"U"

UHF: Radio frequencies above 300 MHz.
Upper Level Low: An area of cold air aloft (20,000 to 30,000 ft) which spins counter-clock-wise.
Umbra: Central dark spot of a "Sunspot" having a temperature of about 4,000 Degs. Kelvin.
Unlimeted Ceiling: Clear skies or scattered cloud conditions.
Ultra Violet Purifying Water System.

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Above picture are the working parts of a "Ultra Violet Light"
for purifying water.

Updraft: Rising air.
Upper Air Chart: Weather maps depicting the upper atmosphere. Ex. 850 mb, 500 mb, 300 mb charts.
Upper Level Low: An area of cold air aloft rotating counter-clockwise.
Upper Level Winds: Winds above the surface.
Upslope Wind: Air which rises towards high terrain.
Upward Vertical Velocity: Rising Air.
Upwelling: Water which rises in the oceans.
Upwind: When walking the wind will be from front to your back.
UTC: Universal Tome
Unstable Air: Air which rises easily.
UV Rays: (UV) Ultraviolet Radiation: Shortwave electromagnetic radition emitted by the Sun.
UV Rays: (UV) Ultraviolet Radiation: Bad between 10am and 2pm for sunbathing.