Weather Glossary - D thru G  

 

Weatherwx

Terminology Page

This Weather Glossary, features an alphabetical listing of over 1000 weather-related terms, phrases and abbreviations covering the environmental sciences. The terms with *asterisks before them are related to Storm Spotting and Chasing. The purpose of this glossary is to aid the general public in better understanding the numerous weather related terms and related NWS products.

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Shortcut...just click on the first letter of the word you're looking for

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P| Q | R | S | T | U | V | W | X| Y | Z

 

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-D-

Daily Mean - The average temperature for a day computed by averaging either the hourly readings or, more commonly, the maximum and minimum temperatures.

Dalton's Law - States that the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of the gases. Formulated by John Dalton, an English physicist.

Data Buoys - Buoys placed throughout the Gulf of Mexico and along the Atlantic and Pacific coasts of the United States that relay information on air and water temperature, wind speed, air pressure, and wave conditions via radio signals.

Dawn - The first appearance of light in the eastern sky before sunrise. It marks the beginning of morning twilight. The visual display is created by the scattering of light reaching the upper atmosphere prior to the sun's rise to the observer's horizon. Also known as daybreak.

Day - Considered a basic unit of time as defined by the earth's motion. It represents the time needed for one complete revolution of the earth about its own axis. Also know as a sidereal day, it is approximately equal to 23 hours, 56 minutes, and 4.09 seconds. See night.

dBZ - A Nondimensional "unit" of radar reflectivity which represents a logarithmic power ratio (in decibels, or dB) with respect to radar reflectivity factor, Z. The value of Z is a function of the amount of radar beam energy that is backscattered by a target and detected as a signal (or echo). Higher values of Z (and dBZ) thus indicate more energy being backscattered by a target. The amount of backscattered energy generally is related to precipitation intensity, such that higher values of dBZ that are detected from precipitation areas generally indicate higher precipitation rates.

However, other factors can affect reflectivity, such as width of the radar beam, precipitation type, drop size, or the presence of ground clutter or AP. WSR-88D radars can detect reflectivities as low as -32 dBZ near the radar site, but significant (measurable) precipitation generally is indicated by reflectivities of around 15 dBZ or more. Values of 50 dBZ or more normally are associated with heavy thunderstorms, perhaps with hail, but as with most other quantities, there are no reliable threshold values to confirm the presence of hail or severe weather in a given situation.

*Debris Cloud - Considered a rotating cloud of debris or dust and debris that is on the ground or near the ground they 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 most likely confirm the presence of a tornado , even in the absence of a condensation funnel.

Deepening - Used in describing the history of a low pressure system or an area of cyclonic circulation, it means a decrease in the central pressure of the system. Although it usually describes the action of a pressure system on a constant pressure chart, it also means a surface low is increasing in cyclonic circulation and acquiring more energy. The opposite of filling.

Degree - A measure of temperature difference representing a single division on a temperature scale. See Celsius, Fahrentheit, and Kelvin scales.

Degree Day - A measure of the departure of the mean daily temperature from a given standard. That is one degree day for each degree (Fahrenheit or Celsius) of departure above or below the standard during one day. Refer to cooling degree day and heating degree day.

Delta T - A simple representation of the mean lapse rate within a layer of the atmosphere, obtained by calculating the difference between observed temperatures at the bottom and top of the layer. Delta Ts often are computed operationally over the layer between pressure levels of 700 mb and 500 mb, in order to evaluate the amount of instability in mid-levels of the atmosphere. Generally, values greater than about 18 indicate sufficient instability for severe thunderstorm development.

Dense Fog Advisory - Advisory issued when fog reduces visibility to 1/8 mile or less, creating possible hazardous conditions.

Density -The ratio of the mass of a substance to the volume it occupies. In oceanography, it is equivalent to specific gravity and represents the ratio of the weight of a given volume of sea water to that of an equal volume of distilled water at 4.0 degrees Celsius or 39.2 degrees Fahrenheit.

Density Altitude - The altitude at which a given density is found in the standard atmosphere. Used in aviation, it is computed from the station pressure at takeoff and the virtual temperature at the particular altitude under consideration.

Depression - In meteorology, it is another name for an area of low pressure, a low, or trough. It also applies to a stage of tropical cyclone development and is known as a tropical depression to distinguish it from other synoptic features.

Derecho - (Pronounced day-RAY-cho), a widespread and usually fast-moving windstorm associated with a line of intense, fast-moving thunderstorms that moves across a great distance. Derechos include any family of downburst clusters produced by an extratropical MCS, and can produce damaging straight-line winds over areas hundreds of miles long and more than 100 miles across. Spanish for straight.

Dew - As the surface of the earth cools at night, Condensation in the form of small water drops that forms on grass and other small objects near the ground when the temperature has fallen to the dew point, this generally occurs during the nighttime hours. Dew is particularly heavy on clear nights, when the earth cools rapidly.

Dew Point or (Dew-point Temperature) - Is a measure of atmospheric moisture. It is the temperature to which air must be cooled at a constant pressure to become saturated.

Diablo Winds - Dry winds in the Diablo mountain range in central California that can exceed 60 miles per hour. Similar to the Santa Ana winds, they develop as the wind flows from high pressure over Nevada to lower pressure along the central California coast.

Diffluence - A pattern of air flow in which wind direction diverges along an axis oriented parallel to the flow along the axis. Considered a form of divergence. The opposite of confluence.

Diffraction - The result of light waves interfering with other after passing through a narrow aperture, causing them to bend or spread.

Differential Motion - Cloud motion that appears to differ relative to other nearby cloud elements, e.g. clouds moving from left to right relative to other clouds in the foreground or background. Cloud rotation is one example of differential motion, but not all differential motion indicates rotation. For example, horizontal wind shear along a gust front may result in differential cloud motion without the presence of rotation.

Directional Shear - The component of shear which is due to a rapid change in wind direction with height, e.g., southeasterly winds at the surface and southwesterly winds aloft. A veering wind with height in the lower part of the atmosphere is a type of directional shear often considered important for tornado.

Discontinuity - Comparatively large contrast in meteorological elements over a relatively small distance or period of time. In oceanography, it is the abrupt change or jump of a variable at a line or surface.

Disturbance This has several applications. It can apply to a low or cyclone that is small in size and influence. It can also apply to an area that is exhibiting signs of cyclonic development. It may also apply to a stage of tropical cyclone development and is known as a tropical disturbance to distinguish it from other synoptic features.

Diurnal - Pertaining to actions or events that occur during a twenty-four hour cycle or typically recur every twenty-four hours. Meteorological elements that are measured diurnally include clouds, precipitation, pressure, relative humidity, temperature, and wind.

Divergence - Wind movement that results in a horizontal net outflow of air into a particular region. The expansion or spreading out of a vector field; usually said of horizontal winds. It is the opposite of convergence. Divergence at lower levels is associated with a downward movement of air from aloft. Divergence at upper levels of the atmosphere enhances upward motion, and hence the potential for thunderstorm development (if other factors also are favorable). Contrast with convergence.

Dog Days - The name given to the very hot summer weather that may persists for four to six weeks between mid-July through early September in the United States. In western Europe, this period may exist from the first week in July to mid-August and is often the period of the greatest frequency of thunder. Named for Sirius, the Dog Star, which lies in conjunction with the sun during this period, it was once believed to intensify the sun's heat during the summer months.

Doldrums - Located between 30 degrees North and 30 degrees South latitudes in the vicinity of the equator, this area typically has calm or light and variable winds. Also a nautical term for the equatorial trough. See Intertropical Convergence Zone (ITCZ) or the Horse Latitudes.

Doppler Radar - Weather radar that measures direction and speed of a moving object, such as drops of precipitation. A Radar that can measure radial velocity, the instantaneous component of motion parallel to the radar beam, by determining whether atmospheric motion is horizontally toward or away from the radar. Using the Doppler effect, it measures the velocity of particles. Named for J. Christian Doppler, an Austrian physicist, who in 1842 explained why the whistle of an approaching train had a higher pitch than the same whistle when the train was going away. See NEXRAD.

*Downburst - A severe localized downdraft from a thunderstorm or shower. This outward burst of cool or colder air creates damaging winds at or near the surface. Downburst winds can produce damage similar to a strong tornado. Although usually associated with thunderstorms, downbursts can occur with showers too weak to produce thunder. See microburst.

*Downdraft - A small-scale column of air that has rapidly descent of and is accompany with cool or cold air that sinks toward the ground. It is, usually with precipitation, and associated with a thunderstorm or shower. A downburst is the result of a strong downdraft. Contrast with an updraft.

Downpour - A heavy rain. See cloudburst.

Downslope Effect - The warming of an air flow as it descends a hill or mountain slope. Contrast with an upslope effect.

Downstream - In the same direction as a stream or other flow, or toward the direction in which the flow is moving.

Drainage Wind - A katabatic wind, it is caused by the cooling of air along the slopes of a mountain. An example is a mountain breeze.

Drifting Snow - Snow particles blown from the ground by the wind to a height of less than six feet.

Drifts - Normally used when referring to snow or sand, particles are deposited behind obstacles or irregularities of the surface or driven into piles by the wind.

Drizzle - Slowly falling precipitation in the form numerous minute finewater droplets,(much smaller than in rain), with diameters less than 0.02 inches or 0.5 millimeters, which appear to float. It falls from stratus clouds and is often associated with low visibility and fog. It is reported as "DZ" in an observation and on the METAR.

Dropsonde - A radiosonde dropped with a parachute from an aircraft rather than lifted by a balloon to measure the atmosphere below.

Drought - An extended period of abnormal dry weather for a specific area that is sufficiently prolonged for the lack of water to cause serious hydrological imbalance.

Dry Adiabat - The lifting of dry air, or air that does not contain water vapor. As a parcel lifts, its pressure decreases, and since pressure decreases with height, the air temperature falls due to expansion. When air is dry and is lifted adiabatically, then the temperature decreases at a rate of 1 degree Celsius per 100 meters (5.5 degrees Fahrenheit per 1,000 feet). It appears as a line of constant potential temperature on a thermodynamic chart. Contrast with a moist adiabat.

Dry Bulb Thermometer - A thermometer used to measure the ambient temperature. The temperature recorded is considered identical to air temperaure. One of the two therometers that make up a psychrometer.

Dry Line - A boundary separating moist and dry air masses, it usually refers to the boundary between the dry desert air mass of the Southwest and the moist air mass from the Gulf of Mexico. It is an important factor in determining severe weather frequency in the Great Plains. It typically lies north-south across the central and southern High Plains states during spring and summer. The dry line typically advances eastward during the afternoon and retreats westward at night. The passage of a dry line over an area results in a sharp decrease in humidity, (hence the name),clearing skies, and a wind shift from southeasterly or south to southwesterly or west. (Blowing dust and rising temperatures also may follow, especially if the dry line passes during the daytime; see dry punch).Its presence influences severe weather development in the Great Plains.

However, a strong storm system can sweep the dry line eastward into the Mississippi Valley, or even further east, regardless of the time of day. These changes occur in reverse order when the dry line retreats westward. Severe and sometimes tornadic thunderstorms often develop along a dry line or in the moist air just to the east of it, especially when it begins moving eastward. See LP storm.

Dry-line Bulge - A bulge in the dry line, representing the area where dry air is advancing most strongly at lower levels (i.e., a surface dry punch). Severe weather potential is increased near and ahead of a dry line bulge.

Dry-line Storm - Generally, any thunderstorm that develops on or near a dry line. The term often is used synonymously with LP storm, since the latter almost always occurs near the dryline.

*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. Compare with wet microburst.

Dry Punch - [Slang], For a surge of drier air; normally a synoptic-scale or mesoscale process. A dry punch at the surface results in a dry line bulge. A dry punch aloft above an area of moist air at low levels often increases the potential for severe weather.

Dry Slot - An area of dry, and usually cloud-free, air that wraps into the southern and eastern sections of a synoptic scale or mesoscale low pressure system. Best seen on a satellite picture, such as a water vapor image.

Dusk - The period of waning light from the time of sunset to dark. See twilight and dawn.

Dust - Small particles of earth or other matter suspended in the air. It is reported as "DU" in an observation and for wide spread dust on the METAR.

Dust Bowl - The term given to the area of the Great Plains including Texas, Oklahoma, Kansas, Colorado, and New Mexico that was most greatly affected during the Great Drought of the 1930's.

Dust Devil - A small, rapidly rotating atmospheric vortex (column) of wind, made visible by the rotating cloud of dust, dirt or debris it picks up.(dust whirl). It usually occurs in arid or semi-arid areas and is most likely to develop on clear, dry, hot afternoons in response to surface heating. It is not associated with a thunderstorm. Also called a whirlwind..

*Dust Plume - A non-rotating "cloud" of dust raised by straight-line winds. Often seen in a microburst or behind a gust front. If rotation is observed, then the term dust whirl or debris cloud should be used.

Duststorm - A severe weather condition characterized by strong winds and dust-filled air over a large area. Visibility is reduced to betwen 5/8ths and 5/16ths statute mile. It is reported as "DS" in an observation and on the METAR.

*Dust Whirl - A rotating column of air rendered visible by dust. Similar to debris cloud; see also Dust Devil, gustnado, tornado.

D-Value - The deviation of actual altitude along a constant pressure surface from the standard atmosphere altitude of that surface.

Dynamics - A branch of mechanics that deals with forces and their relations to patterns of motion. Generally, refers to any forces that produce motion or affect change. In metorology, this relates especially to wind and atmospheric dynamics that produces vertical motion in the atmosphere, or water patterns.

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Earthlight (Earthshine) -The faint illumination of the dark part of the moon's disk produced by sunlight reflected onto the moon from the earth's surface and atmosphere.

Earthquake - A sudden, transient motion or trembling of the earth's crust, resulting from the waves in the earth caused by shifting continental plates, faulting of the rocks or by volcanic activity.

Easterlies -Usually applied to the broad patterns of persistent winds with an easterly component, such as the easterly trade winds.

Easterly Wave - An inverted, migratory wave-like disturbance or trough in the tropical region that moves from east to west, generally creating only a shift in winds and rain. The low level convergence and associated convective weather occur on the eastern side of the wave axis. Normally, it moves slower than the atmospheric current in which it is embedded and is considered a weak trough of low pressure. It is often associated with possible tropical cyclone development and is also known as a tropical wave.

Echo - The energy return of a radar signal after it has hit the target. Also called a radar echo.

Eclispe - The obscuring of one celestial body by another. See lunar eclipse or solar eclipse.

Ecliptic - The sun's apparent path across the sky that tracks a circle through the celestial sphere.

ECMWF - European Center for Medium-Range Weather Forecasting. Operational references in forecast discussions typically refer to the ECMWF's medium-range forecast model.

Ecology - The study of the relationships between living organisms and their environment.

Eddy - A small disturbance of wind in a large wind flow, which can produce turbulent conditions or turbulence. They can also be areas of warmer air north of the main westerlies or colder air south of the westerlies. (See cut-off high or cut-off low.) In oceanic circulation, it is a circular movement of water usually formed where currents pass obstructions, between two adjacent currents flowing counter to each other, or along the edge of a permanent current.

Electromagnetic Radiation - Also called radiation, it is waves of energy propagated though space or through a material media.

Electromagnetic Spectrum - The band of electromagnetic radiation with components that are separated into their relative wave lengths. The portion of the spectrum that the human eye can detect is called visible light, between the longer infrared waves and the shorter ultraviolet waves. The various types of energy comprising the spectrum are (from longest to shortest) radio, infrared, visible, ultraviolet, x-rays, gamma rays, and cosmic rays.

*Elevated Convection - Convection occurring within an elevated layer, i.e., a layer in which the lowest portion is based above the earth's surface. Elevated convection often occurs when air near the ground is relatively cool and stable, e.g., during periods of isentropic lift, when an unstable layer of air is present aloft. In cases of elevated convection, stability indices based on near-surface measurements (such as the lifted index) typically will underestimate the amount of instability present. Severe weather is possible from elevated convection, but is less likely than it is with surface-based convection.

Elevated TVS Detection - An Elevated TVS meets the stringent criteria for a TVS, plus adds in more vertical restrictions, which makes the storm much more likely to produce a tornado. A TVS is no guarantee a tornado will form, but definitely a strong indication that one may soon develop, or already has.

Elevation - The measure of height with respect to a point on the earth's surface above mean sea level. Sometimes referred to as station elevation.

El Nino - Spanish for the "Christ Child". Fishermen in Peru and Ecuador used to use this term to refer to a warming cyclical warming of coastal waters around Christmas time that occurs every few years. It is now used to refer cyclical warming of East Pacific Ocean sea water temperatures off the western coast of South America that can result in significant changes in weather patterns in the United States and elsewhere. This occurs when warm equatorial waters move in and displace the colder waters of the Humbolt Current, cutting off the upwelling event. Because it has important consequenses to climate as well as for ocean states (fishing etc). The US government has a very interesting site dedicated to El Nino.

Energy Helicity Index (or EHI) - An index that incorporates vertical shear and instability, designed for the purpose of forecasting supercell thunderstorms. It is related directly to storm-relative helicity in the lowest 2 km (SRH, in m2/s2) and CAPE (in j/kg) as follows: EHI = (CAPE x SRH)/160,000. Thus, higher values indicate unstable conditions and or strong vertical shear. Since both parameters are important for severe weather development, higher values generally indicate a greater potential for severe weather. Values of 1 or more are said to indicate a heightened threat of tornadoes; values of 5 or more are rarely observed, and are said to indicate potential for violent tornadoes. However, there are no magic numbers or critical threshold values to confirm or predict the occurrence of tornadoes of a particular intensity.

Enhanced V - A pattern seen on satellite infrared photographs of thunderstorms, in which a thunderstorm anvil exhibits a V-shaped region of colder cloud tops extending downwind from the thunderstorm core. The enhanced V indicates a very strong updraft, and therefore a higher potential for severe weather.

Enhanced V should not be confused with V notch, which is a radar signature.

Enhanced Wording - An option used by the SPC in tornado and severe thunderstorm watches when the potential for strong/violent tornadoes, or unusually widespread damaging straight-line winds, is high. The statement "THIS IS A PARTICULARLY DANGEROUS SITUATION WITH THE POSSIBILITY OF VERY DAMAGING TORNADOES" appears in tornado watches with enhanced wording. Severe thunderstorm watches may include the statement "THIS IS A PARTICULARLY DANGEROUS SITUATION WITH THE POSSIBILITY OF EXTREMELY DAMAGING WINDS," usually when a derecho event is occurring or forecast to occur. See PDS watch.

Entrance Region - The region upstream from a wind speed maximum in a jet stream (jet max), in which air is approaching (entering) the region of maximum winds, and therefore is accelerating. This acceleration results in a vertical circulation that creates divergence in the upper-level winds in the right half of the entrance region (as would be viewed looking along the direction of flow). This divergence results in upward motion of air in the right rear quadrant (or right entrance region) of the jet max. Severe weather potential sometimes increases in this area as a result. See also exit region, left exit region.

Environment - The sum total of all the external conditions that effect an organism, community, material , or energy.

Environment Canada - The Canadian federal government department responsible for issuing weather forecasts and weather warnings in Canada.

Enivornmental Protection Agengy (EPA) The United States governmental agency in charge of protecting enivironment, this includes air, water and U.S. citizens. The mission of the Environmental Protection Agency is to protect human health and the environment. Since 1970, EPA has been working for a cleaner, healthier environment for the American people. For more information on Environmental Protection Agency, visit their website.

Equator - The geographic circle at 0 degrees latitude on the earth's surface. It is equal distance from the North and South Poles and divides the Northern Hemisphere from the Southern.

Equatorial Trough - The quasi-continuous area of low pressure between the subtropical high pressure areas in both the Northern and Southern Hemisphere. See Intertropical Convergence Zone (ITCZ) and the doldrums.

Equilibrium Level (or 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. However, strong updrafts will continue to rise past the EL before stopping, resulting in storm tops that are higher than the EL. This process sometimes can be seen visually as an overshooting top or anvil dome.

Equinox - The point at which the ecliptic intersects the celestial equator. Days and nights are most nearly equal in duration. In the Northern Hemisphere, the vernal equinox falls on or about March 20 and the autumnal equinox on or about September 22.

Erosion - The movement of soil or rock from one area to another by the action of the sea, running water, moving ice, precipitation, or wind.

Eta Model - One of the operational numerical forecast models run at NCEP. The Eta is run twice daily, with forecast output out to 48 hours.

Evaporation - The physical process by which a liquid, such as water, is tranformed into a gaseous state, such as water vapor. It is the opposite physical process of condensation.

Evapotranspiration - The total amount of water that is transferred from the earth's surface to the atmosphere. It is made up of the evaporation of liquid or solid water plus the transpiration from plants.

Exit Region - The region downstream from a wind speed maximum in a jet stream (jet max), in which air is moving away from the region of maximum winds, and therefore is decelerating. This deceleration results in divergence in the upper-level winds in the left half of the exit region (as would be viewed looking along the direction of flow). This divergence results in upward motion of air in the left front quadrant (or left exit region) of the jet max. Severe weather potential sometimes increases in this area as a result. See also entrance region, right entrance region.

Exosphere - This region is considered the very outer limits of the earth's atmosphere. Its lower boundary is often called the critical level of escape, where gas atoms are so widely spaced that they rarely collide with one another and have individual orbits. It is estimated to be some 400 plus miles (640 kilometers) above the surface. See the ionosphere.

Exposure - The state or fact of being exposed. In terms of weather the state of a person being exposed unprotected to the elements or specifically a person being exposed unprotected to severe weather event.

Extratropical Cyclone -Any cyclone not of tropical origin. Generally considered to be a migratory frontal cyclone found in the middle and high latitudes. Also called an extratropical low or an extratropical storm.

Eye - The center of a tropical storm or hurricane, characterized by a roughly circular area of light winds and rain-free skies. An eye will usually develop when the maximum sustained wind speeds exceed 78 mph. It can range in size from as small as 5 miles to up to 100 miles, but the average size is 20 miles. In general, when the eye begins to shrink in size, the storm is intensifying.

Eye Wall - An organized band of convection surrounding the eye, or center, of a tropical cyclone. It contains cumulonimbus clouds, intense rainfall and very strong winds.

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F scale - See Fujita Scale.

FAHRENHEIT Temperature Scale - A temperature scale where water at sea level has a freezing point of +32 degrees F (Fahrenheit) and a boiling point of +212 degrees F. More commonly used in areas that observe the English system of measurement. Created in 1714 by Gabriel Daniel Fahrenheit (1696-1736), a German physicist, who also invented the alcohol and mercury thermometers.

Fair - This is a subjective meteorlogical description. Considered as pleasant weather conditions with regard to the time of year and the physical location.

Fathom - The common unit of depth in the ocean for countries using the English system of measurement. It is six feet or 1.83 meters. It can also be used in expressing horizontal distance, since 120 fathoms is equal to one cable or nearly on tenth of a nautical mile.

*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.

In Tropical systems the term describes, the lines or bands of thunderstorms that spiral into and around the center of a tropical system. Also known as outer convective bands, a typical hurricane may have three or more of these bands. They occur in advance of the main rain shield and are usually 40 to 80 miles apart.

Fetch - An area of the water surface over which waves are generated by a wind having a constant direction and speed. Also, the name given to the length of the fetch area, measured in the direction of the wind from which the seas are generated.

Few - The amount of sky cover for a cloud layer between 1/8th and 2/8ths, based on the summation layer amount for that layer.

Filling - Used in describing the history of a low pressure system or an area of cyclonic circulation, it means an increase in the central pressure of the system. Although it usually describes the action of a pressure system on a constant pressure chart, it also means a surface low is decreasing in cyclonic circulation and losing its characteristics. The opposite of deepening.

Firewhirl - A tornado-like rotating column of fire and smoke created by intense heat from a forest fire or volcanic eruption.

First Gust - Another name for the initial wind surge observed at the surface as the result of downdrafts forming the leading edge or gust front of a thunderstorm.

*Flanking Line - A line of attached cumulus or towering cumulus clouds connected to and extending outward from the most active part of a supercell, (this is usually on the southwest side of the storm) The line line normally has a stair-step appearance, with the tallest clouds closest to the main storm, and generally coincides with the pseudo-cold front. clouds appears withof descending height, appearing as stair steps of the most active part of a supercell.

Flash Flood - A flood that rises and falls quite rapidly with little or no advance warning, usually as the result of intense rainfall over a relatively small area. Flash floods can be caused by situations such as a sudden excessive rainfall, the failure of a dam, or the thaw of an ice jam.

Flood - High water flow or an overflow of rivers or streams from their natural or artificial banks, inundating adjacent low lying areas.

Flood Plain - Level land that may be submerged by flood waters.

Flood Stage -The level of a river or stream where overflows it's existing banks and floods onto surrounding low areas.

Flurry - Precipitation in the form of snow from a convective cumulus-type cloud. Flurries are characterized by the suddenness with which they start and stop, by their rapid changes in intensity, and usually by rapid changes in the appearance of the sky.

Foehn - A warm dry wind on the lee side of a mountain range, whose temperature is increased as the wind descends down the slope. It is created when air flows downhill from a high elevation, raising the temperature by adiabatic compression. Examples include the chinook wind and the Santa Ana wind. Classified as a katabatic wind.

Fog - Essensitially a cloud formed near the earth's surface consisting of visible aggregate of minute water droplets suspended in the atmosphere, that reduces horizontal visibility to less than 5/8 statute miles. Under very cold conditions, fog is composed of ice crystals or ice fog. It is created when the temperature and the dew point of the air have become the same, or nearly the same, and sufficient condensation nuclei are present. It is reported as "FG" in an observation and on the METARM.

Fog Bank - A fairly well-defined mass of fog observed in the distance. Most commonly seen at sea, over a lake, or along coastal areas.

Fogbow - A whitish semicircular arc seen opposite the sun in fog. The outer margin has a reddish tinge, its inner margin has a bluish tinge, and the middle of the band is white. An additional bow with reversed colors sometimes appears inside the first.

Forecast - A statement of expected future occurrences. Weather forecasting includes the use of objective models based on certain atmospheric parameters, along with the skill and experience of a meteorologist. Also called a prediction.

*Forward Flank Downdraft - The main region of downdraft in the forward, or leading, part of a supercell, where most of the heavy precipitation is. Compare with rear flank downdraft. See pseudo-warm front

Fractus - The elements of cumulus and stratus clouds that appear in irregular fragments, as if they had been shred or torn. Never appears in cirrus clouds. Also known as scud. See cumulus fractus and stratus fractus.

Freezing Drizzle - Drizzle, falling as a liquid, but freezing on impact with the colder ground or other exposed surfaces. It is reported as "FZDZ" in an observation and on the METAR.

Freezing Fog - Used to describe the phenomena when fog is present and the air temperature is below 0 degrees Celsius. It is reported as "FZFG" in an observation and on the METAR.

Freezing Point/Freeze - The process of changing a liquid to a solid. The temperature at which a liquid solidifies under any given set of conditions. Pure water under atmospheric pressure freezes at 0 degrees Celsius or 32 degrees Fahrenheit. It is the opposite of fusion. In oceanography, the freezing point of water is depressed with increasing salinity.

Freezing Precipitation - Precipitation that is liquid, but freezes upon impact with a solid surface, such as the ground or other exposed surfaces. Examples include freezing rain and freezing drizzle.

Freezing Rain - Rain that falls as liquid and freezes upon impact to form a coating of glaze on the colder ground or other exposed surfaces. A freezing rain warning is usually issued when slippery driving and walking conditions are expected, and/or when freezing rain may damage trees, power lines, or other structures. It is reported as "FZRA" in an observation and on the METAR.

Fresh Water - Water found rivers, lakes, and rain, that is distinguished from salt water by its appreciable lack of salinity.

Friction - In meteorology, it is the turbulent resistence of the earth on the atmosphere. Considered as the resistance of fluids (air and water) to the relative motion of a solid body. The amount is dependent on the size and shape of the body.

Friction Layer - The thin layer of atmosphere adjacent to the earth's surface. Surface friction is effective in slowing down wind up to approximately 1,500 to 3,000 feet above the ground. Above this level, air tends to flow parallel to the isobars. Wind distribution within this layer is determined by vertical temperature gradient and the physical contours of the underlying surface features. Also known as a surface boundary layer or boundary layer.

Front - A boundary or transition zone or interface between two air masses of different densities, which usually means different temperatures. A moving front is named according to the advancing air mass, e.g. For example, the area of convergence between warm, moist air and cool, dry air. See cold frontis the leading edge of an advancing cold air mass and warm front is the trailing edge of a retreatinq cold air mass.

Frontal Passage - It is considered as the passage of a front over a specific point on the surface. It is reflected by the change in dew point and temperature, the shift in wind direction, and the change in atmospheric pressure. Accompanying a passage may be precipitation and clouds. May be referred to as "fropa."

Frontogenesis - The birth or creation of a front. This occurs when two adjacent air masses exhibiting different densities and temperatures are brought together by prevailing winds, creating a front. It could happen when either air mass, or both, move over a surface which strengthens their original properties. However, it occurs most often along the eastern coasts of North America and Asia, when the air mass moving out over the ocean has a weak or no distinct boundary. The opposite of frontolysis.

Frontolysis - The destruction or dying of a front where the transition zone is losing its contrasting properties. The opposite of frontogenesis.

Frost - The covering of ice crystals that forms by direct sublimation on exposed surfaces whose temperature is below freezing. It is not frozen dew. A Killing Frost is a frost severe enough to end the growing season.

Frozen Precipitation -Precipitation that reaches the ground in a frozen state. Examples include snow, snow pellets, snow grains, ice crystals, ice pellets, and hail.

Fujita-Pearson Scale (or F Scale)- A scale that classifies the severity of wind damage intensity based on the degree of destruction as it relates to the wind speed as well as path length and path width of the event. It is normally used to identify the most instense damage exhibited by a tornado. Developed by T. Theodore Fujita and Allen Pearson in 1971. The Fujita Scale was replaced with the Enhanced Fujita Scale in Feb. 2007.

Scale	Wind Estimate (MPH)	Typical Damage
F0	<73	Light damage. Some damage to chimneys; branches broken off trees; shallow-rooted trees pushed over; sign boards damaged.
F1	73-112	Moderate damage. Peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos blown off roads.
F2	113-157	Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars overturned; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground.
F3	158-206	Severe damage. Roofs and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted; heavy cars lifted off the ground and thrown.
F4	207-260	Devastating damage. Well-constructed houses leveled; structures with weak foundations blown away some distance; cars thrown and large missiles generated.
F5	261-318	Incredible damage. Strong frame houses leveled off foundations and swept away; automobile-sized missiles fly through the air in excess of 100 meters (109 yds); trees debarked; incredible phenomena will occur.

All tornadoes, and most other severe local windstorms, are assigned a single number from this scale according to the most intense damage caused by the storm.

*Funnel Cloud - A violent, rotating column of air visibly extending from the base of a towering cumulus or cumulonimbus that extends toward the ground, but not in contact with it. It is sometimes called a condensation funnel. It is reported as "FC" in an observation and on the METAR. A condensation funnel is a tornado, not a funnel cloud, if either a) it is in contact with the ground or b) a debris cloud or dust whirl is visible beneath it.

Fusion - The change of state from a solid to a liquid at the same temperature. The heat of fusion is the number of gram calories of heat necessary to change one gram of a substance from the solid to the liquid state. It is the opposite of freezing.

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-G-

Gale (A strong wind) - On the Beaufort Wind Scale, a wind with speeds from 28 to 55 knots (32 to 63 miles per hour). For marine interests, it can be categorized as a moderate gale (28 to 33 knots), a fresh gale (34 to 40 knots), a strong gale (41 to 47 knots), or a whole gale (48 to 55 knots). In 1964, the World Meteorological Organization defined the categories as near gale (28 to 33 knots), gale (34 to 40 knots), strong gale (41 to 47 knots), and storm (48 to 55 knots). A gale warning is issued when forecasted(expected) winds exceed (32 to 54 miles per hour or 28 to 47 knots).

Gale Warning - warning for marine interests for impending winds from 28 to 47 knots (32 to 54 miles per hour).

Geophysics - The study of the physics or nature of earth and its environment. It deals with the composition and physical phenomena of the earth and its liquid and gaseous envelopes. Areas of studies include the atmospheric sciences and meteorology, geology, seismology, and volcanology, and oceanography and related marine sciences, such as hydrology. By extension, it includes astronomy and the related astro-sciences.

Geosphere - Considered the solid portions of the earth, including the hydrosphere and the lithosphere, as opposed to the atmosphere, which lies above it. At their conjunction is the biosphere.

Geostationary SATellite - An orbiting weather satellite that maintains the same position over the equator during the earth's rotation. Also known as GOES, an acronym for Geostationary Operational Environmental Satellite. Compare with a polar-orbiting satellite.

Geostrophic Wind - A steady horizontal motion of air along straight, parallel isobars or contours in an unchanging pressure or contour field. It is assumed that there is no friction, that the flow is straight with no curvature and there is no divergence or convergence with no vertical acceleration.

Glacier Winds - Air flow that descends from glaciers, occasionally at a high rate of speed. Caused by the temperature difference between the air in contact with the glacier and the air at the same altitude, it reaches maximum instensity in the early afternoon. A form of katabatic wind.

Glaze - A smooth clear icy coating of supercooled water droplets that spread out and freeze onto objects on contact. A storm that produces the accretion of glaze is called an ice storm.

Gradient Wind - A steady horizontal air motion along curved parallel isobars or contours in an unchanging pressure or contour field, assuming there is no friction and no divergence or convergence.

Graupel - A form of frozen precipitation consisting of snowflakes or ice crystals and supercooled water droplets frozen together. Also known as snow pellets.

Gravitation - The mutual attraction between two masses of matter. The rotation of the earth and the atmosphere modifes this attraction to produce the field of gravity.

Gravity - The force of attraction of the earth on an object. The direction is downward relative to the earth, and it decreases with elevation or altitude away from the earth's surface.

Green Flash - A brilliant green coloration of the upper edge of the sun, occasionally seen as the sun's apparent disk is about to set below a clear horizon.

Greenhouse Effect - The greenhouse effect is the name applied to the process which causes the surface of the earth to be warmer than it would have been in the absence of an atmosphere because it receives energy from two sources: the sun and the atmosphere. Radiation is not trapped, and the atmosphere does not behave as a greenhouse and the greenhouse gasses do not behave as a blanket ~ the name "greenhouse effect" is somewhat of a misnomer.

Also interepted to mean the overall warming of the earth's lower atmosphere. It occurs when carbon dioxide and water vapor, which initially permit the sun's rays to heat the earth, then trap the heat near the earth's surface.

Greenwitch Mean Time (GMT) - The name of the twenty-four hour time scale which is used throughout the scientific and military communities. Standard Time begins at Greenwich, England, home of the Royal Observatory which first utilized this method of world time. This is also the Prime Meridian of Longitude. The globe is divided into twenty-four (24) time zones of 15 degrees of arc, or one hour in time apart. To the east of this meridian, time zones are number from 1 to 12 and prefixed with a minus (-), indicting the number of hours to be subtracted to obtain Greenwich Time (GMT). To the west, the time zones are also numbered 1 through 12, but are prefixed with a plus (+), indicating the number of hours to be added to obtain GMT.
Related terms: Universal Time Coordinate (UTC) and Zulu (Z)

Ground Clutter -A pattern of radar echoes reflecting off fixed ground targets such as buildings or hills near the radar. This may hide or confuse the proper return echo signifying actual precipitation echoes near the radar antenna.

Ground Fog - Fog created when radiational cooling at the earth's surface lowers the temperature of the air near the ground to or below its initial dew point. Primarily takes place at night or early morning. Also known as radiation fog.

Growing Season - Considered the period of the year during which the temperature of cultivated vegetation remains sufficiently high enough to allow plant growth. Usually considered the time period between the last killing frost in the spring and the first killing frost of the autumn. The frost-free growing season is between the first and last occurrence of 32 degrees Fahrenheit temperatures in spring and autumn.

*Grunge - [Slang], For anything in the atmosphere that restricts visibility for storm spotting, such as fog, haze, precipitation (steady rain or drizzle), widespread low clouds (stratus), etc...

Gulf Stream - The warm, well-defined, swift, relatively narrow ocean current which exists off the east coast of the United States, beginning near Cape Hatteras. The term also applies to the oceanic system of currents that dominate the western and northern Atlantic Ocean: the Florida current, which flows through the Florida Straits between the Florida Keys and Cuba and northwards; the Gulf Stream, which begins around Cape Hatteras and flows northeasterly off the continental slope into the North Atlantic; and the North Atlantic current, which begins around the Grand Banks off Newfoundland and continues east-northeastwards towards the British Isles.

Gully Washer - A heavy rain shower that occurs suddenly, possibly creating a flash flood.

Gust - A sudden significant increase in wind speed or rapid fluctuations of wind speed,generally less than 20 seconds Peak wind must reach at least 16 knots (18 miles per hour) and the variation between peaks and lulls is at least 10 knots (11.5 miles per hour). The duration is usually less twenty seconds.

*Gust Front - A sudden, increase in wind speed from, the leading edge of the cool, gusty surface winds produced by thunderstorm downdrafts. It is considered the leading edge of a thunderstorm and is sometimes confused with an outflow boundary. Gust fronts can sometimes associated with a shelf cloud or roll cloud Also see first gust, downburst, gustnado and outflow boundary.

Gustnado or (Gustinado) - [Slang], gust front tornado. It is actually a weak, and usually short-lived, tornado that forms along the gust front of a thunderstorm, appearing as a temporary dust whirl or debris cloud. Gustnadoes are not associated with storm-scale rotation (i.e. mesocyclones); they are more likely to be associated visually with a shelf cloud than with a wall cloud.

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