydrologic terms, a floating structure designed to retain ice.
Ice Bridge
In hydrologic terms, a continuous ice cover of limited size extending from shore to shore like a bridge.
A piece of a glacier which has broken off and is floating in the sea.
Material that does not permit fluids to pass through it.
Inflow Bands
Bands of low clouds, arranged parallel to the low-level winds and moving into or toward a thunderstorm. They may indicate the strength of the inflow of moist air into the storm, and, hence, its potential severity. Spotters should be especially wary of inflow bands that are curved in a manner suggesting cyclonic rotation; this pattern may indicate the presence of a mesocyclone
(abbrev. INSTBY)- The tendency for air parcels to accelerate when they are displaced from their original position; especially, the tendency to accelerate upward after being lifted. Instability is a prerequisite for severe weather - the greater the instability, the greater the potential for severe thunderstorms.
Instability- The tendency for air parcels to accelerate when they are displaced from their original position; especially, the tendency to accelerate upward after being lifted. Instability is a prerequisite for severe weather - the greater the instability, the greater the potential for severe thunderstorms.
Intangible Flood Damage
In hydrologic terms, estimates of the damage done by disruption of business, danger to health, shock, and loss of life and in general all costs not directly measurable which require a large element of judgment for estimating.
Interbasin Transfer
In hydrologic terms, the physical transfer of water from one watershed to another.
A line of equal change in atmospheric pressure during a specified time period.
A line connecting points of equal pressure.
Isobaric Chart
A weather map representing conditions on a surface of equal atmospheric pressure. For example, a 500 mb chart will display conditions at the level of the atmosphere at which the atmospheric pressure is 500 mb. The height above sea level at which the pressure is that particular value may vary from one location to another at any given time, and also varies with time at any one location, so it does not represent a surface of constant altitude/height (i.e., the 500 mb level may be at a different height above sea level over Dallas than over New York).
Isobaric Process
Any thermodynamic change of state of a system that takes a place at constant pressure.
In hydrologic terms, an imaginary line on the earth's surface or a line on a map connecting all points which are the same vertical distance above the upper or lower surface of a water-bearing formation or aquifer
Katabatic Wind
A wind that is created by air flowing downhill.
Keetch-Byrum Drought Index
An index used to gage the severity of drought in deep duff and organic soils.
Lake Breeze
A thermally produced wind blowing during the day from the surface of a large lake to the shore, caused by the difference in the rates of heating of the surfaces of the lake and of the land.
Land Breeze
A coastal breeze at night blowing from land to sea, caused by the difference in the rates of cooling of their respective surfaces.
(Great Lakes Weather Broadcast) - A National Weather Service product containing an observation summary prepared to provide Great Lakes mariners with a listing of weather observations along or on the Lakes.
Light Bridge
In solar-terrestrial terms, it is observed in white light, a bright tongue or streaks penetra- ting or crossing sunspot umbrae. The appearance of a light bridge is frequently a sign of impending region division or dissolution
In solar-terrestrial terms, the edge of the solar disk.
Limb Flare
In solar-terrestrial terms, a solar flare seen at the edge (Limb) of the sun.
A convective downdraft with an affected outflow area of at least 2½ miles wide and peak winds lasting between 5 and 20 minutes. Intense macrobursts may cause tornado-force damage of up to F3 intensity.
Magnetic Bay
In solar-terrestrial terms, a relatively smooth excursion of the H (horizontal) component of the geomagnetic field away from and returning to quiet levels.
A voluntary marine observation program of the National Weather Service in the early stages of development whose goal is to solicit meteorological and oceanographic observations in coded format from recreational and small commercial mariners who are not part of the more in-depth Voluntary Observing Ship program.
Maximum Unambiguous Range
The range from the radar at which an echo can be known unquestionably as being at that range. As the radar sends out a pulse of energy, the pulse hits a target and part of the energy bounces back to the radar, but part of the energy may continue to travel away from the radar. The distance to the target is computed by knowing the time that has elapsed since the pulse was emitted. Then a second pulse of energy is transmitted. If some of the energy from the first pulse strikes a target at a far range and returns to the radar when radiation from the second pulse arrives, the RDA misinterprets the returned first pulse as arriving from a target near the returned second pulse. The maximum unambiguous range is related to the amount of time that elapses between successive pulses of emitted energy.
Maximum Unambiguous Velocity
The highest radial velocity that can be measured unambiguously by a pulsed Doppler radar. The maximum unambiguous velocity is related to the radar's successive pulses of emitted energy. When a target's velocity exceeds the maximum unambiguous velocity, the velocity will be "folded" to appear as a different velocity.
Meander Belt
In hydrologic terms, the area between lines drawn tangential to the extreme limits of fully developed meanders
Mercury Barometer
An instrument for measuring atmospheric pressure. The instrument contains an evacuated and graduated glass tube in which mercury rises or falls as the pressure of the atmosphere increases or decreases.
Meteorological Model Ensemble River Forecast (MMEFS)
An automated short-term hydrologic ensemble forecast system which utilizes temperature and precipitation output from the National Centers for Environmental Prediction (NCEP) GEFS and NAEFS meteorological models as inputs to River Forecast Center hydrologic models.
A instrument designed to continuously record a barometer's reading of very small changes in atmospheric pressure.
A convective downdraft with an affected outflow area of less than 2½ miles wide and peak winds lasting less than 5 minutes. Microbursts may induce dangerous horizontal/vertical wind shears, which can adversely affect aircraft performance and cause property damage.
Microwave Burst
In solar-terrestrial terms, a radiowave signal associated with optical and/or X-ray flares
A unit of atmospheric pressure equal to 1/1000 bar, or 1000 dynes per square centimeter.
Minimum Discernible Signal
In a receiver, it is the smallest input signal that will a produce a detectable signal at the output. In radar terms, it is the minimal amount of back scattered energy that is required to produce a target on the radar screen. In other words, MDS is a measure of the radar's sensitivity.
Marine Optical Buoy. It measures solar radiation to calibrate satellite ocean color instruments.
Moist Adiabat
The line on a Skew T-Log P chart that depicts the change in temperature of saturated air as it rises and undergoes cooling due to adiabatic expansion. As saturated air rises, the temperature changes at a rate of 0.55 degrees Celsius per 100 meters (2-3 degrees Fahrenheit per 1,000 feet). Contrast with a dry adiabat.
Moist Adiabatic Lapse Rate
(abbrev. MALR)- The rate at which the temperature of a parcel of saturated air decreases as the parcel is lifted in the atmosphere. The moist adiabatic lapse rate (abbreviated MALR) is not a constant like the dry adiabatic lapse rate but is dependent on parcel temperature and pressure.
(Also known as saturation-adiabatic process.) An adiabatic process for which the air is saturated and may contain liquid water. A distinction is made between the reversible process, in which total water is conserved, and the pseudoadiabatic or irreversible moist adiabatic process, in which liquid water is assumed to be removed as soon as it is condensed.
Movable Bed
In hydrologic terms, a stream bed made up of materials readily transportable by the stream flow
National Ambient Air Quality Standards
In the United States, national standards for the ambient concentrations in air of different air pollutants designed to protect human health and welfare.
National Blend of Models (NBM)
The National Blend of Models (NBM) is a nationally consistent and skillful suite of calibrated forecast guidance based on a blend of both NWS and non-NWS numerical weather prediction model data and post-processed model guidance.
National Digital Forecast Database
(NDFD)- The National Weather Service's NDFD provides access to gridded forecasts of sensible weather elements (e.g., wind, wave height) through the National Digital Forecast Database (NDFD). NDFD contains a seamless mosaic of digital forecasts from NWS field offices working in collaboration with the National Centers for Environmental Prediction (NCEP). The database is made available to all customers and partners from the public, private and academic sectors. Those customers and partners may use this data to create a wide range of text, graphic, gridded and image products of their own.
National Severe Storms Laboratory
This is one of NOAA's internationally known Environmental Research Laboratories, leading the way in investigations of all aspects of severe weather. Headquartered in Norman OK with staff in Colorado, Nevada, Washington, Utah, and Wisconsin, the people of NSSL, in partnership with the National Weather Service, are dedicated to improving severe weather warnings and forecasts in order to save lives and reduce property damage.
National Data Buoy Center
Neutral Stability
An atmospheric condition that exists in unsaturated air when the environmental lapse rate equals the dry adiabatic rate, or in saturated air when the environmental lapse rate equals the moist adiabatic rate.
(abbrev. NS)- A cloud of the class characterized by a formless layer that is almost uniformly dark gray; a rain cloud of the layer type, of low altitude, usually below 8000 ft (2400 m).
Non-Uniform Visibility
A localized visibility which varies from that reported in the body of the report.
North American Ensemble Forecast System (NAEFS)
An atmospheric ensemble of 20 members each from the NCEP GEFS and CMC EPS ensemble systems.
Any atmospheric phenomenon, except clouds, that restricts vertical visibility (e.g., dust, rain, snow, etc.).
Obscuring Phenomena
Any atmospheric phenomenon, except clouds, that restricts vertical visibility (e.g., dust, rain, snow, etc.).
Observation Well
In hydrologic terms, a non-pumping well used for observing the elevation of the water table or piezometric surface
Office of Global Programs
The Office of Global Programs (OGP) sponsors focused scientific research, within approximately eleven research elements, aimed at understanding climate variability and its predictability. Through studies in these areas, researchers coordinate activities that jointly contribute to improved predictions and assessments of climate variability over a continuum of timescales from season to season, year to year, and over the course of a decade and beyond.
Offshore Breeze
A wind that blows from the land towards a body of water. Also known as a land breeze.
Onshore Breeze
A wind that blows from a body of water towards the land. Also known as a seabreeze
Outer Convective Band
Bands in a hurricane that occur in advance of main rain shield and up to 300 miles from the eye of the hurricane. The typical hurricane has two or three bands (and sometimes more) which are comprised of cells resembling ordinary thunderstorms. Wind gusts are usually higher in these bands than in the Pre-Hurricane Squall Line.
Outflow Boundary
A storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature. Outflow boundaries may persist for 24 hours or more after the thunderstorms that generated them dissipate, and may travel hundreds of miles from their area of origin.

New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary (cold front, dry line, another outflow boundary, etc.; see triple point).
Partial Beam Filling
A limitation of the rainfall estimation techniques used by NEXRAD. At far ranges from the radar, a storm may occupy only a portion of the radar beam (which may be several miles across). However, the radiation received by the radar antenna consists of the average reflectivity across the entire beam, so the reflectivity and associated rainfall rates are underestimated.
In solar-terrestrial terms, the sunspot area that may surround the darker umbra or umbrae. It consists of linear bright and dark elements radial from the sunspot umbra.
In hydrologic terms, the ability of a material to transmit fluid through its pores when subjected to a difference in head.
Permeability Coefficient
In hydrologic terms, the rate of flow of a fluid through a cross section of a porous mass under a unit hydraulic gradient, at a temperature of 60 degrees Fahrenheit.
Perturbation Model
A computer model used to calculate air pollution concentrations. A perturbation model produces a wind field from solutions to a simplified set of equations that describe atmospheric motions.
Pilot balloon. A small helium-filled meteorological balloon that is tracked as it rises through the atmosphere to determine how wind speed and direction change with altitude.
Pilot Balloon
(Abbrev. PIBAL)- A small helium-filled meteorological balloon that is tracked as it rises through the atmosphere to determine how wind speed and direction change with altitude.
Pitot Tube
In hydrologic terms, a device for measuring the velocity of flowing water using the velocity head of the stream as an index of velocity. It consists essentially of an orifice held to a point upstream in the water, connected with a tube in which the rise of water due to velocity head may be observed and measured. It also may be constructed with an upstream and downstream orifice, with two water columns, in which case the difference in height of water column in the tubes is the index of velocity.
Planetary Boundary Layer
The layer within the atmosphere between 1 km and the earth's surface where friction affects wind speed and wind direction.
Plume Blight
Visibility impairment caused by air pollution plumes aggregated from individual sources.
Polar Cap Absorption (PCA)
In solar-terrestrial terms, an anomalous condition of the polar ionosphere whereby HF and VHF (3 - 300 MHz) radiowaves are absorbed, and LF and VLF (3 - 300 kHz) radiowaves are reflected at lower altitudes than normal. In practice, the absorption is inferred from the proton flux at energies greater than 10 MeV, so that PCAs and proton events are simultaneous. Transpolar radio paths may still be disturbed for days, up to weeks, following the end of a proton event.
Polar Orbiting Satellite
A weather satellite which travels over both poles each time it orbits the Earth. It orbits about 530 miles (850 km) above the Earth's surface. A satellite with an orbit nearly parallel to the earth's meridian lines which crosses the polar regions on each orbit.
Precipitable Water
Measure of the depth of liquid water at the surface that would result after precipitating all of the water vapor in a vertical column over a given location, usually extending from the surface to 300 mb.
Prescribed Fire
A management ignited or natural wildland fire that burns under specified conditions where the fire is confined to a predetermined area and produces the fire behavior and fire characteristics required to attain planned fire treatment and resource management objectives.
Prevailing Visibility
The visibility that is considered representative of conditions at the station; the greatest distance that can be seen throughout at least half the horizon circle, not necessarily continuous.
Primary Ambient Air Quality Standards
Air quality standards designed to protect human health.
A chance, or likelihood, that a certain event might happen.
Probability Forecast
A forecast of the probability that one or more of a mutually exclusive set of weather conditions will occur.
Probability of Hail
(Abbrev. POH) - a product from the NEXRAD hail detection algorithm that estimates the likelihood that hail is present in a storm.
Probability of Precipitation
(Abbrev. PoP)- The probability that precipitation will be reported at a certain location during a specified period of time.
Probability of Thunderstorms
The probability based on climatology that a thunderstorm will be reported at that location during a specified period of time.
Probability of Tropical Cyclone Conditio
The probability, in percent, that the cyclone center will pass within 50 miles to the right or 75 miles to the left of the listed location within the indicated time period when looking at the coast in the direction of the cyclone's movement.
Public Information Statement
A narrative statement issued by a National Weather Service Forecast Office that can be used for:

1) A current or expected nonhazardous event of general interest to the public that can usually be covered with a single message (e.g., unusual atmospheric phenomena such as sun dogs, halos, rainbows, aurora borealis, lenticular clouds, and stories about a long-term dry/cold/wet/warm spell).

2) Public educational information and activities, such as storm safety rules, awareness activities, storm drills, etc.

3) Information regarding service changes, service limitations, interruptions due to reduced or lost power or equipment outages, or special information clarifying interpretation of NWS data. For example, this product may be used to inform users of radar equipment outages or special information clarifying interpretation of radar data originating from an unusual source which may be mistaken for precipitation (such as chaff drops, smoke plumes, etc., that produces echoes on the radar display.
Public Severe Weather Outlook
These are issued when the Storm Prediction Center (SPC) in Norman, Oklahoma anticipates an especially significant and/or widespread outbreak of severe weather. This outlook will stress the seriousness of the situation, defines the threat area, and provides information on the timing of the outbreak. The lead time on this outlook is normally less than 36 hours prior to the severe weather event.
Radar Beam
The straight line that a radar pulse travels along. As the radar beam gets further away from the radar, it gets wider and wider. In order for a precipitation target to be detected by the radar, it must fill the entire radar beam; therefore, the radar will have a difficult time detecting small showers and thunderstorms at a great distance from the radar.
Rain-free Base
A dark, horizontal cloud base with no visible precipitation beneath it. It typically marks the location of the thunderstorm updraft. Tornadoes may develop from wall clouds attached to the rain-free base, or from the rain-free base itself - especially when the rain-free base is on the south or southwest side of the main precipitation area. Note that the rain-free base may not actually be rain free; hail or large rain drops may be falling. For this reason, updraft base is more accurate.
A luminous arc featuring all colors of the visible light spectrum (red, orange, yellow, green, blue, indigo, and violet). It is created by refraction, total reflection, and the dispersion of light. It is visible when the sun is shining through air containing water spray or raindrops, which occurs during or immediately after a rain shower. The bow is always observed in the opposite side of the sky from the sun.
Radiosonde Observation (Upper-Air Observation)
Rawinsonde Observation
A radiosonde observation which includes wind data.
Red Watch or Red Box
Slang for Tornado Watch.
Remote Observing System Automation
A type of automated data transmitter used by NWS Cooperative Program observers.
Rex Block
A blocking pattern where there is an upper level high located directly north of a closed low.
Ribbon Lightning
Appears to be a broad stream of fire. A succession of strokes, each blown a bit to the side of the previous strokes by wind, but striking so fast that all the strokes are seen at once as a ribbon-like flash.
River Basin
In hydrologic terms, drainage area of a river and its tributaries.
River Observing Station
An established location along a river designated for observing and measuring properties of the river.
Rossby Waves
A series of troughs and ridges on quasi-horizontal surfaces in the major belt of upper tropospheric westerlies. The waves are thousands of kilometers long and have significant latitudinal amplitude.
S-Band Radar
These were in use as network radars in the National Weather Service prior to the installation of the WSR 88-D radars. They were 10-centimeter wavelength radars.
Satellite Analysis Branch, part of NESDIS NESDIS: National Environmental Satellite, Data and Information Service
Surface Based CAPE; CAPE calculated using a Surface based parcel.
Sea Breeze
A thermally produced wind blowing during the day from a cool ocean surface onto the adjoining warm land, caused by the difference in the rates of heating of the surfaces of the ocean and of the land.
Sea Breeze Convergence Zone
The zone at the leading edge of a sea breeze where winds converge. The incoming air rises in this zone, often producing convective clouds.
Sea Breeze Front
The leading edge of a sea breeze, whose passage is often accompanied by showers, a wind shift, or a sudden drop in temperature.
Secondary Ambient Air Quality Standards
Air quality standards designed to protect human welfare, including the effects on vegetation and fauna, visibility and structures.
Sector Boundary
In solar-terrestrial terms, in the solar wind, the area of demarcation between sectors, which are large-scale features distinguished by the predominant direction of the interplanetary magnetic field, toward or away from the sun.
Sector Visibility
The visibility in a specific direction that represents at least a 45º arc of a horizontal circle.
Sectorized Hybrid Scan
A single reflectivity scan composed of data from the lowest four elevation scans. Close to the radar, higher tilts are used to reduce clutter. At further ranges, either the maximum values from the lowest two scans are used or the second scan values are used alone.
Sensible Heat Flux
The flux of heat from the earth's surface to the atmosphere that is not associated with phase changes of water; a component of the surface energy budget.
Severe Weather Probability
This WSR-88D radar product algorithm displays numerical values proportional to the probability that a storm will produce severe weather within 30 minutes. Values determined using a statistical regression equation which analyzes output from the VIL algorithm. It is used to quickly identify the most significant thunderstorms.
A secondary energy maximum located outside the main radar beam. Typically, it contains a small percentage of energy compared to the main lobe, but it may produce erroneous echoes.
Smoothed Sunspot Number
An average of 13 monthly RI numbers, centered on the month of concern.
Snow Accumulation and Ablation Model
In hydrologic terms, a model which simulates snow pack accumulation, heat exchange at the air-snow interface, areal extent of snow cover, heat storage within the snow pack, liquid water retention, and transmission and heat exchange at the ground-snow interface.
A flat, solid, white material, such as painted plywood, approximately two feet square, which is laid on the ground, or snow surface by weather observers to obtain more accurate measurements of snowfall and water content.
Solar Sector Boundary (SSB)
In solar-terrestrial terms, the apparent solar origin, or base, of the interplanetary sector boundary marked by the larger-scale polarity inversion lines.
Special Tropical Disturbance Statement
This statement issued by the National Hurricane Center furnishes information on strong and formative non-depression systems. This statement focuses on the major threat(s) of the disturbance, such as the potential for torrential rainfall on an island or inland area. The statement is coordinated with the appropriate forecast office(s).
Sphere Calibration
Reflectivity calibration of a radar by pointing the dish at a metal sphere of (theoretically) known reflectivity. The sphere is often tethered to a balloon.
The degree of resistance of a layer of air to vertical motion.
Stability Index
The overall stability or instability of a sounding is sometimes conveniently expressed in the form of a single numerical value. Used alone, it can be quite misleading, and at times, is apt to be worthless. The greatest value of an index lies in alerting the forecaster to those soundings which should be examined more closely.
An atmospheric state with warm air above cold air which inhibits the vertical movement of air.
Stable Boundary Layer
The stably-stratified layer that forms at the surface and grows upward, usually at night or in winter, as heat is extracted from the atmosphere's base in response to longwave radiative heat loss from the ground. Stable boundary layers can also form when warm air is advected over a cold surface or over melting ice.
Stable Core
Post-sunrise, elevated remnant of the temperature inversion that has built up overnight within a valley.
Stilling basin
In hydrologic terms, a basin constructed to dissipate the energy of fast-flowing water (e.g., from a spillway or bottom outlet), and to protect the streambed from erosion.
Stratiform Rings and Bands
These occur between the active convective bands of a hurricane outside of the eye wall. Inner stratiform bands often exhibit the bright band aloft, a VIP Level 2, and in the lower layers typically show a VIP Level 1.
Sub-synoptic Low
Essentially the same as mesolow.
The transition of a substance from the solid phase directly to the vapor phase, or vice versa, without passing through an intermediate liquid phase. Thus an ice crystal or icicle sublimes under low relative humidity at temperatures below 0°C. The process is analogous to evaporation of a liquid.
Sublimation of ice
The transition of water from solid to gas without passing through the liquid phase.
The bending of the radar beam in the vertical which is less than under standard refractive conditions. This causes the beam to be higher than indicated, and lead to the underestimation of cloud heights.
1. A descending motion of air in the atmosphere occurring over a rather broad area.
2. In hydrologic terms, sinking down of part of the earth's crust due to underground excavation, such as the removal of groundwater.
Subsidence Inversion
A temperature inversion that develops aloft as a result of air gradually sinking over a wide area and being warmed by adiabatic compression, usually associated with subtropical high pressure areas.
A location where observations are taken or other services are furnished by people not located at NWS offices who do not need to be certified to take observations.
Subsurface Storm Flow
In hydrologic terms, the lateral motion of water through the upper layers until it enters a stream channel. This usually takes longer to reach stream channels than runoff. This also called interflow.
Subtle Heavy Rainfall Signature
This heavy rain signature is often difficult to detect on satellite. These warm top thunderstorms are often embedded in a synoptic-scale cyclonic circulation. Normally, they occur when the 500 mb cyclonic circulation is quasi-stationary or moves slowly to the east or northeast (about 2 degrees per 12 hours). The average surface temperature is 68ºF with northeasterly winds. The average precipitable water (P) value is equal to or greater than 1.34 inches and the winds veer with height, but they are relatively light. The heavy rain often occurs north and east of the vorticity maximum across the lower portion of the comma head about 2 to 3 degrees north or northeast of the 850 mb low.
Subtropical Cyclone
A non-frontal low pressure system that has characteristics of both tropical and extratropical cyclones. This system is typically an upper-level cold low with circulation extending to the surface layer and maximum sustained winds generally occurring at a radius of about 100 miles or more from the center. In comparison to tropical cyclones, such systems have a relatively broad zone of maximum winds that is located farther from the center, and typically have a less symmetric wind field and distribution of convection.
Subtropical Depression
A subtropical cyclone in which the maximum 1-minute sustained surface wind is 33 knots (38 mph) or less.
Subtropical Jet
(Abbrev. STJ) - this jet stream is usually found between 20° and 30° latitude at altitudes between 12 and 14 km.
Subtropical Storm
A subtropical cyclone in which the maximum 1-minute sustained surface wind is 34 knots (39 mph) or more.
Sudden Ionospheric Disturbance (SID)
In solar-terrestrial terms, HF propagation anomalies due to ionospheric changes resulting from solar flares, proton events and geomagnetic storms.
Sunspot Number
In solar-terrestrial terms, a daily index of sunspot activity (R), defined as R = k (10 g + s) where S = number of individual spots, g = number of sunspot groups, and k is an observatory factor.
Surface Energy Budget
The energy or heat budget at the earth's surface, considered in terms of the fluxes through a plane at the earth-atmosphere interface. The energy budget includes radiative, sensible, latent and ground heat fluxes.
Surface-based Convection
Convection occurring within a surface-based layer, i.e., a layer in which the lowest portion is based at or very near the earth's surface. Compare with elevated convection.
Symmetric Double Eye
A concentrated ring of convection that develops outside the eye wall in symmetric, mature hurricanes. The ring then propagates inward and leads to a double-eye. Eventually, the inner eye wall dissipates while the outer intensifies and moves inward.
T -Number
A system used to subjectively estimate tropical cyclone intensity based solely on visible and infrared satellite images. Also called the Dvorak technique.
Tropical Analysis and Forecast Branch (of the TPC)
Thermal Belt
A zone of high nighttime temperatures (and relatively low humidities) that is often experienced within a narrow altitude range on valley sidewalls, especially evident during clear weather with light winds.
Tipping-Bucket Rain Gage
A precipitation gage where collected water is funneled into a two compartment bucket; 0.01, 0.1 mm, or some other designed quantity of rain will fill one compartment and overbalance the bucket so that it tips, emptying into a reservoir and moving the second compartment into place beneath the funnel. As the bucket is tipped, it actuates an electric circuit.
Transverse Bands
Bands of clouds oriented perpendicular to the flow in which they are embedded. They often are seen best on satellite photographs. When observed at high levels (i.e., in cirrus formations), they may indicate severe or extreme turbulence. Transverse bands observed at low levels (called transverse rolls or T rolls) often indicate the presence of a temperature inversion (or cap) as well as directional shear in the low- to mid-level winds. These conditions often favor the development of strong to severe thunderstorms.
Tropical Analysis and Forecast Branch
One of three branches of the Tropical Prediction Center (TPC). It provides year-round products involving marine forecasting, aviation forecasts and warnings (SIGMETs), and surface analyses. The unit also provides satellite interpretation and satellite rainfall estimates for the international community. In addition, TAFB provides support to NHC through manpower and tropical cyclone intensity estimates from the Dvorak technique.
Tropical Disturbance
A discrete tropical weather system of apparently organized convection--generally 100 to 300 mi in diameter--originating in the tropics or subtropics, having a nonfrontal migratory character and maintaining its identity for 24 hours or more. It may or may not be associated with a detectable perturbation of the wind field.
The thickness or opaqueness of water caused by the suspension of matter. The turbidity of rivers and lakes increases after a rainfall.
Irregular motion of the atmosphere, as indicated by gusts and lulls in the wind.
Transcribed Weather Broadcasts - This NWS aviation product is similar to the Area Forecast (AF) except information is contained in a route format. Forecast sky cover (height and amount of cloud bases), cloud tops, visibility (including vertical visibility), weather, and obstructions to vision are described for a corridor 25 miles either side of the route. Cloud bases and tops are always Mean Sea Level (MSL) unless noted. Ceilings are always above ground level.
Two-Ribbon Flare
In solar-terrestrial terms, a flare that has developed as a pair of bright strands (ribbons) on both sides of the main inversion ("neutral") line of the magnetic field of the active region.
U Burst
In solar-terrestrial terms, a fast radio burst spectrum of a flare. It has a U-shaped appear- ance in an intensity-vs.-frequency plot
In solar-terrestrial terms, the dark core or cores (umbrae) in a sunspot with penumbra, or a sunspot lacking penumbra.
Unstable Air
Air that is able to rise easily, and has the potential to produce clouds, rain, and thunderstorms.
Upper Level Disturbance
A disturbance in the upper atmospheric flow pattern which is usually associated with clouds and precipitation. This disturbance is characterized by distinct cyclonic flow, a pocket of cold air, and sometimes a jet streak. These features make the air aloft more unstable and conducive to clouds and precipitation.
Urban and Small Stream Flood Advisory
This advisory alerts the public to flooding which is generally only an inconvenience (not life-threatening) to those living in the affected area. Issued when heavy rain will cause flooding of streets and low-lying places in urban areas. Also used if small rural or urban streams are expected to reach or exceed bankfull. Some damage to homes or roads could occur.
Urban and Small Stream Flooding
Flooding of small streams, streets, and low-lying areas, such as railroad underpasses and urban storm drains. This type of flooding is mainly an inconvenience and is generally not life threatening nor is it significantly damaging to property.
Urban Flash Flood Guidance
A specific type of flash flood guidance which estimates the average amount of rain needed over an urban area during a specified period of time to initiate flooding on small, ungaged streams in the urban area.
Urban Flooding
Flooding of streets, underpasses, low lying areas, or storm drains. This type of flooding is mainly an inconvenience and is generally not life threatening.
Urban Heat Island
The increased air temperatures in urban areas in contrast to cooler surrounding rural areas.
Variable Infiltration Capacity (VIC)
A large-scale, semi-distributed hydrologic model that solves full water and energy balances. As such, it shares several basic features with other land surface models that are commonly coupled to global circulation models.
Variable Wind
Same as Variable Wind Direction; a condition when
(1) the wind direction fluctuates by 60° or more during the 2-minute evaluation period and the wind speed is greater than 6 knots; or
(2) the direction is variable and the wind speed is less than 6 knots.
Variable Wind Direction
A condition when
(1) the wind direction fluctuates by 60° or more during the 2-minute evaluation period and the wind speed is greater than 6 knots; or
(2) the direction is variable and the wind speed is less than 6 knots.
The distance at which a given standard object can be seen and identified with the unaided eye
Visibility Protection Program
The program specified by the Clean Air Act to achieve a national goal of remedying existing impairments to visibility and preventing future visibility impairment throughout the United States.
Visible Infra-Red Imaging Radiometer (VIIRS)
A medium-resolution sensor flown aboard the NOAA-20 and Suomi-NP satellites.
Visible Satellite Imagery
This type of satellite imagery uses reflected sunlight (this is actually reflected solar radiation) to see things in the atmosphere and on the Earth's surface. Clouds and fresh snow are excellent reflectors, so they appear white on the imagery. Clouds can be distinguished from snow, because clouds move and snow does not move. Meanwhile, the ground reflects less sunlight, so it appears black on the imagery. The satellite uses its 0.55 to 0.75 micrometer (um) channel to detect this reflected sunlight. Since this imagery relies on reflected imagery, it cannot be used during night.
Voluntary Observing Ship Program
(VOS) - An international voluntary marine observation program under the auspices of the World Meteorological Organization (WMO). Observations are coded in a special format known as the ships synoptic code, or "BBXX" format. They are then distributed for use by meteorologists in weather forecasting, by oceanographers, ship routing services, fishermen, and many others.
Visible Satellite Imagery
Watch Box
(or simply "Box") - slang for a Severe Thunderstorm Watch or Tornado Watch issued by the SPC.
Water Table
The level below the earth's surface at which the ground becomes saturated with water. The water table is set where hydrostatic pressure equals atmospheric pressure.
Wet Bulb Zero - the height where the wet-bulb temperature goes below 0°C. It is important because WBZ heights between 7000 ft and 10,500 ft (above ground level) correlate well with large hail at the surface when storms develop in an airmass primed for strong convection. Higher values infer mid and upper level stability and also indicate a large melting area for falling hail. Lower WBZ heights indicate that the low level atmosphere is often too cool and stable to support large hail.
West African Disturbance Line
A line of convection about 300 miles long, similar to a squall line. It forms over west Africa north of the equator and south of 15 degrees North latitude. It moves faster than an Easterly Wave between 20 and 40 mph. They move off the African coast every 4 to 5 days mainly in the summer. Some reach the American tropics and a few develop into tropical cyclones.
Wet Bulb Zero
(Abbrev. WBZ) - the height where the wet-bulb temperature goes below 0°C. It is important because WBZ heights between 7000 ft and 10,500 ft (above ground level) correlate well with large hail at the surface when storms develop in an airmass primed for strong convection. Higher values infer mid and upper level stability and also indicate a large melting area for falling hail. Lower WBZ heights indicate that the low level atmosphere is often too cool and stable to support large hail.
Wet Microburst
A microburst accompanied by heavy precipitation at the surface. A rain foot may be a visible sign of a wet microburst.
Wet-Bulb Temperature
The lowest temperature that can be obtained by evaporting water into the air.
Wolf Number
A historic term for Sunspot Number. In 1849, R. Wolf of Zurich originated the general procedure for computing the sunspot number
A frequency band of microwave radiation in which radars operate.
X-Ray Background
In solar-terrestrial terms, a daily average background X-ray flux in the 1 to 8 angstrom range. It is a midday minimum designed to reduce the effects of flares.
X-Ray Burst
In solar-terrestrial terms, a temporary enhancement of the X-ray emission of the sun. The time-intensity profile of soft X-ray bursts is similar to that of the H-alpha profile of an associated flare.
Expendable Bathythermograph
Zoned Embankment Dam
In hydrologic terms, an embankment dam which is comprised of zones of selected materials having different degrees of porosity, permeability and density.

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