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 UKMET United Kingdom forecast model. (Available via AccuWeather.com Professional.) 
Ultraviolet radiation Electromagnetic radiation of shorter wavelength than visible radiation but longer than x-rays, between 0.02 and 0.4 micron (200 and 4000 angstrom).
Uncertainty The standard deviation of a sufficiently large number of measurements of the same quantity by the same instrument or method. The non-correctable part of the inaccuracy of an instrument, it represents the limit of measurement precision. The uncertainty of an instrument is caused by the unpredictable effects upon its performance of such factors as friction, backlash, and electronic noise.
Unprotected thermometer A reversing thermometer (for seawater temperature) which is not protected against hydrostatic pressure. The mercury bulb is therefore squeezed, and the amount of mercury broken off on reversal is a function of both temperature and of hydrostatic pressure.
Updraft A relatively small-scale, upward moving current of air.
Upper air That portion of the atmosphere which is above the lower troposphere. Generally applied to levels above 850 mb.
Upper air observation A measurement of atmospheric conditions aloft, above the effective range of a surface weather observation. Elements evaluated include temperature, humidity, pressure, wind speed, and wind direction.
Upward total radiation Solar and terrestrial radiation directed upward (away From the earth's surface); outgoing radiation.
Upwelling The rising of cold water from the deeper areas of the ocean to the surface. This phenomena often occurs along the California coast during the spring and summer.
Upwind In the direction from which the wind is blowing.
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