Undulatus: A cloud composed of separate or merged elements and organized in undulations. Applies mainly to cirrocumulus, cirrostratus, altocumulus, altostratus, stratocumulus, and stratus.

Unstable Air: Air that does not resist vertical displacement. If it is lifted, its temperature will not cool as rapidly as the surrounding environment and so it will continue to rise on its own. See also Absolute Instability.

Upper Cold Front: The leading edge of dry air which may move ahead of the surface cold front and overrun the warm conveyor belt. Sometimes called a 'split cold front'.

Upper Trough: A trough in the contours of a pressure surface high above the ground but not necessarily present at the surface. Associated with cold unstable air aloft, and frequently follows a cold front.

Upslope Fog: Fog which forms when moist air flows up the windward side of high ground. The air is cooled below its dew point by adiabatic expansion during lifting.

Upslope Precipitation: Precipitation that forms due to moist, stable air gradually rising along an elevated plain. Upslope precipitation is common over the western Great Plains, especially east of the Rocky Mountains.

Upwelling: The rising of water (usually cold) toward the surface from the deeper regions of a body of water. If the summertime winds along the northern California coastline stopped blowing from the northwest, upwelling would diminish and the average coastal water temperature would rise between 10° and 15°F.

Urban Heat Island: The increased air temperatures in urban areas as contrasted to the cooler surrounding rural areas.

UT (Universal Time): Defined by the rotation of Earth, it used to be called GMT (Greenwich Mean Time) and is distributed by broadcast time signals.

UV (Ultraviolet) Radiation: Light whose wavelength is shorter than the visible spectrum, often associated with sunburn. It is strongly absorbed by Ozone in the stratosphere but in recent years has penetrated to low levels over polar regions, where an Ozone hole developed during the winter months. Wavelengths are longer than X-rays but shorter than visible light. Wavelengths range from 0.2 to 0.4 micrometer.