Adirondack Type Snow Sampling SetIn hydrologic terms, a snow sampler consisting of a 5-foot fiberglass tube, 3 inches in diameter, with a serrated-edge steel cutter at one end and a twisting handle at the other. This sampler has a 60-inch snow depth capacity. Advanced Baseline Imager (ABI)A moderate resolution imaging system on the GOES and Himawari family of satellites.Air Quality ModelMathematical or conceptual model used to estimate present or future air quality.Alberta ClipperA 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".ALIASINGThe process by which frequencies too high to be analyzed with the given sampling interval appear at a frequency less than the Nyquist frequency.AmplifierA device used to increase the strength of an analog signalAmplitudeThe maximum magnitude of a quantity. Often used to refer to the maximum height of a wave.Annual Exceedance Probability (AEP)The probability that a stream reach will have a flow of a certain magnitude in any given year.AnthelionA luminous white spot that appears on the parhelic circle at the same altitude as the sun and 180 degrees from it in azimuth. AphelionThe point on the annual orbit of a body (about the sun) that is farthest from the sun; at present, the earth reaches this point (152 million kilometer from the sun) on about 5 July. Opposite of perihelion.Aurora AustralisSame as Aurora Borealis, but in the Southern Hemisphere. Also known as the southern 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.Aurora BorealisAlso 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.Baroclinic leaf shieldA cloud pattern on satellite images - frequently
noted in advance of formation of a low pressure center.Baroclinic ZoneA 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.BaroclinityA measure of the state of stratification in a fluid in which surfaces of constant pressure (isobaric) intersect surfaces of constant density (isosteric).BEAM FILLINGThe 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.Blizzard(abbrev. BLZD)- A blizzard means that the following conditions are expected to prevail for a period of 3 hours or longer:
Blizzard WarningIssued 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.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.CalibrationIn hydrologic terms, the process of using historical data to estimate parameters in a hydrologic forecast technique such as SACSMA, routings, and unit hydrographs.Catalina EddyA Catalina Eddy (coastal eddy) forms when upper level large-scale flow off Point Conception interacts with the complex topography of the Southern California coastline. As a result, a counter clockwise circulating low pressure area forms with its center in the vicinity of Catalina Island. This formation is accompanied by a southerly shift in coastal winds, a rapid increase in the depth of the marine layer, and a thickening of the coastal stratus. Predominately these eddies occur between April and September with a peak in June. A typical Catalina eddy will allow coastal low clouds and fog to persist into the afternoon. A strong Catalina eddy may extend to 6000 feet and these clouds will move through the inland valleys and reach as far as Palmdale.Ceiling(Abbrev. CIG) - The height of the cloud base for the lowest broken or overcast cloud layer. ChannelizationIn hydrologic terms, the modification of a natural river channel; may include deepening, widening, or straightening.Client AgencyAs used in connection with reimbursable National Weather Service (NWS) fire
weather services, a public fire service or wildlands management agency, Federal or non-Federal,
which requires and uses NWS fire and forestry meteorological servicesClimateThe composite or generally prevailing weather conditions of a region, throughout the year, averaged over a series of years.Climate ChangeA non-random change in climate that is measured over several decades or longer. The change may be due to natural or human-induced causes.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.Climate Diagnostics Center(CDC) - The mission of NOAA's Climate Diagnostics Center is to identify the nature and causes for climate variations on time scales ranging from a month to centuries.Climate ModelMathematical model for quantitatively describing, simulating, and analyzing the interactions between the atmosphere and underlying surface (e.g., ocean, land, and ice).Climate OutlookA climate outlook issued by the CPC gives probabilities that conditions, averaged over a specified period, will be below-normal, normal, or above-normal.Climate Prediction CenterThis Center is one of several centers under the National Centers for Environmental Prediction (NCEP) part of the National Weather Service (NWS) in the National Oceanic and Atmospheric Administration (NOAA). The Center serves the public by assessing and forecasting the impacts of short-term climate variability, emphasizing enhanced risks of weather-related extreme events, for use in mitigating losses and maximizing economic gains.Climate SystemThe system consisting of the atmosphere (gases), hydrosphere (water), lithosphere (solid rocky part of the Earth), and biosphere (living) that determine the Earth's climate.Climatological OutlookAn outlook based upon climatological statistics for a region, abbreviated as CL on seasonal outlook maps. CL indicates that the climate outlook has an equal chance of being above normal, normal, or below normal.ClimatologyThe science that deals with the phenomena of climates or climatic conditions.CLIMOClimatology/ClimatologicalClimometerAn instrument that measures angles of inclination; used to measure cloud ceiling heights.Cloud CeilingSame as Ceiling; the height of the cloud base for the lowest broken or overcast cloud layer. Consolidated Ice CoverIn hydrologic terms, ice cover formed by the packing and freezing together of floes, brash ice and other forms of floating ice.Cooling Degree Days(Abbrev. CDD) - A form of Degree Day used to estimate energy requirements for air conditioning or refrigeration. Typically, cooling degree days are calculated as how much warmer the mean temperature at a location is than 65°F on a given day. For example, if a location experiences a mean temperature of 75°F on a certain day, there were 10 CDD (Cooling Degree Days) that day because 75 - 65 = 10.Coriolis ForceA fictitious force used to account for the apparent deflection of a body in motion with respect to the earth, as seen by an observer on the earth. The deflection (to the right in the Northern Hemisphere) is caused by the rotation of the earth.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. CumuliformDescriptive of all clouds with vertical development in the form of rising mounds, domes, or towersCumuliform AnvilA thunderstorm anvil with visual characteristics resembling cumulus-type clouds (rather
than the more typical fibrous appearance associated with cirrus). A cumuliform anvil arises from rapid
spreading of a thunderstorm updraft, and thus implies a very strong updraft. See anvil rollover, knuckles,
mushroom.Cyclic StormA thunderstorm that undergoes cycles of intensification and weakening (pulses)
while maintaining its individuality. Cyclic supercells are capable of producing multiple tornadoes (i.e.,
a tornado family) and/or several bursts of severe weather.Daily Climatological ReportAs the name indicates, this climatological product is issued daily by each National Weather Service office. Most of the climatological data in this report are
presented in a tabular form; however, some narrative statements may also be used in the product. The report is organized so that similar items are grouped together (i.e., temperature,
precipitation, wind, sunrise and sunset times, etc.).DeclinationThe latitude that the sun is directly over at a given time. The declination is ~23°N at the summer solstice, ~23°S at the winter solstice, and 0° (over the equator) at the spring and autumn equinoxes.Dividing StreamlineIn the blocked flow region upwind of a mountain barrier, the streamline that separates the blocked flow region near the ground from the streamlines above which go over the barrier.Dividing Streamline HeightThe height above ground of the dividing streamline, as measured far upwind of a mountain barrier. See dividing streamline.Downwelling RadiationThe component of radiation directed toward the earth's surface from the sun or the atmosphere, opposite of upwelling radiation.Drains (Relief Wells)In hydrologic terms, a vertical well or borehole, usually downstream of impervious cores, grout curtains or cutoffs, designed to collect and direct
seepage through or under a dam to reduce uplift pressure under or within a dam. A line of such wells forms a "drainage curtain".Dry LineA boundary separating moist and dry air masses, and an important factor in
severe weather frequency in the Great Plains. It typically lies north-south across
the central and southern high Plains states during the spring and early summer,
where it separates moist air from the Gulf of Mexico (to the east) and dry desert
air from the southwestern states (to the west). The dry line typically advances
eastward during the afternoon and retreats westward at night. 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.
A typical dry line passage results in a sharp drop in humidity (hence the name),
clearing skies, and a wind shift from south or southeasterly to west or
southwesterly. (Blowing dust and rising temperatures also may follow, especially
if the dry line passes during the daytime. 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.Dry Line BulgeA 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 Line StormAny thunderstorm that develops on or near a dry line.Dynamic LiftingThe forced uplifting of air from various atmospheric processes, such as weather fronts, and
cyclones.EasterliesAny winds with components from the east.Energy Helicity IndexAn index that incorporates vertical shear and instability, designed for the
purpose of forecasting supercell thunderstormsEqui-Potential LineIn hydrologic terms, a line, in a field of flow, such that the total head is the same for all points on the line, and therefore the direction of flow is
perpendicular to the line at all points.Equilibrium DrawdownIn 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 DischargeIn 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 rateEquilibrium TimeIn 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.Fall LineA skiing term, indicating the line of steepest descent of a slope.FillingThe opposite of deepening. A general increase in the central pressure of a low pressure system.Firn LineIn hydrologic terms, the highest level to which the fresh snow on a glacier's surface retreats during the melting season. The line separating the
accumulation area from the ablation areaFlanking LineA line of cumulus or towering cumulus clouds connected to and extending
outward from the most active part of a supercell, normally on the southwest side. The line normally
has a stair-step appearance, with the tallest clouds closest to the main stormFlash MultiplicityThe number of return strokes in a lightning flash. Flow SplittingThe splitting of a stable airflow around a mountain barrier, with branches going around the left and right edges of the barrier, often at accelerated speeds.Forced ChannelingChanneling of upper winds along a valley's axis when upper winds are diverted by the underlying topography. Compare pressure-driven channeling.Forecast valid forThe period of time the forecast is in effect
beginning at a given day, date and time, and ending at a given day, date
and time.FunnellingThe process whereby wind is forced to flow through a narrow opening between adjacent land areas, resulting in increased wind speed. Geostationary SatelliteA satellite that rotates at the same rate as the earth, remaining over the same spot above the equator.Gravity Recovery and Climate Experiment (GRACE)NASA satellites that detect small changes in the Earth’s gravitational field caused by the redistribution of water on and beneath the land surface.Green LineThe green line is one of the strongest (and first-recognized)
visible coronal lines. It identifies moderate temperature regions
of the CORONA.Ground Blizzard WarningWhen blizzard conditions are solely caused by blowing and drifting snow. Hague LineThe North Atlantic boundary between the U.S. and Canada fishing waters as determined by the World Court in The Hague, Netherlands.Heat LightningLightning that occurs at a distance such that thunder is no longer audible.HelicityA property of a moving fluid which represents the potential for helical flow (i.e. flow which follows the pattern of a corkscrew) to evolve. Helicity is proportional to the strength of the flow, the amount of vertical wind shear, and the amount of turning in the flow (i.e. vorticity).
Atmospheric helicity is computed from the vertical wind profile in the lower part of the atmosphere (usually from the surface up to 3 km), and is measured relative to storm motion. Higher values of helicity (generally, around 150 m2/s2 or more) favor the development of mid-level rotation (i.e. mesocyclones). Extreme values can exceed 600 m2/s2.
High Water ProbabilityThe probability (%) that a stream will reach its high water flow within the forecast period.Hydraulic Fill DamIn hydrologic terms, a dam constructed of materials, often dredged, that are conveyed and placed by suspension in flowing waterHydraulic FlowAtmospheric flow that is similar in character to the flow of water over an obstacle.Hydraulic Grade LineIn hydrologic terms, a line whose plotted ordinate position represents the sum of pressure head plus elevation head for the various positions along a
given fluid flow path, such as along a pipeline or a ground water streamline.Hydraulic HeadIn hydrologic terms,
- Sustained wind or frequent gusts to 35 miles an hour or greater; and
- Considerable falling and/or blowing snow (i.e., reducing visibility frequently to less than ¼ mile)
(1) The height of the free surface of a body of water above a given point beneath the surface.
(2) The height of the water level at
the headworks, or an upstream point, of a waterway, and the water surface at a given point downstream.
(3) The height of a
hydraulic grade line above the center line of a pressure pipe, at a given point.Hydraulic JumpA steady disturbance in the lee of a mountain, where the airflow passing over the mountain suddenly changes from a region of low depth and high velocity to a region of high depth and low velocity.Hydraulic PermeabilityIn 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.In-Cloud Lightning(abbrev. IC) Lightning that takes place within the cloud.Infrared Satellite ImageryThis satellite imagery senses surface and cloud top temperatures by measuring the wavelength of electromagnetic radiation emitted from these objects.
This energy is called "infrared". High clouds are very cold, so they appear white. Mid-level clouds are somewhat warmer, so they will be a light gray shade. Low cloud are warmer still, so
they appear as a dark shade of gray or black. Often, low clouds are the same temperature as the surrounding terrain and cannot be distinguished at all. The satellite picks up this infrared
energy between 10.5 and 12.6 micrometer (um) channels. Instability(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.Instrument Flight RulesRefers to the general weather conditions pilots can expect at the surface and applies to the weather situations at an airport during which a pilot must use instruments to assist take off and landing. IFR conditions for fixed wing aircraft means the minimum cloud ceiling is greater than 500 feet and less than 1,000 feet and/or visibility is greater than 1 mile and less than 3 miles.International Date LineThe line of longitude located at 180 degrees East or West (with a few local deviations) where the date changes by a day. West of the line it is one day later than east of the line.Isentropic LiftLifting of air that is traveling along an upward-sloping isentropic surface.
Isentropic lift often is referred to erroneously as overrunning, but more accurately describes the physical process by which the lifting occurs. Situations involving isentropic lift often are characterized by widespread stratiform clouds and precipitation, but may include elevated convection in the form of embedded thunderstorms.LCD (Local Climatological Data)This National Climatic Data Center (NCDC) publication is produced monthly and annually for some 270 United States cities and it's territories. The
LCD summarizes temperature, relative humidity, precipitation, cloudiness, wind speed and direction observation. LILifted Index. A common measure of atmospheric instability. Its value is obtained by computing the temperature that air near the
ground would have if it were lifted to some higher level (around 18,000 feet, usually) and comparing that
temperature to the actual temperature at that level. Negative values indicate instability - the more negative, the
more unstable the air is, and the stronger the updrafts are likely to be with any developing thunderstorms.
However there are no "magic numbers" or threshold LI values below which severe weather becomes imminent. LidA layer of warm air several thousand feet above the earth's surface which suppresses or delays the development of thunderstorms.Lid(Also called cap.) A region of negative buoyancy below an existing level of free convection (LFC) where energy must be supplied to the parcel to maintain its ascent.
This tends to inhibit the development of convection until some physical mechanism can lift a parcel to its LFC. The intensity of the cap is measured by its convective inhibition. The term capping inversion is sometimes used, but an inversion is not necessary for the conditions producing convective inhibition to exist.LIFRLow Instrument Flight RulesLifted Index(abbrev. LI)- A common measure of atmospheric instability. Its value is obtained by computing the temperature that air near the ground would have if it were lifted to some higher level (around 18,000 feet, usually) and comparing that temperature to the actual temperature at that level. Negative values indicate instability - the more negative, the more unstable the air is, and the stronger the updrafts are likely to be with any developing thunderstorms. However there are no "magic numbers" or threshold LI values below which severe weather becomes imminent.Lifting Condensation Level(LCL) - The level at which a parcel of moist air becomes saturated when it is lifted dry adiabatically.Light BridgeIn 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 dissolutionLightning(abbrev. LTNG) A visible electrical discharge produced by a thunderstorm. The discharge may occur within or between clouds, between the cloud and air, between a cloud and the ground or between the ground and a cloud.Lightning ChannelThe irregular path through the air along which a lightning discharge occurs.
A typical discharge of flash between the ground and the cloud is actually a composite flash
which is composed of several sequential lightning strokes, each of which is initiated by a leader
and terminated by a return streamer.Lightning DischargeThe series of electrical processes by which charge is transferred along a
channel of high ion density between electrical charge centers of opposite sign. This can be
between a cloud and the Earth's surface of a cloud-to-ground discharge.Lightning StrokeAny of a series of repeated electrical discharges comprising a single lightning
discharge (strike). Specifically, in the case of a cloud-to-ground discharge, a leader plus its
subsequent return streamer. Likely(abbrev. LKLY) In probability of precipitation statements, the equivalent of a 60 or 70 percent chance.LimbIn solar-terrestrial terms, the edge of the solar disk.Limb FlareIn solar-terrestrial terms, a solar flare seen at the edge (Limb) of the sun.LimnologyIn hydrologic terms, the branch of hydrology that pertains to the study of lakesLine Echo Wave Pattern(abbrev. LEWP) A radar echo pattern formed when a segment of a line of thunderstorms surges forward at an accelerated rate.Line SourceAn array of pollutant sources along a defined path that can be treated in dispersion models as an aggregate uniform release of pollutants along a line. Example: the sum of emissions from individual cars traveling down a highway can be treated as a line source. Compare area source and point source.Liquid Water EquivalentSame as Water Equivalent; the liquid content of solid precipitation that has accumulated on the ground (snow depth). The accumulation may consist of snow, ice formed by freezing precipitation, freezing liquid precipitation, or ice formed by the refreezing of melted snow. LithometeorAtmospheric phenomena which affect the state of the atmosphere. They constitute dry particles that hang suspended in the atmosphere, such as dust, smoke, sand, and haze. LithosphereIn hydrologic terms, that part of the earth which is composed predominantly of rocks (either coherent or incoherent, and including the disintegrated
rock materials known as soils and subsoils), together with everything in this rocky crust.Littoral ZoneIn hydrologic terms, the area on, or near the shore of a body waterLive CapacityIn hydrologic terms, the total amount of storage capacity available in a reservoir for all purposes, from the dead storage level to the normal water or
normal pool level surface level. Does not include surcharge, or dead storage, but does include inactive storage, active conservation
storage and exclusive flood control storage.LIVVLifted Index Vertical VelocityMadden-Julian Oscillation(abbrev. MJO)- Tropical rainfall exhibits strong variability on time scales shorter than the seasonal El Niño-Southern Oscillation (ENSO). These fluctuations in tropical rainfall often go through an entire cycle in 30-60 days, and are referred to as the Madden-Julian Oscillation or intraseasonal oscillations. The intraseasonal oscillations are a naturally occurring component of our coupled ocean-atmosphere system. They significantly affect the atmospheric circulation throughout the global Tropics and subtropics, and also strongly affect the wintertime jet stream and atmospheric circulation features over the North Pacific and western North America. As a result, they have an important impact on storminess and temperatures over the United States. During the summer these oscillations have a modulating effect on hurricane activity in both the Pacific and Atlantic basins.Marginal Visual Flight Rules(Abbrev. MVFR) - In an aviation product, refers to the general weather conditions pilots can expect at the surface. VFR stands for Visual Flight Rules and MVFR means Minimum or Marginal Visual Flight Rules. MVFR criteria means a ceiling between 1,000 and 3,000 feet and/or 3 to 5 miles visibility.Measured CeilingA ceiling classification applied when the ceiling value has been determined by an instrument, such as a ceilometer or ceiling light, or by the known heights of unobscured portions of objects, other than natural landmarks, near the runway. See variable ceiling. MesoclimateThe climate of a small area of the earth's surface which may differ from the general climate of the district.MicroclimateThe climate of a small area such as a cave, house, city or valley that may be different from that in the general region.Mid-level CoolingLocal cooling of the air in middle levels of the atmosphere (roughly 8 to 25 thousand feet), which can lead to destabilization of the entire atmosphere if all other factors are equal.MillibarA unit of atmospheric pressure equal to 1/1000 bar, or 1000 dynes per square centimeter.MLLIMean Layer Lifted Index - Lifted Index (LI) calculated using a parcel consisting of Mean Layer values of temperature and moisture from the lowest 100 mb above ground level. See Lifted Index (LI).Monthly Climatological ReportThis climatological product is issued once a month by each
National Weather Service office. It is a mix of tabular and narrative information. It is organized so
that similar items are grouped together (i.e., temperature, precipitation, wind, heating/cooling
degree information, etc.). Mud SlideFast moving soil, rocks and water that flow down mountain slopes and canyons during a heavy downpour of rain.MULIMost Unstable Lifted Index - Lifted Index (LI) calculated using a parcel from the pressure level that results in the Most Unstable value (lowest value) of LI possible.National Ambient Air Quality StandardsIn the United States, national standards for the ambient concentrations in air of different air pollutants designed to protect human health and welfare.National Climatic Data CenterThe agency that archives climatic data from the National Oceanic and Atmospheric Administration, as well as other climatological organizations.Nautical TwilightThe time after civil twilight, when the brighter stars used for celestial navigation have appeared and the horizon may still be seen. It ends when the center of the sun is 12 degrees below the horizon, and it is too difficult to perceive the horizon, preventing accurate sighting of stars.Neutral LineThe line that separates longitudinal magnetic fields of opposite polarity.Neutral StabilityAn 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.Non-Uniform VisibilityA localized visibility which varies from that reported in the body of the report.Northern LightsCommon name for Aurora Borealis; 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 colours. Orographic LiftingSame as Upslope Flow; occurs when air is forced to rise and cool due to terrain features such as hills or mountains. If the cooling is sufficient, water vapor condenses into clouds. Additional cooling results in rain or snow. It can cause extensive cloudiness and increased amounts of precipitation in higher terrain.Orographic UpliftSame as Orographic Lifting; occurs when air is forced to rise and cool due to terrain features such as hills or mountains. If the cooling is sufficient, water vapor condenses into clouds. Additional cooling results in rain or snow. It can cause extensive cloudiness and increased amounts of precipitation in higher terrain. ParhelionThe scientific name for sun dogs. Either of two colored luminous spots that appear at roughly 22 degrees on both sides of the sun at the same elevation. They are caused by the refraction of sunlight passing through ice crystals. They are most commonly seen during winter in the middle latitudes and are exclusively associated with cirriform clouds. They are also known as mock suns. Partial Beam FillingA 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.PerihelionThe point on the annual orbit of a body (about the sun) that is closest to the sun; at present, the earth reaches this point on about 5 January. Opposite of aphelion.PermeabilityIn hydrologic terms, the ability of a material to transmit fluid through its pores when subjected to a difference in head.Permeability CoefficientIn 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.Plume BlightVisibility impairment caused by air pollution plumes aggregated from individual sources.Polar Orbiting SatelliteA 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.Positive Cloud to Ground LightningA CG flash that delivers positive charge to the ground, as opposed to the more common negative charge. Positive CGs have been found to occur more frequently in some severe thunderstorms. Their occurrence is detectable by most lightning detection networks, but visually it is not considered possible to distinguish between a positive CG and a negative CG. (Some claim to have observed a relationship between staccato lightning and positive CGs, but this relationship is as yet unproven.)Pre-Frontal Squall LineA line of thunderstorms that precedes an advancing cold front.Pre-Hurricane Squall LineIt is often the first serious indication that a hurricane is approaching. It is a generally a straight line and resembles a squall-line that occurs with a mid-latitude cold front. It is as much as 50 miles or even more before the first ragged rain echoes of the hurricane's bands and is usually about 100 to 200 miles ahead of the eye, but it has been observed to be as much as 500 miles ahead of the eye in the largest hurricanes.Preliminary ReportNow known as the "Tropical Cyclone Report". A report summarizing the life history
and effects of an Atlantic or eastern Pacific tropical cyclone. It contains a summary of
the cyclone life cycle and pertinent meteorological data, including the post-analysis
best track (six-hourly positions and intensities) and other meteorological statistics. It
also contains a description of damage and casualties the system produced, as well
as information on forecasts and warnings associated with the cyclone. NHC writes
a report on every tropical cyclone in its area of responsibility.Pressure Falling RapidlyA decrease in station pressure at a rate of 0.06 inch of mercury or more
per hour which totals 0.02 inch or more.Pressure-driven ChannelingChanneling of wind in a valley by synoptic-scale pressure gradients superimposed along the valley's axis. Compare forced channeling.Prevailing VisibilityThe 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.Prevailing WesterliesThe westerly winds that dominant in middle latitudes.Prevailing WindsA wind that consistently blows from one direction more than from any other.Primary Ambient Air Quality StandardsAir quality standards designed to protect human health.ProbabilityA chance, or likelihood, that a certain event might happen.Probability ForecastA 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 ThunderstormsThe probability based on climatology that a thunderstorm will be reported at that location during a specified period of time.Probability of Tropical Cyclone ConditioThe 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 StatementA 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 OutlookThese 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.Quality of SnowThe amount of ice in a snow sample expressed as a percent of the weight of the sample.Radiational CoolingThe cooling of the Earth's surface. At night, the Earth suffers a net heat loss to space due to terrestrial cooling. This is more pronounced when you have a clear sky.Range NormalizationA receiver gain function in the radar which compensates for the effect of
range (distance) on the received power for an equivalent reflectivity.Ribbon LightningAppears 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.Salinity (SAL)In oceanography, conductivity is measured and converted to salinity by a known functional relationship between the measured electrical conductivity of seawater temperature and pressure.Sampling FrequencyThe rate at which sensor data is read or sampled.Satellite Hydrology ProgramA NOHRSC program that uses satellite data to generate areal extent of snow cover data over large areas of the western United States.Secondary Ambient Air Quality StandardsAir quality standards designed to protect human welfare, including the effects on vegetation and fauna, visibility and structures.Sector VisibilityThe visibility in a specific direction that represents at least a 45º arc of a horizontal circle.Severe Weather ProbabilityThis 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.Short-Term Prediction Research and Transition Center - Land Information System (SPoRT-LIS)Provides high-resolution (~3 km) gridded soil moisture products in real-time to support regional and local modeling and improve situational awareness.Slight ChanceIn probability of precipitation statements, usually equivalent to a 20 percent chance.Slight Risk(of severe thunderstorms)- Severe thunderstorms are expected to affect between 2 and 5
percent of the area. A slight risk generally implies that severe weather events are expected to be isolated.Sling PsychrometerAn instrument used to measure the water vapor content of the atmosphere in which wet and dry bulb thermometers are mounted on a frame connected to a handle at one end by means of a bearing or a length of chain. The psychrometer is whirled by hand to provide the necessary ventilation to evaporate water from the wet bulb.Sphere CalibrationReflectivity calibration of a radar by pointing the dish at a metal sphere of (theoretically) known reflectivity. The sphere is often tethered to a balloon.Split FlowA flow pattern high in the atmosphere characterized by diverging winds. Storms moving along in this type of flow pattern usually weaken.Splitting StormA thunderstorm which splits into two storms which follow diverging paths (a left mover and a right mover). The left mover typically moves faster than the original storm, the right mover, slower. Of the two, the left mover is most likely to weaken and dissipate (but on rare occasions can become a very severe anticyclonic-rotating storm), while the right mover is the one most likely to reach supercell status.Squall LineA line of active thunderstorms, either continuous or with breaks, including contiguous precipitation areas resulting from the existence of the thunderstorms.StabilityThe degree of resistance of a layer of air to vertical motion.Stability IndexThe 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. Staccato LightningA Cloud to Ground (CG) lightning discharge which appears as a single very bright, short-duration stroke, often with considerable branching.Stilling basinIn 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.Straight-Line HodographThe name pretty well describes what it looks like on the hodograph. What causes this shape is a steady increase of winds with height (vertical wind shear). This shape of hodograph favors multicell thunderstorms.Straight-line WindsGenerally, any wind that is not associated with rotation, used mainly to differentiate them from tornadic winds.Stream lineArrows on a weather chart showing wind speed and direction. The head of the arrow points toward where the wind is blowing and the length of the arrow is proportional to the wind speed. Sometimes shows wind direction and trajectory only. SublimationThe 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 iceThe transition of water from solid to gas without passing through the liquid phase.Supercooled Liquid WaterIn the atmosphere, liquid water can survive at temperatures colder than 0 degrees Celsius; many vigorous storms contain large amounts of supercooled liquid water at cold temperatures. Important in the formation of graupel and hail.Tail-End CharlieSlang for the thunderstorm at the southernmost end of a squall line or other line or band of thunderstorms. Since low-level southerly inflow of warm, moist air into this storm is relatively unimpeded, such a storm often has a higher probability of strengthening to severe levels than the other storms in the line.TheodoliteAn instrument used in surveying to measure horizontal and vertical angles with a small telescope that can move in the horizontal and vertical planes. It used to track the movements of either a ceiling balloon or a radiosonde.ThermoclineAs one descends from the surface of the ocean, the temperature remains nearly the same as it was at the surface, but at a certain depth temperature starts decreasing rapidly with depth. This boundary is called the thermocline. In studying the tropical Pacific Ocean, the depth of 20ºC water ("the 20ºC isotherm") is often used as a proxy for the depth of the thermocline. Along the equator, the 20ºC isotherm is typically located at about 50 m depth in the eastern Pacific, sloping downwards to about 150 m in the western Pacific.Thin Line EchoA narrow, elongated, non-precipitating echo. It is usually associated with
thunderstorm outflows, fronts, or other density discontinuities. It is also known as a Fine Line.Tidal PilingOccurs when unusually high water levels occur as the result of an accumulation of successive incoming tides that do not completely drain due to
opposing strong winds and/or waves.TwilightThe average time of civil twilight, which is the time between civil dawn and sunrise in the morning, and between sunset and civil dusk in the evening.UpwellingIn ocean dynamics, the upward motion of sub-surface water toward the surface of the ocean. This is often a source of cold, nutrient-rich water. Strong upwelling occurs along the equator where easterly winds are present. Upwelling also can occur along coastlines, and is important to fisheries and birds in California and Peru.Valid TimeThe period of time during which a forecast or warning, until it is updated or superseded by a new forecast issuance, is in effect. Valid Time Event Code(VTEC) - The Valid Time Event Code (VTEC) always is used in conjunction with, and provides supplementary information to, the Universal Geographic Code (UGC), to further aid in the automated delivery of National Weather Service text products to users. The VTEC is included in many event driven or non-routine products and in some routine Marine forecasts. The VTEC provides information on the event, while the UGC describes the affected geographic area. VisibilityThe distance at which a given standard object can be seen and identified with the unaided eyeVisibility Protection ProgramThe 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 Satellite ImageryThis 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.West African Disturbance LineA 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.WesterliesThe prevailing winds that blow from the west in the mid-latitudes.White Light (WL)Sunlight integrated over the visible portion of the spectrum (4000 - 7000 angstroms) so that all colors are blended to appear white to the eye.White Light FlareIn solar-terrestrial terms, a major flare in which small parts become visible in
white light. Such flares are usually strong X-ray, radio, and
particle emitters.Wind Shift LineA long, but narrow axis across which the winds change direction (usually veer).
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