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.
UNIVERSAL TIME COORDINATE
One of several names for the twenty-four hour time which is used throughout the scientific and military communities.
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.
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.
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.
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.
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.