Acid Precipitation
Precipitation, such as rain, snow or sleet, containing relatively high concentrations of acid-forming chemicals that have been released into the atmosphere and combined with water vapor; harmful to the environment.
Acid Rain
Rain containing relatively high concentrations of acid-forming chemicals that have been released into the atmosphere and combined with water vapor; harmful to the environment.
Active Storage Capacity
In hydrologic terms, the total amount of reservoir capacity normally available for release from a reservoir below the maximum storage level. It is total or reservoir capacity minus inactive storage capacity. More specifically, it is the volume of water between the outlet works and the spillway crest.
Antedecent Precipitation Index
(Abbrev. API) - an index of moisture stored within a drainage basin before a storm.
Arctic Oscillation
(abbrev. AO)- The Arctic Oscillation is a pattern in which atmospheric pressure at polar and middle latitudes fluctuates between negative and positive phases. The negative phase brings higher-than-normal pressure over the polar region and lower-than-normal pressure at about 45 degrees north latitude. The negative phase allows cold air to plunge into the Midwestern United States and western Europe, and storms bring rain to the Mediterranean. The positive phase brings the opposite conditions, steering ocean storms farther north and bringing wetter weather to Alaska, Scotland and Scandinavia and drier conditions to areas such as California, Spain and the Middle East. In recent years research has shown, the Arctic Oscillation has been mostly in its positive phase. Some researchers argue that the North Atlantic Oscillation is in fact part of the AO.
Area-Capacity Curve
In hydrologic terms, a graph showing the relation between the surface area of the water in a reservoir, the corresponding volume, and elevation.
Artificial Control
In hydrologic terms, a weir or other man-made structure which serves as the control for a stream-gaging station.
ASSOCIATED PRINCIPAL USER
A Principal User with dedicated communications to a WSR-88D unit.
Atmospheric Circulation Model
A mathematical model for quantitatively describing, simulating, and analyzing the structure of the circulation in the atmosphere and the underlying causes. Sometimes referred to as Atmospheric General Circulation Models or AGCMs.
CCITT
Consultative Committee for International Telephone and Telegraph
CI
Cirrus clouds- High-level clouds (16,000 feet or higher), composed of ice crystals and appearing in the form of white, delicate filaments or white or mostly white patches or narrow bands. Cirrus clouds typically have a fibrous or hairlike appearance, and often are semi-transparent. Thunderstorm anvils are a form of cirrus cloud, but most cirrus clouds are not associated with thunderstorms.
CIG
Ceiling- The height of the lowest layer of clouds, when the sky is broken or overcast.
CIN
Convective INhibition. A measure of the amount of energy needed in order to initiate convection. Values of CIN typically reflect the strength of the cap. They are obtained on a sounding by computing the area enclosed between the environmental temperature profile and the path of a rising air parcel, over the layer within which the latter is cooler than the former. (This area sometimes is called negative area.) See CAPE.
CIO
Chief Information Officer
Circulation
The flow, or movement, of a fluid (e.g., water or air) in or through a given area or volume.
Cirriform
High altitude ice clouds with a very thin wispy appearance.
Cirrocumulus
A cirriform cloud characterized by thin, white patches, each of which is composed of very small granules or ripples. These clouds are of high altitude (20,000-40,000 ft or 6000 -12,000 m).
Cirrostratus
A cloud of a class characterized by a composition of ice crystals and often by the production of halo phenomena and appearing as a whitish and usually somewhat fibrous veil, often covering the whole sky and sometimes so thin as to be hardly discernible. These clouds are of high altitude (20,000-40,000 ft or 6000 -12,000 m).
Cirrus
(abbrev. CI) High-level clouds (16,000 feet or higher), composed of ice crystals and appearing in the form of white, delicate filaments or white or mostly white patches or narrow bands. Cirrus clouds typically have a fibrous or hairlike appearance, and often are semi-transparent. Thunderstorm anvils are a form of cirrus cloud, but most cirrus clouds are not associated with thunderstorms.
Civil Dawn
The time of morning at which the sun is 6 degrees below the horizon. At this time, there is enough light for objects to be distiguishable and that outdoor activities can commence.
Civil Dusk
The time at which the sun is 6 degrees below the horizon in the evening. At this time objects are distinguishable but there is no longer enough light to perform any outdoor activities.
Civil Emergency Message
(Abbrev. CEM) - A message issued by the National Weather Service in coordination with Federal, state or local government to warn the general public of a non-weather related time-critical emergency which threatens life or property, e.g. nuclear accident, toxic chemical spill, etc.
Collection Efficiency
The fraction of droplets approaching a surface that actually deposit on that surface.
Cyclonic Circulation
Circulation (or rotation) which is in the same sense as the Earth's rotation, i.e., counterclockwise (in the Northern Hemisphere) as would be seen from above. Nearly all mesocyclones and strong or violent tornadoes exhibit cyclonic rotation, but some smaller vortices, such as gustnadoes, occasionally rotate anticyclonically (clockwise). Compare with anticyclonic rotation.
Eccentricity
A dimensionless quantity describing the elliptical shape of a planet's orbit.
Effective Precipitation
1) That part of the precipitation that produces runoff.

2) A weighted average of current and antecedent precipitation that is "effective" in correlating with runoff.

3) That part of the precipitation falling on an irrigated area that is effective in meeting the consumptive use requirements.
Energy Helicity Index
An index that incorporates vertical shear and instability, designed for the purpose of forecasting supercell thunderstorms
Exclusive Flood Control Storage Capacity
In hydrologic terms, the space in a reservoir reserved for the sole purpose of regulating flood inflows to abate flood damage
Field (Moisture) Capacity
The amount of water held in soil against the pull of gravity
Field Moisture Deficiency
The quantity of water, which would be required to restore the soil moisture to field moisture capacity.
Flash Multiplicity
The number of return strokes in a lightning flash.
Float Recording Precipitation gage
In hydrologic terms, a rain gage where the rise of a float within the instrument with increasing rainfall is recorded. Some of these gages must be emptied manually, while others employ a self-starting siphon to empty old rainfall amounts.
General Circulation
The totality of large-scale organized motion for the entire global atmosphere.
General Circulation Models
(GCMs) - These computer simulations reproduce the Earth's weather patterns and can be used to predict change in the weather and climate.
Glaciation
The transformation of cloud particles from water drops to ice crystals. Thus, a cumulonimbus cloud is said to have a "glaciated" upper portion.
Glacier
In hydrologic terms, bodies of land ice that consist of recrystallized snow accumulated on the surface of the ground, and that move slowly downslope.
Glacier Dammed Lake
In hydrologic terms, the lake formed when a glacier flows across the mouth of an adjoining valley and forms an ice dam.
Glacier Wind
A shallow downslope wind above the surface of a glacier, caused by the temperature difference between the air in contact with the glacier and the free air at the same altitude. The glacier wind does not reverse diurnally like slope and along-valley wind systems.
Great Circle Track
A great-circle track is the shortest distance between two points on a sphere, and when viewed on a 2-dimensional map the track will appear curved. Swell waves travel along routes that mark out great circles.
Group Velocity
The speed at which a particular wave front or swell train advances.
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.
Hourly Precipitation Data (HPD)
It contains data on nearly 3,000 hourly precipitation stations (National Weather Service, Federal Aviation Administration, and cooperative observer stations) in inches to tenths or inches to hundredths at local standard time. HPD includes maximum precipitation for nine (9) time periods from 15 minutes to 24 hours, for selected stations.
Icing
A coating of ice on a solid object.
Inactive Storage Capacity
In hydrologic terms, the portion of capacity below which the reservoir is not normally drawn, and which is provided for sedimentation, recreation, fish and wildlife, aesthetic reasons, or for the creation of a minimum controlled operational or power head in compliance with operating agreements or restrictions.
Infiltration Capacity
In hydrologic terms, the maximum rate at which water can enter the soil at a particular point under a given set of conditions.
Intraseasonal Oscillation
Oscillation with variability on a timescale less than a season. One example is the Madden-Julian Oscillation.
Jet Stream Cirrus
A loose term for filamentous cirrus that appears to radiate from a point in the sky, and exhibits characteristics associated with strong vertical wind shear, such as twisted or curved filaments.
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.
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.
MARC Velocity Signature
A Doppler radar-velocity based precursor towards forecasting the initial onset of damaging straight-line winds in a linear Quasi_Linear Convective System (QLCS) or bowing convective system.
Maximum Unambiguous Velocity
The highest radial velocity that can be measured unambiguously by a pulsed Doppler radar. The maximum unambiguous velocity is related to the radar's successive pulses of emitted energy. When a target's velocity exceeds the maximum unambiguous velocity, the velocity will be "folded" to appear as a different velocity.
Mean Areal Precipitation
(abbrev. MAP)- The average rainfall over a given area, generally expressed as an average depth over the area.
Mean Doppler Velocity
Reflectivity-weighted average velocity of targets in a given pulse resolution volume. Usually determined from a large number of successive radar pulses. Also called mean radial (towards or away from the antenna) velocity. Doppler velocity refers to spectral density first moment, radial velocity to base data.
Mixed Precipitation
Any of the following combinations of freezing and frozen precipitation: snow and sleet, snow and freezing rain, or sleet alone. Rain may also be present.
Municipal Use of Water
In hydrologic terms, the various uses to which water is put to use developed urban areas, including domestic use, industrial use, street sprinkling, fire protection, etc.
Negative Vorticity Advection
(Abbrev. NVA) - the advection of lower values of vorticity into an area.
North Atlantic Oscillation
(Abbrev. NAO) - the NAO is a large-scale fluctuation in atmospheric pressure between the subtropical high pressure system located near the Azores in the Atlantic Ocean and the sub-polar low pressure system near Iceland and is quantified in the NAO Index. The surface pressure drives surface winds and wintertime storms from west to east across the North Atlantic affecting climate from New England to western Europe as far eastward as central Siberia and eastern Mediterranean and southward to West Africa.
North Pacific High
A semi-permanent, subtropical area of high pressure in the North Pacific Ocean. It is strongest in the Northern Hemispheric summer and is displaced towards the equator during the winter when the Aleutian Low becomes more dominate. Comparable systems are the Azores High and the Bermuda High.
Orographic Precipitation
Precipitation which is caused by hills or mountain ranges deflecting the moisture-laden air masses upward, causing them to cool and precipitate their moisture.
Oscillation
A shift in position of various high and low pressure systems that in climate terms is usually defined as an index (i.e., a single numerically-derived number, that represents the distribution of temperature and pressure over a wide ocean area, such as the El Niño-Southern Oscillation, North Atlantic Oscillation, and Pacific Decadal Oscillation).
Pacific Decadal Oscillation
(Abbrev. PDO) - a recently described pattern of climate variation similar to ENSO though on a timescale of decades and not seasons. It is characterized by SST anomalies of one sign in the north-central Pacific and SST anomalies of another sign to the north and east near the Aleutians and the Gulf of Alaska. It primarily affects weather patterns and sea surface temperatures in the Pacific Northwest, Alaska, and northern Pacific Islands.
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.
Point Precipitation
Precipitation at a particular site, in contrast to the mean precipitation over an area.
Positive Vorticity Advection
(Abbrev. PVA) - Advection of higher values of vorticity into an area, which often is associated with upward motion (lifting) of the air. PVA typically is found in advance of disturbances aloft (i.e., shortwaves), and is a property which often enhances the potential for thunderstorm development.
Potential Vorticity
This plays an important role in the generation of vorticity in cyclogenesis, especially along the polar front. It is also very useful in tracing intrusions of stratospheric air deep into the troposphere in the vicinity of jet streaks.
Precipitable Water
Measure of the depth of liquid water at the surface that would result after precipitating all of the water vapor in a vertical column over a given location, usually extending from the surface to 300 mb.
Precipitation
The process where water vapor condenses in the atmosphere to form water droplets that fall to the Earth as rain, sleet, snow, hail, etc.
Precipitation Attenuation
The loss of energy that radar beam experiences as it passes through an area of precipitation.
Precipitation Mode
The standard, or default, operational mode of the WSR-88D. The radar automatically switches into precipitation mode from clear-air mode if the measured reflectivity exceeds a specific threshold value. The precipitation mode of NEXRAD is more sensitive than previous weather radars. The minimum detectable reflectivity in NEXRAD's precipitation mode is 5 dBZ, compared to 28 dBZ with the old WSR-57.
Precipitation Processing System
The WSR-88D system that generates 1-hour running, 3-hourly, and running storm total precipitation accumulations. Five functional steps are performed to calculate the best estimate of precipitation: 1) development of a sectorized hybrid scan, 2) conversion to precipitation rate, 3) precipitation accumulation, 4) adjustment using rain gages, 5) product update.
Precision
The accuracy with which a number can be represented, i.e., the number of digits used to represent a number.
Probability of Precipitation
(Abbrev. PoP)- The probability that precipitation will be reported at a certain location during a specified period of time.
Quantitative Precipitation Estimate (QPE)
A spatial and temporal analysis estimating the amount of precipitation that has occurred using a variety of techniques including observational and remote sensing data.
Quantitative Precipitation Forecast
A spatial and temporal precipitation forecast that will predict the potential amount of future precipitation for a specified region, or area.
Radial Velocity
Component of motion toward or away from a given location. As "seen" by Doppler radar, it is the component of motion parallel to the radar beam. (The component of motion perpendicular to the beam cannot be seen by the radar. Therefore, strong winds blowing strictly from left to right or from right to left, relative to the radar, can not be detected.)
Relative Vorticity
The sum of the rotation of an air parcel about the axis of the pressure system and the rotation of the parcel about its own axis.
Sediment Storage Capacity
In hydrologic terms, the volume of a reservoir planned for the deposition of sediment.
Severe Icing
The rate of ice accumulation on an aircraft is such that de-icing/anti-icing equipment fails to reduce or control the hazard. Immediate diversion is necessary.
Small Craft Should Exercise Caution
Precautionary statement issued to alert mariners with small, weather sensitive boats.
Southern Oscillation
(SO) - a "see-saw" in surface pressure in the tropical Pacific characterized by simultaneously opposite sea level pressure anomalies at Tahiti, in the eastern tropical Pacific and Darwin, on the northwest coast of Australia. The SO was discovered by Sir Gilbert Walker in the early 1920's. Walker was among the first meteorologists to use the statistical techniques to analyze and predict meteorological phenomena. Later, the three-dimensional east-west circulation related to the SO was discovered and named the "Walker Circulation". The SO oscillates with a period of 2-5 years. During one phase, when the sea level pressure is low at Tahiti and High at Darwin, the El Nino occurs. The cold phase of the SO, called "La Nina" by some, is characterized by high pressure in the eastern equatorial Pacific, low in the west, and by anomalously cold sea surface temperature (SST) in the central and eastern Pacific. This is called El Nino Southern Oscillation or ENSO.
Southern Oscillation Index
A numerical index measuring the state of the Southern Oscillation. The SOI is based on the (atmospheric) pressure difference between Tahiti and Darwin, Australia. It is highly correlated with tropical sea surface temperature anomaly indices recorded in Niño3.
Special Avalanche Warning
Issued by the National Weather Service when avalanches are imminent or occurring in the mountains. It is usually issued for a 24 hour period.
Special Fire Weather
Meteorological services uniquely required by user agencies which cannot be provided at an NWS office during normal working hours. Examples are on-site support, weather observer training, and participation in user agency training activities.
Special Marine Warning
(SMW) A warning product issued for potentially hazardous weather conditions usually of short duration (up to 2 hours) producing sustained marine thunderstorm winds or associated gusts of 34 knots or greater; and/or hail 3/4 inch or more in diameter; and/or waterspouts affecting areas included in a Coastal Waters Forecast, a Nearshore Marine Forecast, or an Great Lakes Open Lakes Forecast that is not adequately covered by existing marine warnings. Also used for short duration mesoscale events such as a strong cold front, gravity wave, squall line, etc., lasting less than 2 hours and producing winds or gusts of 34 knots or greater.
Special Tropical Disturbance Statement
This statement issued by the National Hurricane Center furnishes information on strong and formative non-depression systems. This statement focuses on the major threat(s) of the disturbance, such as the potential for torrential rainfall on an island or inland area. The statement is coordinated with the appropriate forecast office(s).
Specific Gravity
The ratio of the density of any substance to the density of water.
Specific Humidity
In a system of moist air, the ratio of the mass of water vapor to the total mass of the system.
Specific Yield
In hydrologic terms, the ratio of the water which will drain freely from the material to the total volume of the aquifer formation. This value will always be less than the porosity.
Storm Relative Mean Radial Velocity Map
(SRM): This WSR-88D radar product depicts a full 360º sweep of radial velocity data with the average motion of all identified storms subtracted out. It is available for every elevation angle sampled. It is used to aid in displaying shear and rotation in storms and storm top divergence that might otherwise be obscured by the storm's motion, investigate the 3-D velocity structure of a storm, and help with determining rotational features in fast and uniform moving storms.
Storm Relative Mean Radial Velocity Regi
(SRR): This WSR-88D radar product depicts a 27 nm by 27 nm region of storm relative mean radial velocity centered on a point which the operator can specify anywhere within a 124 nm radius of the radar. The storm motion subtracted defaults to the motion of the storm closest to the product center, or can be input by the operator. It is used to examine the 3-dimensional storm relative flow of a specific thunderstorm (radar operator centers product on a specific thunderstorm; aid in displaying shear and rotation in thunderstorms and storm top divergence that might otherwise be obscured by storm motion; and gain higher resolution velocity product
Storm Total Precipitation
This radar image is an estimate of accumulated rainfall since the last time there was a one-hour, or more, break in precipitation. It is used to locate flood potential over urban or rural areas, estimate total basin runoff and provide rainfall accumulations for the duration of the event and is available only for the short range (out to 124 nm). To determine accumulated precipitation at greater distances you should link to an adjacent radar.
Summation Principle
This principle states that the sky cover at any level is equal to the summation of the sky cover of the lowest layer plus the additional sky cover provided at all successively higher layers up to and including the layer in question.
Surcharge Capacity
In hydrologic terms, the volume of a reservoir between the maximum water surface elevation for which the dam is designed and the crest of an uncontrolled spillway, or the normal full-pool elevation of the reservoir with the crest gates in the normal closed position.
Thermally Driven Circulation
A diurnally reversing closed cellular wind current resulting from horizontal temperature contrasts caused by different rates of heating or cooling over adjacent surfaces; includes along-slope, cross-valley, along-valley, mountain-plain and sea breeze circulations.
Variable Infiltration Capacity (VIC)
A large-scale, semi-distributed hydrologic model that solves full water and energy balances. As such, it shares several basic features with other land surface models that are commonly coupled to global circulation models.
Velocity Azimuth Display
A WSR 88-D product which shows the radar derived wind speeds at various heights. This radar product shows the wind speeds from 2,000 to 55,000 feet above the ground. VAD and EVAD (Extended VAD) are methods of guessing the large scale two-dimensional winds from one-dimensional radial velocity data. They are essentially multivariate regressions which fit a simple, large scale wind model to the observed winds. EVAD also estimates the large scale horizontal divergence and particle fall speed. See VWP.
Velocity Cross Section
This WSR-88D radar product displays a vertical cross section of velocity on a grid with heights up to 70,000 feet on the vertical axis and distance up to 124 nm on the horizontal axis. The two end points to create cross section are radar operator selected along a radial or from one AZRAN to another AZRAN within 124 nm of the radar that are less than 124 nm apart.
It is used to:
1) Examine storm structure features such as location of updrafts/downdrafts, strength of storm top divergence, and the depth of mesocyclones;
2) Locate areas of convergence/divergence (when generated along a radial; and
3) Analyze areas of rotation (when generated from one AZRAN to another).
Velocity Zones
In hydrologic terms, areas within the floodplain subject to potential high damage from waves. These sometimes appear on flood insurance rate maps
Vertical Velocity
The component of velocity (motion) in the vertical. The evaluation of areas of upward vertical velocity is key to forecasting areas of active weather.
Volume Velocity Processing
A way to guess the large-scale 2-dimensional winds, divergence and fall speeds from one-dimensional radial velocity data. Essentially a multivariate regression which fits a simple wind model to the observed radial velocities. Very similar to VAD and EVAD, except it uses different functions for the fit.
Vorticity
A measure of the rotation of air in a horizontal plane. Positive (counter-clockwise or cyclonic) vorticity can be correlated with surface low development and upward vertical motion (in areas of positive vorticity advection).
Weighing-Type Precipitation Gage
A rain gage that weighs the rain or snow which falls into a bucket set on a platform of a spring or lever balance. The increasing weight of its contents plus the bucket are recorded on a chart. The record thus shows the accumulation of precipitation.
XCITED
Excited

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