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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.

Topic: 
climatology and meteorology

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.
Topic: 
snow

unfrozen ground

soil or rock that does not contain any ice.
Topic: 
frozen ground or permafrost

unfrozen water content

the amount of unfrozen (liquid) water contained in frozen soil or rock.
Topic: 
frozen ground or permafrost

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).
Topic: 
frozen ground or permafrost

unstable air

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

Topic: 
climatology and meteorology

upper air observation

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

Topic: 
climatology and meteorology

upper atmosphere

the general term applied to the atmosphere above the mesopause.

Topic: 
climatology and meteorology

upward freezing

the advance of a freezing front upwards from the permafrost table during annual freezing of the active layer.
Topic: 
frozen ground or permafrost