Adirondack Type Snow Sampling Set
In 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 Model
Mathematical or conceptual model used to estimate present or future air quality.
Alberta Clipper
A fast moving low pressure system that moves southeast out of Canadian Province of Alberta (southwest Canada) through the Plains, Midwest, and Great Lakes region usually during the winter. This low pressure area is usually accompanied by light snow, strong winds, and colder temperatures. Another variation of the same system is called a "Saskatchewan Screamer".
ALIASING
The process by which frequencies too high to be analyzed with the given sampling interval appear at a frequency less than the Nyquist frequency.
Amplifier
A device used to increase the strength of an analog signal
Amplitude
The 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.
Anthelion
A luminous white spot that appears on the parhelic circle at the same altitude as the sun and 180 degrees from it in azimuth.
Aphelion
The 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 Australis
Same 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 Borealis
Also known as the northern lights; the luminous, radiant emission from the upper atmosphere over middle and high latitudes, and centred around the earth's magnetic poles. These silent fireworks are often seen on clear winter nights in a variety of shapes and colors.
Baroclinic leaf shield
A cloud pattern on satellite images - frequently noted in advance of formation of a low pressure center.
Baroclinic Zone
A region in which a temperature gradient exists on a constant pressure surface. Baroclinic zones are favored areas for strengthening and weakening systems; barotropic systems, on the other hand, do not exhibit significant changes in intensity. Also, wind shear is characteristic of a baroclinic zone.
Baroclinity
A measure of the state of stratification in a fluid in which surfaces of constant pressure (isobaric) intersect surfaces of constant density (isosteric).
BEAM FILLING
The measure of variation of hydrometeor density throughout the radar sampling volume. If there is no variation in density, the beam is considered to be filled.
Blizzard
(abbrev. BLZD)- A blizzard means that the following conditions are expected to prevail for a period of 3 hours or longer:
Blizzard Warning
Issued for winter storms with sustained or frequent winds of 35 mph or higher with considerable falling and/or blowing snow that frequently reduces visibility to 1/4 of a mile or less. These conditions are expected to prevail for a minimum of 3 hours.
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.
Calibration
In hydrologic terms, the process of using historical data to estimate parameters in a hydrologic forecast technique such as SACSMA, routings, and unit hydrographs.
Catalina Eddy
A 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.
Channelization
In hydrologic terms, the modification of a natural river channel; may include deepening, widening, or straightening.
Client Agency
As 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 services
Climate
The composite or generally prevailing weather conditions of a region, throughout the year, averaged over a series of years.
Climate Change
A 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 Model
Mathematical model for quantitatively describing, simulating, and analyzing the interactions between the atmosphere and underlying surface (e.g., ocean, land, and ice).
Climate Outlook
A 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 Center
This 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 System
The 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 Outlook
An 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.
Climatology
The science that deals with the phenomena of climates or climatic conditions.
CLIMO
Climatology/Climatological
Climometer
An instrument that measures angles of inclination; used to measure cloud ceiling heights.
Cloud Ceiling
Same as Ceiling; the height of the cloud base for the lowest broken or overcast cloud layer.
Consolidated Ice Cover
In 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 Force
A 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.
Cumuliform
Descriptive of all clouds with vertical development in the form of rising mounds, domes, or towers
Cumuliform Anvil
A 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 Storm
A 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 Report
As 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.).
Declination
The 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 Streamline
In 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 Height
The height above ground of the dividing streamline, as measured far upwind of a mountain barrier. See dividing streamline.
Downwelling Radiation
The 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 Line
A boundary separating moist and dry air masses, and an important factor in severe weather frequency in the Great Plains. It typically lies north-south across the central and southern high Plains states during the spring and early summer, where it separates moist air from the Gulf of Mexico (to the east) and dry desert air from the southwestern states (to the west). 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 Bulge
A bulge in the dry line, representing the area where dry air is advancing most strongly at lower levels. Severe weather potential is increased near and ahead of a dry line bulge.
Dry Line Storm
Any thunderstorm that develops on or near a dry line.
Dynamic Lifting
The forced uplifting of air from various atmospheric processes, such as weather fronts, and cyclones.
Easterlies
Any winds with components from the east.
Energy Helicity Index
An index that incorporates vertical shear and instability, designed for the purpose of forecasting supercell thunderstorms
Equi-Potential Line
In 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 Drawdown
In hydrologic terms, the ultimate, constant drawdown for a steady rate of pumped discharge.
Equilibrium Level
(EL) - On a sounding, the level above the level of free convection (LFC) at which the temperature of a rising air parcel again equals the temperature of the environment. The height of the EL is the height at which thunderstorm updrafts no longer accelerate upward. Thus, to a close approximation, it represents the height of expected (or ongoing) thunderstorm tops.
Equilibrium Surface Discharge
In hydrologic terms, the steady rate of surface discharge which results from a long-continued, steady rate of net rainfall, with discharge rate equal to net rainfall rate
Equilibrium Time
In hydrologic terms, the time when flow conditions become substantially equal to those corresponding to equilibrium discharge or equilibrium drawdown.
Eruptive Prominence on Limb (EPL)
In solar-terrestrial terms, a solar prominence that becomes activa- ted and is seen to ascend from the sun.
Fall Line
A skiing term, indicating the line of steepest descent of a slope.
Filling
The opposite of deepening. A general increase in the central pressure of a low pressure system.
Firn Line
In 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 area
Flanking Line
A 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 storm
Flash Multiplicity
The number of return strokes in a lightning flash.
Flow Splitting
The 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 Channeling
Channeling of upper winds along a valley's axis when upper winds are diverted by the underlying topography. Compare pressure-driven channeling.
Forecast valid for
The 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.
Funnelling
The process whereby wind is forced to flow through a narrow opening between adjacent land areas, resulting in increased wind speed.
Geostationary Satellite
A 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 Line
The green line is one of the strongest (and first-recognized) visible coronal lines. It identifies moderate temperature regions of the CORONA.
Ground Blizzard Warning
When blizzard conditions are solely caused by blowing and drifting snow.
Hague Line
The North Atlantic boundary between the U.S. and Canada fishing waters as determined by the World Court in The Hague, Netherlands.
Heat Lightning
Lightning that occurs at a distance such that thunder is no longer audible.
Helicity
A 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 Probability
The probability (%) that a stream will reach its high water flow within the forecast period.
Hydraulic Fill Dam
In hydrologic terms, a dam constructed of materials, often dredged, that are conveyed and placed by suspension in flowing water
Hydraulic Flow
Atmospheric flow that is similar in character to the flow of water over an obstacle.
Hydraulic Grade Line
In 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 Head
In hydrologic terms,
(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 Jump
A 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 Permeability
In hydrologic terms, the flow of water through a unit cross-sectional area of soil normal to the direction of flow when the hydraulic gradient is unity.
In-Cloud Lightning
(abbrev. IC) Lightning that takes place within the cloud.
Infrared Satellite Imagery
This 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 Rules
Refers 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 Line
The 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 Lift
Lifting 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.
LI
Lifted 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.
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.
Lid
A layer of warm air several thousand feet above the earth's surface which suppresses or delays the development of thunderstorms.
LIFR
Low Instrument Flight Rules
Lifted 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 Bridge
In solar-terrestrial terms, it is observed in white light, a bright tongue or streaks penetra- ting or crossing sunspot umbrae. The appearance of a light bridge is frequently a sign of impending region division or dissolution
Lightning
(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 Channel
The 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 Discharge
The 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 Stroke
Any 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.
Limb
In solar-terrestrial terms, the edge of the solar disk.
Limb Flare
In solar-terrestrial terms, a solar flare seen at the edge (Limb) of the sun.
Limnology
In hydrologic terms, the branch of hydrology that pertains to the study of lakes
Line 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 Source
An 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 Equivalent
Same 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.
Lithometeor
Atmospheric 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.
Lithosphere
In 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 Zone
In hydrologic terms, the area on, or near the shore of a body water
Live Capacity
In 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.
LIVV
Lifted Index Vertical Velocity
Madden-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 Ceiling
A 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.
Mesoclimate
The climate of a small area of the earth's surface which may differ from the general climate of the district.
Microclimate
The 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 Cooling
Local 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.
Millibar
A unit of atmospheric pressure equal to 1/1000 bar, or 1000 dynes per square centimeter.
MLLI
Mean 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 Report
This 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 Slide
Fast moving soil, rocks and water that flow down mountain slopes and canyons during a heavy downpour of rain.
MULI
Most 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 Standards
In the United States, national standards for the ambient concentrations in air of different air pollutants designed to protect human health and welfare.
National Climatic Data Center
The agency that archives climatic data from the National Oceanic and Atmospheric Administration, as well as other climatological organizations.
Nautical Twilight
The 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 Line
The line that separates longitudinal magnetic fields of opposite polarity.
Neutral Stability
An atmospheric condition that exists in unsaturated air when the environmental lapse rate equals the dry adiabatic rate, or in saturated air when the environmental lapse rate equals the moist adiabatic rate.
Non-Uniform Visibility
A localized visibility which varies from that reported in the body of the report.
Northern Lights
Common 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 Lifting
Same 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 Uplift
Same 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.
Parhelion
The 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 Filling
A limitation of the rainfall estimation techniques used by NEXRAD. At far ranges from the radar, a storm may occupy only a portion of the radar beam (which may be several miles across). However, the radiation received by the radar antenna consists of the average reflectivity across the entire beam, so the reflectivity and associated rainfall rates are underestimated.
Perihelion
The 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.
Permeability
In hydrologic terms, the ability of a material to transmit fluid through its pores when subjected to a difference in head.
Permeability Coefficient
In hydrologic terms, the rate of flow of a fluid through a cross section of a porous mass under a unit hydraulic gradient, at a temperature of 60 degrees Fahrenheit.
Plume Blight
Visibility impairment caused by air pollution plumes aggregated from individual sources.
Polar Orbiting Satellite
A weather satellite which travels over both poles each time it orbits the Earth. It orbits about 530 miles (850 km) above the Earth's surface. A satellite with an orbit nearly parallel to the earth's meridian lines which crosses the polar regions on each orbit.
Positive Cloud to Ground Lightning
A 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 Line
A line of thunderstorms that precedes an advancing cold front.
Pre-Hurricane Squall Line
It 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 Report
Now 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 Rapidly
A 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 Channeling
Channeling of wind in a valley by synoptic-scale pressure gradients superimposed along the valley's axis. Compare forced channeling.
Prevailing Visibility
The visibility that is considered representative of conditions at the station; the greatest distance that can be seen throughout at least half the horizon circle, not necessarily continuous.
Prevailing Westerlies
The westerly winds that dominant in middle latitudes.
Prevailing Winds
A wind that consistently blows from one direction more than from any other.
Primary Ambient Air Quality Standards
Air quality standards designed to protect human health.
Probability
A chance, or likelihood, that a certain event might happen.
Probability Forecast
A forecast of the probability that one or more of a mutually exclusive set of weather conditions will occur.
Probability of Hail
(Abbrev. POH) - a product from the NEXRAD hail detection algorithm that estimates the likelihood that hail is present in a storm.
Probability of Precipitation
(Abbrev. PoP)- The probability that precipitation will be reported at a certain location during a specified period of time.
Probability of Thunderstorms
The probability based on climatology that a thunderstorm will be reported at that location during a specified period of time.
Probability of Tropical Cyclone Conditio
The probability, in percent, that the cyclone center will pass within 50 miles to the right or 75 miles to the left of the listed location within the indicated time period when looking at the coast in the direction of the cyclone's movement.
Public Information Statement
A narrative statement issued by a National Weather Service Forecast Office that can be used for:

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

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

3) Information regarding service changes, service limitations, interruptions due to reduced or lost power or equipment outages, or special information clarifying interpretation of NWS data. For example, this product may be used to inform users of radar equipment outages or special information clarifying interpretation of radar data originating from an unusual source which may be mistaken for precipitation (such as chaff drops, smoke plumes, etc., that produces echoes on the radar display.
Public Severe Weather Outlook
These are issued when the Storm Prediction Center (SPC) in Norman, Oklahoma anticipates an especially significant and/or widespread outbreak of severe weather. This outlook will stress the seriousness of the situation, defines the threat area, and provides information on the timing of the outbreak. The lead time on this outlook is normally less than 36 hours prior to the severe weather event.
Quality of Snow
The amount of ice in a snow sample expressed as a percent of the weight of the sample.
Radiational Cooling
The 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 Normalization
A receiver gain function in the radar which compensates for the effect of range (distance) on the received power for an equivalent reflectivity.
Ribbon Lightning
Appears to be a broad stream of fire. A succession of strokes, each blown a bit to the side of the previous strokes by wind, but striking so fast that all the strokes are seen at once as a ribbon-like flash.
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 Frequency
The rate at which sensor data is read or sampled.
Satellite Hydrology Program
A 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 Standards
Air quality standards designed to protect human welfare, including the effects on vegetation and fauna, visibility and structures.
Sector Visibility
The visibility in a specific direction that represents at least a 45º arc of a horizontal circle.
Severe Weather Probability
This WSR-88D radar product algorithm displays numerical values proportional to the probability that a storm will produce severe weather within 30 minutes. Values determined using a statistical regression equation which analyzes output from the VIL algorithm. It is used to quickly identify the most significant thunderstorms.
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 Chance
In 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 Psychrometer
An 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 Calibration
Reflectivity calibration of a radar by pointing the dish at a metal sphere of (theoretically) known reflectivity. The sphere is often tethered to a balloon.
Split Flow
A flow pattern high in the atmosphere characterized by diverging winds. Storms moving along in this type of flow pattern usually weaken.
Splitting Storm
A 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 Line
A line of active thunderstorms, either continuous or with breaks, including contiguous precipitation areas resulting from the existence of the thunderstorms.
Stability
The degree of resistance of a layer of air to vertical motion.
Stability Index
The overall stability or instability of a sounding is sometimes conveniently expressed in the form of a single numerical value. Used alone, it can be quite misleading, and at times, is apt to be worthless. The greatest value of an index lies in alerting the forecaster to those soundings which should be examined more closely.
Staccato Lightning
A Cloud to Ground (CG) lightning discharge which appears as a single very bright, short-duration stroke, often with considerable branching.
Stilling basin
In hydrologic terms, a basin constructed to dissipate the energy of fast-flowing water (e.g., from a spillway or bottom outlet), and to protect the streambed from erosion.
Straight-Line Hodograph
The 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 Winds
Generally, any wind that is not associated with rotation, used mainly to differentiate them from tornadic winds.
Stream line
Arrows 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.
Sublimation
The transition of a substance from the solid phase directly to the vapor phase, or vice versa, without passing through an intermediate liquid phase. Thus an ice crystal or icicle sublimes under low relative humidity at temperatures below 0°C. The process is analogous to evaporation of a liquid.
Sublimation of ice
The transition of water from solid to gas without passing through the liquid phase.
Supercooled Liquid Water
In 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 Charlie
Slang 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.
Theodolite
An 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.
Thermocline
As 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 Echo
A 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 Piling
Occurs 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.
Twilight
The 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.
Upwelling
In 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 Time
The 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.
Visibility
The distance at which a given standard object can be seen and identified with the unaided eye
Visibility Protection Program
The program specified by the Clean Air Act to achieve a national goal of remedying existing impairments to visibility and preventing future visibility impairment throughout the United States.
Visible Satellite Imagery
This type of satellite imagery uses reflected sunlight (this is actually reflected solar radiation) to see things in the atmosphere and on the Earth's surface. Clouds and fresh snow are excellent reflectors, so they appear white on the imagery. Clouds can be distinguished from snow, because clouds move and snow does not move. Meanwhile, the ground reflects less sunlight, so it appears black on the imagery. The satellite uses its 0.55 to 0.75 micrometer (um) channel to detect this reflected sunlight. Since this imagery relies on reflected imagery, it cannot be used during night.
West African Disturbance Line
A line of convection about 300 miles long, similar to a squall line. It forms over west Africa north of the equator and south of 15 degrees North latitude. It moves faster than an Easterly Wave between 20 and 40 mph. They move off the African coast every 4 to 5 days mainly in the summer. Some reach the American tropics and a few develop into tropical cyclones.
Westerlies
The 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 Flare
In 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 Line
A long, but narrow axis across which the winds change direction (usually veer).

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