Weather Glossary - T thru Z  

 

Weatherwx

Terminology Page

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

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

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*Tail Cloud - A horizontal, tail-shaped cloud (not a funnel cloud) at low levels extending from the precipitation cascade region of a supercell toward the wall cloud (i.e., it usually is observed extending from the wall cloud toward the north or northeast). The base of the tail cloud is about the same as that of the wall cloud. Cloud motion in the tail cloud is away from the precipitation and toward the wall cloud, with rapid upward motion often observed near the junction of the tail and wall clouds. Compare with beaver tail, which is a form of inflow band that normally attaches to the storm's main updraft (not to the wall cloud) and has a base at about the same level as the updraft base (not the wall cloud).

*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.

Teleconnections - Information used by forecasters to determine what the weather might be elsewhere when compared with past weather conditions at the same degree of longitude.

Temperate Climate - Climates with distinct winter and summer seasons, typical of regions found between the Tropics of Cancer and Capricorn and the Arctic and Antarctic Circles. Considered the climate of the middle latitudes.

Temperature - The measure of molecular motion or the degree of heat of a substance. It is measured on an arbitrary scale from absolute zero, where the molecules theoretically stop moving. In general, the degree of hotness or coldness measured against some definite scale by means of a thermometer. In surface observations, it refers primarily to the free air or ambient temperature close to the surface of the earth.

Terminal Doppler Weather Radar (TDWR) - Doppler radar installed at major airports throughout the United States to detect microbursts.

Terrestrial Radiation - Long wave radiation that is emitted by the earth back into the atmosphere. Most of it is absorbed by the water vapor in the atmosphere, while less than ten percent is radiated directly into space.

Texas Norther - A local name in the south-central Great Plains for strong winter winds blowing north or northwest following a sharp cold front with dropping temperatures. Marked by a dark, blue-black sky. Also called a Blue Norther.

Thaw - A warm spell of weather when ice and snow melt. To free something from the binding action of ice by warming it to a temperature above the melting point of ice.

Theodolite - An optical instrument used to track the motion of a pilot balloon, or pibal, by measuring the elevation and azimuth angles.

Themograph Is essentially, a self-recording thermometer. A thermometer that continuously records the temperature on a chart.

Thermal Low - Also known as (Heat Low), it is an area of low pressure due to the high temperatures caused by intensive heating at the surface. It tends to remain stationary over its source area, with weak cyclonic circulation. There are no fronts associated with it. An example is the low that develops over southwestern United States and northwestern Mexico during the summer months.

Thermocline - A vertical negative temperature gradient in some layer of a body of water which is appreciably greater than the gradients above and below it. In the ocean, this may be seasonal, due to the heating of the surface water in the summer, or permanent.

Thermodynamic Chart (or Thermodynamic Diagram) - A chart containing contours of pressure, temperature, moisture, and potential temperature, all drawn relative to each other such that basic thermodynamic laws are satisfied. Such a chart typically is used to plot atmospheric soundings, and to estimate potential changes in temperature, moisture, etc... if air were displaced vertically from a given level. A thermodynamic chart thus is a useful tool in diagnosing atmospheric instability

Thermodynamics - In general is the study of, the relationships between heat and other properties (such as temperature, pressure, density, etc.) In forecast discussions, thermodynamics usually refers to the distribution of temperature and moisture (both vertical and horizontal) as related to the diagnosis of atmospheric instability.

Thermohaline - In oceanography, it pertains to when both temperature and salinity act together. An example is thermohaline circulation which is vertical circulation induced by surface cooling, which causes convective overturning and consequent mixing.

Thermometer - An instrument used for measuring temperature. The different scales used in meteorology are; Celsius, Fahrenheit, and Kelvin or Absolute.

Thermosphere - A thermal classification, it is the layer of the atmosphere located between the mesosphere and outer space. It is a region of steadily increasing temperature with altitude, and includes all of the exosphere and most, if not all, of the ionosphere.

Theta-e (or Equivalent Potential Temperature) - The temperature a parcel of air would have if a) it was lifted until it became saturated, b) all water vapor was condensed out, and c) it was returned adiabatically (i.e., without transfer of heat or mass) to a pressure of 1000 millibars. Theta-e, which typically is expressed in degrees Kelvin, is directly related to the amount of heat present in an air parcel. Thus, it is useful in diagnosing atmospheric instability.

Theta-e Ridge - An axis of relatively high values of theta-e. Severe weather and excessive rainfall often occur near or just upstream from a theta-e ridge.

Thickness - The thickness of a layer in the atmosphere is proportional to the mean temperature of that whole layer. The layer most often used in meteorology is between 1000 and 500 millibars. There can be different temperature profiles in the lowest layer of the atmosphere with the same 1000-500 millibar thickness value, depending on what is happening above that lowest layer. For example, if the lower levels are warming but higher levels are cooling, the overall mean temperature, the thickness, could remain the same. Likewise, on a sunny day, the amount of incoming solar radiation, affects the temperature right at the earth's surface, without necessarily having much effect on the thickness of the whole layer.

Thunder - The sound emitted by rapidly expanding gases along the channel of a lightning discharge. Over three-quarters of lightning's electrical discharge is used in heating the gases in the atmosphere in and immediately around the visible channel. Temperatures can rise to over 10,000 degrees Celsius in microseconds, resulting in a violent pressure wave, composed of compression and rarefaction. The rumble of thunder is created as one's ear catches other parts of the discharge, the part of the lightning flash nearest registering first, then the parts further away.

Thunder Snow - A wintertime thunderstorm from which falls snow instead of rain. Violent updrafts and at or below freezing temperatures throughout the atmosphere, from surface to high aloft, discourage the melting of snow and ice into rain. Intense snowfall rates often occur during these situations.

Thunderstorm - or thundershower.. A local storm, produced by a cumulonimbus cloud, it is a storm of relatively short duration characterized by and accompanied by thunder, and lightning ,and gusty surface winds, and turbulence. The storm may be can produce hail, icing, and heavyprecipitation, with moderate to extreme up and downdrafts, and under the most severe conditions,the thunderstorm may produce microbursts or tornadoes.

Tide - The periodic rising and falling of the earth's oceans and atmosphere. It is the result of the tide-producing forces of the moon and the sun acting on the rotating earth. This propagates a wave through the atmosphere and along the surface of the earth's waters.

Tilt" - The inclination to the vertical of a significant feature of the pressure pattern or of the field of moisture or temperature. For example, midlatitide troughs tend to display a westward tilt with altitude through the troposphere.

Tilt Sequence - A Radar term indicating that the radar antenna is scanning through a series of antenna elevations in order to obtain a volume scan.

Tilted Storm or Tilted Updraft - A thunderstorm or cloud tower which is not purely vertical but instead exhibits a slanted or tilted character. It is a sign of vertical wind shear, a favorable condition for severe storm development.

Tiros - A series of Television InfraRed Observation Satellites that demonstrated the feasibility and capability of observing the cloud cover and weather patterns of earth from space. An experimental program, it was the first spaceborne system that allowed meteorologists to acquire information that was immediately put to use in an operational setting. The first U.S. weather satellite, TIROS I, was launched on April 1, 1960, and TIROS X, the last of the series, was launched on July 2, 1965.

*Tornado - A tornado appears as a violently rotating column of air in contact with the ground and extending from the base of a thunderstorm. It is funnel-shaped wind vortex in the lower atmosphere with upward spiralling winds of high speeds - spawned by severe thunderstorms. The condensation funnel does not need to reach to the ground for a tornado to be present; a debris cloud beneath a thunderstorm is all that is needed to confirm the presence of a tornado, even in the total absence of a condensation funnel. The tornado usually appears from a bulge in the base of a cumulonimbus cloud. It has a typical width of tens to hundreds of meters and a lifespan of minutes to hours. In area, it is one of the least extensive of all storms, but in violence, it is the world's most severe. More tornadoes occur in the United States than in any other country. In Canada, when they do occur, it is mainly in the Prairies and southern Ontario.

Tornado Alley - This is the geographic corridor in the United States which stretches north from Texas to Nebraska and Iowa. In terms of sheer numbers, this section of the United States receives more tornadoes than any other place on Earth.

Tornado Family - A series of tornadoes produced by a single supercell, resulting in damage path segments along the same general line.

Tornado Warning - This is issued when a tornado is indicated by the WSR-88D radar or sighted by spotters; therefore, people in the affected area should seek safe shelter immediately. They can be issued without a Tornado Watch being already in effect. They are usually issued for a duration of around 30 minutes.
A Tornado Warning is issued by your local National Weather Service office (NWFO). It will include where the tornado was located and what towns will be in its path. If the tornado will affect the nearshore or coastal waters, it will be issued as the combined product--Tornado Warning and Special Marine Warning. If the thunderstorm which is causing the tornado is also producing torrential rains, this warning may also be combined with a Flash Flood Warning. If there is an ampersand (&) symbol at the bottom of the warning, it indicates that the warning was issued as a result of a severe weather report.
After it has been issued, the affected NWFO will followed it up periodically with Severe Weather Statements. These statements will contain updated information on the tornado and they will also let the public know when warning is no longer in effect.

Tornado Watch - This is issued by the National Weather Service when conditions are favorable for the development of tornadoes in and close to the watch area. Their size can vary depending on the weather situation. They are usually issued for a duration of 4 to 8 hours. They normally are issued well in advance of the actual occurrence of severe weather. During the watch, people should review tornado safety rules and be prepared to move a place of safety if threatening weather approaches.
A Tornado Watch is issued by the Storm Prediction Center (SPC) in Norman, Oklahoma. Prior to the issuance of a Tornado Watch, SPC will usually contact the affected local National Weather Forecast Office (NWFO) and they will discuss what their current thinking is on the weather situation. Afterwards, SPC will issue a preliminary Tornado Watch and then the affected NWFO will then adjust the watch (adding or eliminating counties/parishes) and then issue it to the public. After adjusting the watch, the NWFO will let the public know which counties are included by way of a Watch Redefining Statement. During the watch, the NWFO will keep the public informed on what is happening in the watch area and also let the public know when the watch has expired or been cancelled.

Total-Totals Index - A stability index and severe weather forecast tool, equal to the temperature at 850 mb plus the dew point at 850 mb, minus twice the temperature at 500 mb. The total-totals index is the arithmetic sum of two other indices: the Vertical Totals Index (temperature at 850 mb minus temperature at 500 mb) and the Cross Totals Index (dew point at 850 mb minus temperature at 500 mb). As with all stability indices there are no magic threshold values, but in general, values of less than 50 or greater than 55 are considered weak and strong indicators, respectively, of potential severe storm development.

*Tower - (Short for towering cumulus), a cloud element showing appreciable upward vertical development.

*Towering Cumulus - (Same as congestus.) - A large cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil shaped top of a Cb. (Often shortened to "towering cu," and abbreviated TCU.)

Trace - Generally, an unmeasureable amount or insignificant quantity of precipitation in amount of less than 0.005 inch.

Trade Winds - Are two belts of prevailing winds that blow easterly from the subtropical high pressure centers towards the equatorial trough. They are primarily lower level winds, they are characterized by their great consistency of direction. In the Northern Hemisphere, the trades blow from the northeast, and in the Southern Hemisphere, the trades blow from the southeast.

Trajectory - The curve that a body, such as a celestial object, describes in space. This applies to air parcel movement also.

Translucent - Not transparent, but clear enough to allow light to pass through.

Transmissometer - An electronic instrument system which provides a continuous record of the atmospheric transmission between two fixed points. By showing the transmissivity of light through the atmosphere, the horizontal visibility may be determined.

Transparent - A condition where a material is clear enough not to block the passage of radiant energy, especially light.

Transpiration - The process by which water in plants is transferred as water vapor to the atmosphere. Refer to: evapotranspiration.

Triple Point - The intersection between two atmospheric boundaries, such as a dry line,gust front,cold front or outflow boundary, and often a focal point for thunderstorm development. Triple point also may refer to a point on the gust front of a supercell, where the warm moist inflow, the rain-cooled outflow from the forward flank downdraft, and the rear flank downdraft all intersect; this point is a favored location for tornado development (or redevelopment). It also refers to the condition of temperature and pressure under which the gaseous, liquid, and solid forms of a substance can exist in equilibrium.

Tropics/Tropical - The region of the earth located between the Tropic of Cancer, at 23.5 degrees North latitude, and the Tropic of Capricorn, at 23.5 degrees South latitude. It encompasses the equatorial region, an area of high temperatures and considerable precipitiation during part of the year.

Tropical Air Mass - An air mass that forms in the tropics or subtropics over the low latitudes. Maritime tropical air is produced over oceans and is warm and humid, while continental tropical air is formed over arid regions and is very hot and dry.

Tropical Cyclone - The term used for cyclones that originate over tropical waters. It is a warm core low pressure system which is non-frontal and has organized circulation. Depending on sustained surface winds, the system is classified as a tropical disturbance, a tropical depression, a tropical storm, or a hurricane or typhoon.

Tropical Depression - A tropical cyclone in which the maximum sustained surface winds of at least 38 miles per hour (33 knots) or less. Characteristically having one or more closed isobars, it may form slowly from a tropical disturbance or an easterly wave which has continued to organize.

Tropical Disturbance - An area of organized convection, originating in the tropics and occasionally the subtropics, that maintains its identity for a least 24 hours or more. The cloudiness and showers may or may not be associated with a tropical wave, but the system is non-frontal in nature. It is often the first developmental stage of any subsequent tropical depression, tropical storm, or hurricane.

Tropical Prediction Center (TPC) - A division of the National Centers for Environmental Prediction, the Center issues watches, warnings, forecasts, and analyses for the potential of hazardous weather conditions in the tropics for both domestic and international communities. The National Hurricane Center is a branch.
For further information, contact the TCP, located in Miami, Florida.

Tropical Storm - A tropical cyclone in which the maximum sustained surface winds are from 39 miles per hour (34 knots) to 73 miles per hour (63 knots). At this point, the tropical system is given a name to identify and track it.

Tropical Wave - Another name for an easterly wave, it is an area of relatively low pressure moving westward through the trade wind easterlies. Generally, it is associated with extensive cloudiness and showers, and may be associated with possible tropical cyclone development.

Tropic of Cancer - The most northern point on the earth where the sun is directly overhead on the Summer Solstice in the Northern Hemisphere, it is located at approximately 23.5 degrees North latitude.

Tropic of Capricorn - The most southern point on the earth where the sun is directly overhead on the Winter Solticw in the Northern Hemisphere, it is located at approximately 23.5 degrees South latitude.

Tropopause - The upper boundary zone or transition layer between the troposphere and the stratosphere. This boundary layer is characterized by an abrupt change in temperature with height, (lapse rate), from positive (decreasing temperature with height) to a more neutral or negative (temperature constant or increasing with height). At this boundary layer there is little or no change in lapse rate with increasing altitude.

Troposphere - The layer of the atmosphere from the earth's surface up to the tropopause, approximately 11 miles (17 kilometers) into the atmosphere it is characterized by decreasing temperature with height. It's the layer of the atmosphere where most of the clouds,vertical wind motion, appreciable water vapor content, and sensible weather (clouds, rain, etc.), occurs. The decreasing temperature may ocassionally be disrupted, in thin layers by - (see inversion or cap)

Trough - An elongated area of relatively low atmospheric pressure not associated with a closed circulation, it usually extends from the center of a low pressure region. It is the opposite of a ridge.

Tsunami - An ocean wave with a long period that is formed by an underwater earthquake or landslide, or volcanic eruption. It may travel unnoticed across the ocean for thousands of miles from its point of origin and builds up to great heights over shallower waters. Also known as a seismic sea wave, and incorrectly, as a tidal wave.

Tsunami Advisory - For products of the Pacific Tsunami Warning Center (PTWC - Pacific (except Alaska, British Columbia and Western States) Hawaii, Caribbean (except Puerto Rico, Virgin Is.), Indian Ocean): The third highest level of tsunami alert. Advisories are issued to coastal populations within areas not currently in either warning or watch status when a tsunami warning has been issued for another region of the same ocean. An Advisory indicates that an area is either outside the current warning and watch regions or that the tsunami poses no danger to that area. The Center will continue to monitor the event, issuing updates at least hourly. As conditions warrant, the Advisory will either be continued, upgraded to a watch or warning, or ended. For products of the West Coast/Alaska Tsunami Warning Center (WC/ATWC - Alaska, British Columbia and Western States, Canada, Eastern and Gulf States, Puerto Rico, U.S Virgin Islands): A tsunami advisory is issued due to the threat of a potential tsunami which may produce strong currents or waves dangerous to those in or near the water. Coastal regions historically prone to damage due to strong currents induced by tsunamis are at the greatest risk. The threat may continue for several hours after the arrival of the initial wave, but significant widespread inundation is not expected for areas under an advisory. Appropriate actions to be taken by local officials may include closing beaches, evacuating harbors and marinas, and the repositioning of ships to deep waters when there is time to safely do so. Advisories are normally updated to continue the advisory, expand/contract affected areas, upgrade to a warning, or cancel the advisory.

Tsunami Information Statement - A tsunami information statement is issued to inform emergency management officials and the public that an earthquake has occurred, or that a tsunami warning, watch or advisory has been issued for another section of the ocean. In most cases, information statements are issued to indicate there is no threat of a destructive tsunami and to prevent unnecessary evacuations as the earthquake may have been felt in coastal areas. An information statement may, in appropriate situations, caution about the possibility of destructive local tsunamis. Information statements may be re-issued with additional information, though normally these messages are not updated. However, a watch, advisory or warning may be issued for the area, if necessary, after analysis and/or updated information becomes available.

Tsunami Warning - For products of the Pacific Tsunami Warning Center (PTWC - Pacific (except Alaska, British Columbia and Western States) Hawaii, Caribbean (except Puerto Rico, Virgin Is.), Indian Ocean): The highest level of tsunami alert. Warnings are issued due to the imminent threat of a tsunami from a large undersea earthquake or following confirmation that a potentially destructive tsunami is underway. They may initially be based only on seismic information as a means of providing the earliest possible alert. Warnings advise that appropriate actions be taken in response to the tsunami threat. Such actions could include the evacuation of low-lying coastal areas and the movement of boats and ships out of harbors to deep water. Warnings are updated at least hourly or as conditions warrant to continue, expand, restrict, or end the warning. For products of the West Coast/Alaska Tsunami Warning Center (WC/ATWC - Alaska, British Columbia and Western States, Canada, Eastern and Gulf States, Puerto Rico, U.S Virgin Islands): A tsunami warning is issued when a potential tsunami with significant widespread inundation is imminent or expected. Warnings alert the public that widespread, dangerous coastal flooding accompanied by powerful currents is possible and may continue for several hours after arrival of the initial wave. Warnings also alert emergency management officials to take action for the entire tsunami hazard zone. Appropriate actions to be taken by local officials may include the evacuation of low-lying coastal areas, and the repositioning of ships to deep waters when there is time to safely do so. Warnings may be updated, adjusted geographically, downgraded, or canceled. To provide the earliest possible alert, initial warnings are normally based only on seismic information.

Tsunami Watch - For products of the Pacific Tsunami Warning Center (PTWC - Pacific (except Alaska, British Columbia and Western States) Hawaii, Caribbean (except Puerto Rico, Virgin Is.), Indian Ocean): The second highest level of tsunami alert. Watches are issued based on seismic information without confirmation that a destructive tsunami is underway. It is issued as a means of providing an advance alert to areas that could be impacted by destructive tsunami waves. Watches are updated at least hourly to continue them, expand their coverage, upgrade them to a Warning, or end the alert. A Watch for a particular area may be included in the text of the message that disseminates a Warning for another area. For products of the West Coast/Alaska Tsunami Warning Center (WC/ATWC - Alaska, British Columbia and Western States, Canada, Eastern and Gulf States, Puerto Rico, U.S Virgin Islands): A tsunami watch is issued to alert emergency management officials and the public of an event which may later impact the watch area. The watch area may be upgraded to a warning or advisory - or canceled - based on updated information and analysis. Therefore, emergency management officials and the public should prepare to take action. Watches are normally issued based on seismic information without confirmation that a destructive tsunami is underway.

Tule Fog - A Ground fog that is commom in the central valley of California and is the leading cause of weather-related casualties in that state. It forms at night and in the early morning when the ground cools, lowering the air temperature near the ground to or below its initial dew point. It is a form of radiation fog.

Turbulence - The irregular and instantaneous motions of air which is made up of a number of small of eddies that travel in the general air current. Atmospheric turbulence is caused by random fluctuations in the wind flow. It can be caused by thermal or convective currents, differences in terrain and wind speed, along a frontal zone, or variation in temperature and pressure. This is the same vertical motion of the air, that at times can cause violent, up-and-down movements we feel on a plane.

*Turkey Tower - [Slang], For narrow, individual cloud tower that develops and falls apart rapidly. The sudden development of turkey towers from small cumulus clouds may signify the breaking of a cap.

TVS - Tornadic Vortex Signature. - A Doppler radar signature in the radial velocity field indicating intense, concentrated rotation - more so than a mesocyclone. Like the mesocyclone, specific criteria involving strength, vertical depth, and time continuity must be met in order for a signature to become a TVS. Existence of a TVS strongly increases the probability of tornado occurrence, but does not guarantee it. A TVS is not a visually observable feature.

Twilight - Often called dusk, it is the evening period of waning light from the time of sunset to dark. The time of increasing light in the morning is called dawn. Twilight ends in the evening or begins in the morning at a specific time and can be categorized into three areas of decreasing light. Civil twilight is the time in the evening when car headlights need to be turned on to be seen by other drivers. Nautical twilight is when the bright stars used by navigators have appeared and the horizon may still be seen. Astronomical twilight is when the sunlight is still shining on the higher levels of the atmosphere, yet it is dark enough for astronomical work to begin. During dawn, the reverse order occurs until full daylight.

Twister - A slang term used in the United States for a tornado.

Typhoon - The name for a tropical cyclone with sustained winds of 74 miles per hour (65 knots) or greater in the western North Pacific Ocean. This ia same tropical cyclone is known as a hurricane in the eastern North Pacific and North Atlantic Ocean, and as a cyclone in the Indian Ocean.

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UKMET - A medium-range numerical weather prediction model operated by the United Kingdom METeorological Agency.

UltraViolet - Electromagnetic radiation that has a wavelength shorter than visible light and longer than x-rays. Although it accounts for only 4 to 5 percent of the total energy of insolation, it is responsible for many complex photochemical reactions, such as fluorescence and the formation of ozone.

Undercast - In aviation, it is an opaque cloud layer viewed from an observation point above the layer. From the ground, it would be considered an overcast.

United States Weather Bureau - The official name of the National Weather Service prior to 1970.

Universal Coordinated Time - One of several names for the twenty-four hour time which is used throughout the scientific and military communities. Related terms: Zulu (Z) and Greenwich Mean Time (GMT)

University Corporation For Atmospheric Research (UCAR) - A non-profit university membership consortium which carries out programs to benefit atmospheric, oceanic, and related sciences around the globe. Among other activites, UCAR operates the National Center for Atmospheric Research (NCAR) with National Science Foundation sponsorship. For further information, contact UCAR,> located in Boulder, Colorado.

Unstable/ Instability - Occurs when a rising air parcel becomes less dense than the surrounding air. Since its temperature will not cool as rapidly as the surrounding environment, it will continue to rise on its own. Refer to instability. Contrast with stable air.

*Updraft - A small scale current of air with vertical motion. If there is enough moisture, then it may condense, forming a cumulus cloud, or an individual tower of a towering cumulus or Cb this is the first step towards thunderstorm development. Contrast with a downdraft.

*Updraft Base - A alternate term used for a rain-free base.

Upper Level System - A general term for any large-scale or mesoscale disturbance capable of producing upward motion (lift) in the middle or upper parts of the atmosphere. This term sometimes is used interchangeably with impulse or shortwave.

Upslope Effect - The cooling of an air flow as it ascends a hill or mountain slope. If there is enough moisture and the air is stable, stratiform clouds and precipitation may form. If the air is unstable, there might be an increased chance of thunderstorm development. Contrast with a downslope effect.

Upslope Flow - Air that flows toward higher terrain, and hence is forced to rise. The added lift often results in widespread low cloudiness and stratiform precipitation if the air is stable, or an increased chance of thunderstorm development if the air is unstable.

Upslope Fog - Fog that forms when warm, moist surface air is forced up a slope by the wind. It is adiabatically cooled to below its initial dew point, which means the air cools by expansion as it rises. It forms best where there is a gradual slope, and it can become quite deep, requiring considerable time to dissipate. An example is Cheyenne Fog.

Upper Air/Upper Level - The portion of the atmosphere which is above the lower troposphere. It is generally applied to the levels above 850 millibars. Therefore, upper level lows and highs, troughs, winds, observations, and charts all apply to atmospheric phenomena above the surface.

Upstream - Toward the source of the flow, or located in the area from which the flow is coming.

Upwelling - The process by which water rises from a lower to a higher depth, usually as a result of divergence and offshore currents. It influences climate by bringing colder, more nutrient-rich water to the surface. A vital factor of the El Nino event.

Upwind - The direction from which the wind is blowing. Also the windward side of an object. The opposite of the downwind or leeward side.

UV Index (or Ultraviolet Index) - The UV Index, which was developed by the National Weather Service and the U.S. Environmental Protection Agency (EPA), provides important information to help you plan your outdoor activities to prevent overexposure to the sun's rays. The UV Index provides an indicator of the expected risk of overexposure to the sun. The Index measures UV intensity levels on a scale of 1 to 11+, where low indicates a minimal risk of overexposure and 11+ means an extreme risk. The UV Index takes into account clouds and other local conditions that affect the amount of UV radiation reaching the ground.

UVM (or UVV) - Upward Vertical Motion (or Velocity) - Air flow that has a vertical flow or motion.

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VAD - Velocity Azimuth Display. A radar display on which mean radial velocity is plotted as a function of azimuth. Also see VWP.

Valley Breeze - An anabatic wind, it is formed during the day by the heating of the valley floor. As the ground becomes warmer than the surrounding atmosphere, the lower levels of air heat and rise, flowing up the mountainsides. It blows in the opposite direction of a mountain breeze.

Vapor Pressure - The pressure exerted by the molecules of a given vapor. In meteorology, it is considered as the part of total atmospheric pressure due to the water vapor content. It is independent of other gases or vapors.

Vapor Trail - A cloudlike streamer or trail often seen behind aircraft flying in clear, cold, humid air. A vapor trail is created when the water vapor from the engine exhaust gases are added to the atmosphere. Also called a contrail, for condensation trail.

Variable Ceiling - Occurs when the height of a ceiling layer increases and decreases rapidly, The ascribed height is the average of all the varying values.

Vault- Refers to radar echo signatures with low reflectivity a in the center, surrounded by higher reflectivity. It is associated with strong updrafts and is found in the inflow region of a thunderstorm. And it cannot be seen visually. Is another name for and the same as a BWER.

VEERING or Veering Winds A change in wind direction that shifts clockwise in the Northern Hemisphere at a certain location. In the Southern Hemisphere, it is counterclockwise. This can either happen in the horizontal or the vertical (with height). For example, the wind shifts from the north to the northeast to the east or from southerly to westerly, or which change direction in a clockwise sense with height (e.g., southeasterly at the surface turning to southwesterly aloft) The latter example is a form of directional shear which is important for tornado formation. They are the opposite of backing winds.

Vernal Equinox - Taking place in the Northern Hemispheric spring, it is the point at which the ecliptic intersects the celestial equator. Days and nights are most nearly equal in duration. It falls on or about March 20 and is considered the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. It is the astronomical opposite of the autumnal equinox.

Vertical Temperature Profile - A series of temperature measurements taken at various levels in the atmosphere that show the thermal structure of the atmosphere over a specific location. Obtained through a rawinsonde sounding or comparable method, and exhibited in a skew t-log p diagram.

Vertical Visibility - The distance an observer can see vertically into an undefined ceiling, or the height corresponding to the top of a ceiling light projector beam, or the height at which a ceiling balloon disappears during the presence of an indefinite ceiling.

Vertical Wind Profile - A series of wind direction and wind speed measurements taken at various levels in the atmosphere that show the wind structure of the atmosphere over a specific location. Obtained through a rawinsonde sounding or comparable method, and exhibited in a skew t-log p diagram.

VIL - Vertically-Integrated Liquid water. A property computed by RADAP II and WSR-88D units that takes into account the three-dimensional reflectivity of an echo. The maximum VIL of a storm is useful in determining its potential severity, especially in terms of maximum hail size.

Vertically-stacked System - A low-pressure system, usually a closed low or cutoff low, which is not tilted with height, i.e., located similarly at all levels of the atmosphere. Such systems typically are weakening and are slow-moving, and are less likely to produce severe weather than tilted systems. However, cold pools aloft associated with vertically-stacked systems may enhance instability enough to produce severe weather.

VIP - Video Integrator and Processor, which contours radar reflectivity (in dBZ) into six VIP levels:

• VIP 1 (Level 1, 18-30 dBZ) - Light precipitation
• VIP 2 (Level 2, 30-38 dBZ) - Light to moderate rain.
• VIP 3 (Level 3, 38-44 dBZ) - Moderate to heavy rain.
• VIP 4 (Level 4, 44-50 dBZ) - Heavy rain
• VIP 5 (Level 5, 50-57 dBZ) - Very heavy rain; hail possible.
• VIP 6 (Level 6, >57 dBZ) - Very heavy rain and hail; large hail possible.

*Virga - Streaks or wisps of precipitation, such as water or ice particles, that fall from clouds but evaporatebefore reaching the ground. From a distance, the event sometimes may be mistaken for a funnel cloud or tornado. Typically, it may fall from altocumulus, altostratus,or high based cumuonimbus. In certain cases, shafts of virga may precede a microburst; see dry microburst.

Visibility - A measure of the opacity of the atmosphere, and therefore, the greatest distance one can see prominent objects with normal eyesight. The National Weather Service has various terms for visibility. Surface visibility is the prevailing visibility determined from the usual point of observation. Prevailing visibility is considered representative of visibility conditions at the station. Sector visibility is the visibility in a specified direction that represents at least a 45 degree arc of the horizon circle. Tower visibility is the prevailing visibility determined from the airport traffic control tower (ATCT) at stations that also report surface visibility.

Visible Light - The portion of the electromagnetic spectrum that can be detected by the human eye. It travels at the same speed as all other radiation, that is at 186,000 mile per second. It has a wave length longer than ultraviolet light and shorter than x-rays. Refer to light waves.

Visual Flight Rules (VFR) - Refers to the general weather conditions pilots can expect at the surface. VFR criteria means a ceiling greater than 3,000 feet and greater than 5 miles visibility. Contrast with MVFR and IFR.

V Notch - A radar reflectivity signature seen as a V-shaped notch in the downwind part of a thunderstorm echo. The V-notch often is seen on supercells, and is thought to be a sign of diverging flow around the main storm updraft (and hence a very strong updraft). This term should not be confused with inflow notch or with enhanced V, although the latter is believed to form by a similar process.

Volume Scan - A radar scanning strategy in which sweeps are made at successive antenna elevations (i.e., a tilt sequence), and then combined to obtain the three-dimensional structure of the echoes. Volume scans are necessary to determine thunderstorm type, and to detect features such as WERs, BWERs, and overhang.

Vort Max - (Slang; short for vorticity maximum), a center, or maximum, in the vorticity field of an airmass. (see Vorticity Maximum

Vortex - Any circular or rotary flow in the atmosphere that possesses vorticity.

Vorticity - A measure of the local rotation in a fluid flow. In weather analysis and forecasting, it usually refers to the vertical component of rotation (i.e., rotation about a vertical axis) of a small air parcel. It has vorticity when the parcel spins as it moves along its path. It is used most often in reference to synoptic scale or mesoscale weather systems. In convention, positive values indicate there is cyclonic rotation.

Although the axis of the rotation can extend in any direction, meteorologists are primarily concerned with the rotational motion about an axis that is perpendicular to the earth's surface. If it does not spin, it is said to have zero vorticity. In the Northern Hemisphere, the vorticity is positive when the parcel has a counterclockwise, or cyclonic, rotation. It is negative when the parcel has clockwise, or anticyclonic, rotation. An example is relative vorticity. In weather analysis and forecasting, it usually refers to the vertical component of rotation (i.e., rotation about a vertical axis) and is used most often in reference to synoptic scale or mesoscale weather systems. By convention, positive values indicate cyclonic rotation.

Vorticity Maximum - A center of vorticity, or the maximum of the vorticity field of a fluid. In Meteorology it refers to the center, or maximum, in the vorticity field of an airmass.

VWP - VAD Wind Profile. A radar plot of horizontal winds, derived from VAD data, as a function of height above a Doppler Radar. The display is plotted with height as the vertical axis and time as the horizontal axis (a so-called time-height display), which then depicts the change in wind with time at various heights. This display is useful for observing local changes in vertical wind shear, such as backing of low-level winds, increases in speed shear, and development or evolution of nearby jet streams (including low-level jets).

This product often is referred to erroneously as a VAD.

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*Wall Cloud - A localized, persistent, often abrupt lowering of a cloud from its rain-free base,(no visible precipitation underneath), often it is observed with a cumulonimbus or supercell. Wall clouds can range from a fraction of a mile up to nearly five miles in diameter, and normally are found on the south or southwest (inflow) side of the thunderstorm. When seen from within several miles, many wall clouds exhibit rapid upward motion and cyclonic rotation. They form in the area of a thunderstorm updraft, or inflow area, it exhibits rapid upward movement and cyclonic rotation. However, not all wall clouds rotate. Rotating wall clouds usually develop before strong or violent tornadoes, by anywhere from a few minutes up to nearly an hour. Wall clouds should be monitored visually for signs of persistent, sustained rotation and/or rapid vertical motion. "Wall cloud" also is used occasionally in tropical meteorology to describe the inner cloud wall surrounding the eye of a tropical cyclone, but the proper term for this feature is eyewall.

Warm - To have or give out heat to a moderate or adequate degree. A subjective term for temperatures between cold and hot. In meteorology, an air parcel that is warm is only so in relation to another parcel.

Warm Advection - The horizontal movement or transport of warmer air into a location. Low-level warm advection sometimes is referred to (erroneously) as overrunning. Although the two terms are not properly interchangeable, both imply the presence of lifting in low levels.Contrast with cold advection.

Warm Front - The leading edge of an advancing warm air mass that is replacing a retreating relatively colder air mass. Generally, with the passage of a warm front, the temperature and humidity increase, the pressure usually rises, and although the wind shifts (usually from the southwest to the northwest in the Northern Hemisphere), it is not as pronounced as with a cold frontal passage. Precipitation, in the form of rain, snow, or drizzle, is generally found ahead of the surface front, as well as convective showers and thunderstorms. Fog is common in the cold air ahead of the front. Although clearing usually occurs after passage, some conditions may produced fog in the warm air. See occluded front and cold front.

Warm High - A high pressure system that has its warmest temperatures at or near the center of circulation. Contrast with a cold high. Examples include a cut-off high and an omega block.

Warm Low - A low pressure system that has its coldest temperatures at or near the center of circulation. Contrast with a cold low.

*Warning - A forecast product issued by N.O.A.A. when severe weather has developed, is already occurring and reported, or is detected on radar. Warnings state a particular hazard or imminent danger, such as tornadoes, severe thunderstorms, flash and river floods, winter storms, heavy or blowing snow, etc. A warning indicates the need to take action to protect life and property. Warnings are broadcast to the media, on the N.O.A.A. Weather radio System. Also See short-fuse warning.

Wasatch Winds - Strong winds blowing easterly out of the Wasatch Mountains in Utah, sometimes reaching speeds greater than 75 miles per hour.

*Watch - A N.O.A.A. weather service forecast product issued well in advance of a severe weather event to alert the public of the possibility of a particular hazard or that conditions are more favorable than usual for its occurrence. These products may be issued by the SPC or your local weather service office. This product is issued when the threat for severe weather such as tornadoes, severe thunderstorms, flash and river floods, winter storms or heavy snows. A WATCH is a recommendation to the public for planning, preparation, and increased awareness (i.e., to be alert for changing weather, listen for furtherinformation, and think about what to do if the danger materializes).

*Watch Box (or Box) [Slang], For a severe thunderstorm or tornado watch.

Water (H₂O) - Refers to the chemical compound, H2O, as well as its liquid form. Depending on atmospheric temperatures and pressures, it can exist in all three phases: solid (ice), liquid (water), and gaseous (water vapor). It is a vital, life-sustaining part of life on earth.

Water Cycle - Also called the hydrologic cycle, it is the vertical and horizontal transport of water in all its states between the earth, the atmosphere, and the seas.

*Waterspout - In general, a small, weak tornado, which is not formed by a storm-scale rotation. It is generally weaker than a supercell tornado and is not associated with a wall cloud or mesocyclone. Specifically, it normally refers to a small, relatively weak rotating column of air over water beneath cumulonimbus or towering cumulus clouds. But there is sufficient justification for calling virtually any rotating column of air a waterspout if it is in contact with a water surface. Waterspouts are most common over tropical or subtropical waters. The exact definition of waterspout is debatable. In most cases the term is reserved for small vortices over water that are not associated with storm-scale rotation (i.e., they are the water-based equivalent of landspouts).

Water Vapor (H₂O) - Water in gaseous form. It is one of the most import constituents of the atmosphere. Due to its molecular content, air containing water vapor is lighter than dry air. This contributes to the reason why moist air has a tendency to rise.

Wave(S) - In general, any pattern with some roughly identifiable periodicity in time and/or space. It is also considered as a disturbance that moves through or over the surface of the medium with speed dependent on the properties of the medium. In meteorology, this applies to atmospheric waves or the intersection of warm and cold fronts, such as long waves, short waves, Rossby waves, and cyclonic waves. In oceanography, this applies to waves generated by mechanical means, such as currents, turbidity, and the wind.

Wave Cyclone - A cyclone which forms and moves along a front. The circulation around the cyclone's center produces a wavelike deformation on the front. May also be call a migratory cyclone or low.

Wave Length - The least distance between particles moving in the same phase of oscillation of a wave. In oceanography, it is the horizontal distance between the highest parts of two successive wave crests above the still water level, separated by a trough that is below the still water level, and it is measured in meters.

Weather - The state of the atmosphere at a specific time and with respect to its effect on life and human activities. It is the short term variations of the atmosphere, as opposed to the long term, or climatic, changes. Also, weather is the meteorological day-to-day variations of the atmosphere and it's effects on life and human activity. It is often referred to in terms of brightness, cloudiness, humidity, precipitation, temperature, visibility, and wind.

Weathering - The decay and breakup of rocks on the earth's surface by natural chemical and mechanical processes. The mechanical action includes large changes of temperature, high temperatures, frost, or the impact of windborne sand or water. Chemical action includes the chemical reactions between atmospheric constituents in a moist environments or in rain water. Biological agents are mainly fungi which attack organic material.

Weather Balloon - large balloons filled with helium or hydrogen and carry radiosondes (weather instruments) aloft to measure temperature pressure and humidity as the balloon rises through the air. The whole contraption is attached to a small parachute so that when the balloon inevitably breaks, the radiosone doesn't hurtle back to earth dangerously quickly.

Weather Surveillance Radar (WSR-88D) - The newest generation of Doppler radars, the 1988 Doppler weather radar. The radar units, with help from a set of computers, show very detailed images of precipitation and other phenomena, including air motions within a storm.

Weather Vane - Originally used as a wind vane, it is an instrument that indicates the wind direction. The name developed based on observations on what kind of weather occurred with certain wind directions. Creative designs often adorn the tops of barns and houses.

Wedge High - Primarily refers to an elongated area of shallow high pressure at the earth's surface. It is generally associated with cold air east of the Rockies or Appalachians. It is another name for a ridge, ridge line, or ridge axis. Contrast with a trough. Wedge is also a slang term for a large, wide tornado with a wedge-like shape.

*Wedge or (Wedge Tornado) - [Slang], a large tornado with a condensation funnel that is at least as wide (horizontally) at the ground as it is tall (vertically) from the ground to cloud base. The term "wedge" often is used somewhat loosely to describe any large tornado. However, not every large tornado is a wedge. A true wedge tornado, with a funnel at least as wide at the ground as it is tall, is very rare. Wedges often appear with violent tornadoes (F4 or F5 on the Fujita Scale), but many documented wedges have been rated lower. And some violent tornadoes may not appear as wedges (e.g., Xenia, OH on 3 April 1974, which was rated F5 but appeared only as a series of suction vortices without a central condensation funnel).

Whether or not a tornado achieves "wedge" status depends on several factors other than intensity - in particular, the height of the environmental cloud base and the availability of moisture below cloud base. Therefore, spotters should not estimate wind speeds or F-scale ratings based on visual appearance alone. However, it generally is safe to assume that most (if not all) wedges have the potential to produce strong (F2/F3) or violent (F4/F5) damage.

WER - Weak Echo Region. Radar term for a region of relatively weak (reflectivity at low levels on the inflow side of a thunderstorm echo, topped by stronger reflectivity in the form of an echo overhang directly above it . The WER is a sign of a strong updraft on the inflow side of a storm, within which precipitation is held aloft. When the area of low reflectivity extends upward into, and is surrounded by, the higher reflectivity aloft, it becomes a BWER.

Westerlies - Usually applied to the broad patterns of persistent winds with a westerly component. It is the dominant persistent atmospheric motion, centered over the midlatitudes of each hemisphere. Near the earth's surface, the westerlies extend from approximately 35 to 65 degrees latitude, while in the upper levels they extend further polarward and equatorward.

West Virigina High - An area of stagnant high pressure located over West Virginia during a Indian Summer event.

Wet Bulb Depression - Dependent on the temperature and the humidity of the air, it is the difference between the dry bulb and the wet bulb readings.

Wet Bulb Thermometer - A thermometer used to measure the lowest temperature in the ambient atmosphere in its natural state by evaporating water from a wet muslin-covered bulb of a thermometer. The wet bulb temperature is used to compute dew point and relative humidity. One of the two therometers that make up a psychrometer.

*Wet Microburst - A microburst accompanied by heavy precipitation at the surface. A rain foot may be a visible sign of a wet microburst. See dry microburst.

Whirlwind - (Dust Devil) - A small-scale, rapidly rotating column of wind, formed thermally and most likely to develop on clear, dry, hot afternoons. Often called a dust devil when visible by the dust, dirt or debris it picks up. Also slang for a landspout or a tornado.</9>

Whiteout - When visibility is near zero due to blizzard conditions or occurs on sunless days when clouds and surface snow seem to blend, erasing the horizon and creating a completely white vista.

Wind - Air that flows in relation to the earth's surface, generally horizontally. There are four areas of wind that are measured: direction, speed, character (gusts and squalls), and shifts. Surface winds are measured by wind vanes and anemometers, while upper level winds are detected through pilot balloons, rawin, or aircraft reports.

Wind Chill - The combined cooling effect of wind and temperature is called wind chill. The wind chill factor is a measure of this cooling effect. The larger the wind chill factor, the faster the rate of cooling. Note, however, that an object will not be cooled below the actual air temperature, it will just get there faster.

Wind Chill Index - The calculation of temperature that takes into consideration the effects of wind and temperature on the human body. Describes the average loss of body heat and how the temperature feels. This is not the actual air temperature. For an example, check out the wind chill chart.

Wind Direction - The direction from which the wind is blowing. For example, an easterly wind is blowing from the east, not toward the east. It is reported with reference to true north, or 360 degrees on the compass, and expressed to the nearest 10 degrees, or to one of the 16 points of the compass (N, NE, etc.).

Wind Shear - The rate of wind speed or direction change with distance. Vertical wind shear is the rate of change of the wind with respect to altitude. Horizontal wind shear is the rate of change on a horizontal plane. Also see shear.

Wind Shift - The term applied to a change in wind direction of 45 degrees or more, which takes place in less than 15 minutes. It may the result of a frontal passage, from katabatic winds, sea breezes,or thunderstorms, and in some instances, the change may be gradual or abrupt.

Wind Speed - The rate of the motion of the air on a unit of time. It can be measured in a number of ways. In observing, it is measured in knots, or nautical miles per hour. The unit most often used in the United States is miles per hour.

Wind Vane - An instrument that indicates the wind direction. The end of the vane which offers the greatest resistence to the motion of the air moves to the downwind poisition. See weather vane.

Windward - The direction from which the wind is blowing. Also the upwind side of an object. The opposite of the downwind or leeward side.

Wind Wave - An ocean or lake wave resulting from the action of wind on the water's surface. After it leaves its fetch area, it is considered a swell.

Winter - Astronomically, this is the period between the winter solstice and the vernal equinox. It is characterized as having the coldest temperatures of the year, when the sun is primarily over the opposite hemisphere. Customarily, this refers to the months of December, January, and February in the North Hemisphere, and the months of June, July, and August in the Southern Hemisphere.

Winter Solstice - Astronomically, this is the date when the sun reaches it southern most point or furtherest point from the the celestial equator, theTropic of Capricorn. The point at which sun is at it's maximum distance from the equator and days and nights are most unequal in duration. the winter solstice, occurs in the Northern Hemisphere, on or about December 21.

Winter Storm - Any one of several storm systems that develop during the late fall to early spring and deposit wintry precipitation, such as snow, freezing rain, or ice. Examples include blizzards, ice storms, and nor'easters.

World Meteorlogical Organization (WMO) - From weather prediction to air pollution research, climate change related activities, ozone layer depletion studies and tropical storm forecasting, the World Meteorological Organization coordinates global scientific activity to allow increasingly prompt and accurate weather information and other services for public, private and commercial use, including international airline and shipping industries. Established by the United Nations in 1951, it is composed of 184 members. For more information, contact the WMO, located in Geneva, Switzerland.

*Wrapping Gust Front - A gust front which wraps around a mesocyclone, cutting off the inflow of warm moist air to the mesocyclone circulation and resulting in an occluded mesocyclone.

WSR-57, WSR-74 - NWS Weather Surveillance Radar units, replaced by WSR-88D units.

WSR-88D - Another name for Weather Surveillance Radar - 1988 The newest generation ofDoppler; also see Weather Survellance radar and NEXRAD radar unit.

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X-Rays - The portion of the electromagnetic spectrum that has a very short wave length. It has a wave length longer than gamma rays, yet shorter than visible light. X-rays can penetrate various thicknesses of all solids, and when absorbed by a gas, can result in ionization.

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Year - The interval required for the earth to complete one revolution around the sun. A sidereal year, which is the time it take for the earth to make one absolute revolution around the sun, is 365 days, 6 hours, 9 minutes, and 9.5 seconds. The calendar year begins at 12 o'clock midnight local time on the night of December 31st-January 1st. Currently, we operate under the Gregorian calendar of 365 days, with 366 days every four years, a leap year. The tropical year, also called the mean solar year, is dependent on the seasons. It is the interval between two consecutive returns of the sun to the vernal equinox. In 1900, that took 365 days, 5 hours, 48 minutes, and 46 seconds, and it is decreasing at the rate of 0.53 second per century

Yellow Snow - Snow that is given golden, or yellow, appearance by the presence of pine or cypress pollen in it.

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Zenith - The point which is elevated 90 degrees from all points on a given observer's astronomical horizon. The point on any given observer's celestial sphere that lies directly above him. The opposite of nadir.

Zodiac - The position of the sun during the course of the year as it appears to move though successive constellations. Also, the band where the ecliptic runs centrally through the celestial sphere and contains the sun, the moon, and all the planets except Venus and Pluto.

Zonal Flow - Large-scale atmospheric flow in which the east-west component (i.e., latitudinal) is or dominant. The accompanying meridional (north-south) component often is weaker than normal. The flow of air along a latitudinal component of existing flow, normally from west to east. Contrast with meridional flow.

Zonal Index - The measure of the strength of the westerly winds of the middle latitudes. It is expressed as the horizontal pressure difference between 35 degrees and 55 degrees latitude, or as the corresponding geostrophic wind.

ZULU Time - One of several names for the twenty-four hour time which is used throughout the scientific and military communities. Related terms: Universal Time Coordinate (UTC) and Greenwich Mean Time (GMT)

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