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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 TIME COORDINATE: 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 activities, 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.; Related terms: instability and 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, the first step towards thunderstorm development.; Related term: downdraft
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.
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.; Related term: downslope effect
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.; Related term: Cheyenne Fog
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 Niño 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.