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a medium-range (3-7 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 days and longer).

Ultraviolet Radiation (UV)

radiation having a wavelength shorter than wavelengths of visible light and longer than those of X-rays

Universal Coordinated Time (UTC)

a global time scale based on the local time observed on the Prime Meridian; also called Greenwich Mean Time (GMT) or Zulu (Z) time

University Corporation for Atmospheric Research (UCAR)

a nonprofit, Colorado-based corporation that receives and manages funds and administrative functions for NCAR. It includes several universities that are dedicated to the advancement of atmospheric and related sciences




a colloquial term used to describe conditions in the atmosphere favorable to the creation of precipitation. This term typically is associated with the passage of surface or upper level low pressure systems, fronts, or other phenomenon where precipitation is expected.

Unstable Air

generally, air that will continue to rise and accelerate when briefly forced upward


unstable; see Unstable Air


a relatively small-scale current of air with marked upward motion



Upslope Flow (UPSLP)

air that flows toward higher terrain and, hence, is forced to rise

Upslope Fog

a fog that forms as moist air is forced up a hill or mountain.

Upslope Precipitation

precipitation that forms due to moist, stable air gradually forced upward along an elevated plain (e.g., the Rocky Mountains)


upslope; see Upslope Flow

Upstream (UPSTRM)

toward the source of the flow, or located in the area from which the flow is coming


see Upstream

Upward Vertical Velocity (UVV)

rising air


when cool, nutrient-rich water rises from deep in the ocean up to near the surface.

Upwelling Radiation

the component of radiation (either reflected solar or emitted terrestrial) directed upward from the earth’s surface; opposite of downwelling radiation




upward vertical motion; see Upward Vertical Velocity


upper level winds