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.
UCAR
operates the National Center for Atmospheric Research (NCAR) with National Science Foundation sponsorship.
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 Nino 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.
