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U Burst In solar-terrestrial terms, a fast radio burst spectrum of a flare. It has a U-shaped appear- ance in an intensity-vs.-frequency plot
U.S. Geological Survey (Abbrev. USGS)- The Federal Agency chartered in 1879 by congress to classify public lands, and to examine the geologic structure, mineral resources, and products of the national domain. As part of its mission, the USGS provides information and data on the Nation?ǨѢs rivers and streams that are useful for mitigation of hazards associated with floods and droughts.
UCP (Unit Control Position): The WSR-88D radar operator uses this to control the entire radar system. One of the main things that the radar operator will do at the UCP is change volume scan strategies of the antenna. These volume scan strategies tell the radar how many elevation angles will be used during a single volume scan (a volume scan is the completion of a sequence of elevation angles), and the amount of time it will take to complete that sequence of elevation cuts, each one being a single rotation of the antenna's 1 degree beam at selected elevation angles. The WSR-88D uses 3 scan strategies. They are the following: 14 elevation angles in 5 minutes (this is used during severe weather situations), 9 elevation angles in 6 minutes (this is used when there is precipitation within 248 nautical miles of the radar), and 5 elevation angles in 10 minutes (this is used when there is no precipitation within 248 nautical miles). The radar operator at the UCP can also adjust the radar products and help the users out with their communication problems.
UKMET A medium-range (3 to 7 day) numerical weather prediction model operated by the United Kingdom METeorological Agency. It has a resolution of 75 kilometers and covers the entire northern hemisphere. Forecasters use this model along with the ECMWF and GFS in making their extended forecasts (3 to 7 days).
ULJ Upper Level Jet
Ultra High Frequency (UHF) Those radio frequencies exceeding 300 MHz
Ultraviolet Index This index provides important information to help you plan your outdoor activities in ways that prevent overexposure to the sun's rays. It was designed by the National Weather Service (NWS) and the Environmental Protection Agency (EPA). Unlike some countries' indices, the United States UV Index is not based upon surface observations. Rather, it is computed using forecasted ozone levels, a computer model that relates ozone levels to UV incidence on the ground, forecasted cloud amounts, and the elevation of the forecast cities. The calculation starts with measurements of current total ozone amounts for the entire globe, obtained via two satellites operated by the National Oceanic and Atmospheric Administration (NOAA). These data are then used to produce a forecast of ozone levels for the next day at various points around the country.

CategoryUV IndexTime to BurnActions to Take
Minimal0 - 260 min. +Apply SPF sunscreen.
Low3 - 445 min.Apply SPF sunscreen, wear a hat.
Moderate5 - 630 min.Apply SPF 15, wear a hat.
High7 - 915 - 24 min.Apply SPF 15 to 30, wear a hat and sunglasses. Limit midday exposure.
Very High10+10 min.Apply SPF 30; wear a hat, sunglasses, and protective clothing; limit midday exposure.
Ultraviolet Radiation Electromagnetic radiation of shorter wavelength than visible radiation but longer than x-rays.
Umbra In solar-terrestrial terms, the dark core or cores (umbrae) in a sunspot with penumbra, or a sunspot lacking penumbra.
Undercurrent In hydrologic terms, a current below the upper currents or surface of a fluid body.
Underflow The lateral motion of water through the upper layers until it enters a stream channel. This usually takes longer to reach stream channels than runoff. This also called subsurface storm flow.
Undersun An optical effect seen by an observer above a cloud deck when looking toward the sun, as sunlight is reflected upwards off the faces of ice crystals in the cloud deck. [Also known as subsun.]
Unimodal A distribution having only one localized maximum, i.e., only one peak.
Unit Control Position The WSR-88D radar operator uses this to control the entire radar system. One of the main things that the radar operator will do at the UCP is change volume scan strategies of the antenna. These volume scan strategies tell the radar how many elevation angles will be used during a single volume scan (a volume scan is the completion of a sequence of elevation angles), and the amount of time it will take to complete that sequence of elevation cuts, each one being a single rotation of the antenna's 1 degree beam at selected elevation angles. The WSR-88D uses 3 scan strategies. They are the following: 14 elevation angles in 5 minutes (this is used during severe weather situations), 9 elevation angles in 6 minutes (this is used when there is precipitation within 248 nautical miles of the radar), and 5 elevation angles in 10 minutes (this is used when there is no precipitation within 248 nautical miles). The radar operator at the UCP can also adjust the radar products and help the users out with their communication problems.
Unit Hydrograph The discharge hydrograph from one inch of surface runoff distributed uniformly over the entire basin for a given time period
Universal Time (UT) See Coordinated Universal Time
UNSBL Unseasonable
Unsettled In solar-terrestrial terms, with regard to geomagnetic levels, a descriptive word specifically meaning that 8 is less than or equal to the Ap Index which is less than or equal to 15.
Unstable Air Air that is able to rise easily, and has the potential to produce clouds, rain, and thunderstorms.
UNSTBL Unstable
Up-valley Wind A diurnal thermally driven flow directed up a valley's axis, usually occurring during daytime; part of the along-valley wind system.
Updraft A small-scale current of rising air. If the air is sufficiently moist, then the moisture condenses to become a cumulus cloud or an individual tower of a towering cumulus or Cb.
Upper Level In weather observing, the term applies to the portion of the atmosphere that is above the lower troposphere, generally 850 hPa and above.
Upper Level Disturbance A disturbance in the upper atmospheric flow pattern which is usually associated with clouds and precipitation. This disturbance is characterized by distinct cyclonic flow, a pocket of cold air, and sometimes a jet streak. These features make the air aloft more unstable and conducive to clouds and precipitation.
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.
Upper-air Weather Chart Weather maps that are produced for the portion of the atmosphere above the lower troposphere, generally at and above 850 mb. Isolines on these maps usually represent the heights of a constant pressure surface, such as the 500 mb surface.
UPR Upper
Upslope Flow Same as Orographic Lifting; 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 A fog that forms when moist, stable air is carried up a mountain slope.
UPSLP Upslope
Upstream Towards the source of flow, or located in the area from which the flow is coming.
Upstream Slope The part of the dam which is in contact with the reservoir water. On earthen dams, this slope must be protected from the erosive action of waves by rock riprap or concrete.
UPSTRM Upstream
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.
Urban and Small Stream Flood Advisory This advisory alerts the public to flooding which is generally only an inconvenience (not life-threatening) to those living in the affected area. Issued when heavy rain will cause flooding of streets and low-lying places in urban areas. Also used if small rural or urban streams are expected to reach or exceed bankfull. Some damage to homes or roads could occur.
Urban and Small Stream Flooding Flooding of small streams, streets, and low-lying areas, such as railroad underpasses and urban storm drains. This type of flooding is mainly an inconvenience and is generally not life threatening nor is it significantly damaging to property.
Urban Flash Flood Guidance A specific type of flash flood guidance which estimates the average amount of rain needed over an urban area during a specified period of time to initiate flooding on small, ungaged streams in the urban area.
Urban Flooding Flooding of streets, underpasses, low lying areas, or storm drains. This type of flooding is mainly an inconvenience and is generally not life threatening.
Urban Heat Island The increased air temperatures in urban areas in contrast to cooler surrounding rural areas.
UTC Universal Coordinated Time (same as Greenwich Mean Time)
UV Index Ultraviolet Index- This index provides important information to help you plan your outdoor activities in ways that prevent overexposure to the sun's rays. It was designed by the National Weather Service (NWS) and the Environmental Protection Agency (EPA). Unlike some countries' indices, the United States UV Index is not based upon surface observations. Rather, it is computed using forecasted ozone levels, a computer model that relates ozone levels to UV incidence on the ground, forecasted cloud amounts, and the elevation of the forecast cities. The calculation starts with measurements of current total ozone amounts for the entire globe, obtained via two satellites operated by the National Oceanic and Atmospheric Administration (NOAA). These data are then used to produce a forecast of ozone levels for the next day at various points around the country.

CategoryUV IndexTime to BurnActions to Take
Minimal0 - 260 min. +Apply SPF sunscreen.
Low3 - 445 min.Apply SPF sunscreen, wear a hat.
Moderate5 - 630 min.Apply SPF 15, wear a hat.
High7 - 915 - 24 min.Apply SPF 15 to 30, wear a hat and sunglasses. Limit midday exposure.
Very High10+10 min.Apply SPF 30; wear a hat, sunglasses, and protective clothing; limit midday exposure.
UVM Upward Vertical Motion (also known as Upward Vertical Velocity)
UVV Upward Vertical Velocity (rising air)
UWNDS Upper Winds