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ultraviolet radiation

electromagnetic radiation of shorter wavelength than visible radiation but longer than x-rays; roughly, radiation in the wavelength interval from 10 to 4000 angstroms.

unbreakable crust

a hard snow surface lying upon a softer layer; may be formed by sun, rain or wind, and is described as breakable crust or unbreakable crust, depending upon wether it will break under the weight of a turning skier.
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unfrozen ground

soil or rock that does not contain any ice.

unfrozen water content

the amount of unfrozen (liquid) water contained in frozen soil or rock.

uniaxial compression test

a method to determine the short-term strength of rocks; conducted by employing a constant loading rate until failure; the failure load is used to calculate the failure stress; can also be used to determine the elastic constants of rocks (i.e. Young's modulus, Poisson's ratio).

unstable air

air in which static instability prevails; this condition is determined by the vertical gradients of air temperature and humidity.

upper air observation

an observation made in the free atmosphere either directly or indirectly.

upper atmosphere

the general term applied to the atmosphere above the mesopause.

upward freezing

the advance of a freezing front upwards from the permafrost table during annual freezing of the active layer.