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thomson crystal

a large ice crystal found in deep, stagnant water-filled cavities of a glacier.
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thufur

perennial hummocks formed in either the active layer in permafrost areas, or in the seasonally frozen ground in non-permafrost areas, during freezing of the ground.
These big lumps in the ground are called thufur. Thufur can form when the ground freezes and thaws. These thufur are in Iceland. —Credit: Reinhold Richter

tide crack

the fissure at the line of junction between an immovable icefoot or ice wall and fast ice, the latter being subject to the rise and fall of the tide.

tidewater glacier

mountain glacier that terminates in the ocean.
Holgate Glacier, Kenai Peninsula, Alaska
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timber line

(1) in mountainous regions, the line above which climatic conditions do not allow the upright growth of trees (2) the poleward limit of tree growth, also known as the tree line.

tongue

a projection of the ice edge up to several km in length caused by wind and current; usually forms when a valley glacier moves very quickly into a lake or ocean.
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total annual freezing index

the cumulative number of degree-days, calculated by adding all the negative mean daily air temperatures (degrees Celsius) for a specific station during a calendar year.

total annual thawing index

the cumulative number of degree-days, calculated by adding all the positive mean daily air temperatures (degrees Celsius) for a specific station during a calendar year.

total water content (of frozen ground)

the total amount of water (unfrozen water plus ice) contained in soil or rock.

transient talik

a layer or body of unfrozen ground (in a permafrost area) that is gradually being eliminated by freezing.

tributary glacier

a small glacier that flows into a larger glacier.
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Tropic of Cancer

the northern parallel of maximum solar declination, approximately 23° 27′ N latitude; the farthest northern latitude at which the sun can appear directly overhead, which occurs on the day of the summer solstice.

Tropic of Capricorn

the southern parallel of maximum solar declination, approximately 23° 27′ S latitude; the farthest southern latitude at which the sun can appear directly overhead, which occurs on the day of the summer solstice.

tropopause

the boundary layer between the troposphere and stratosphere, where an abrupt change in temperature lapse rate usually occurs; it is defined as the lowest level at which the lapse rate decreases to 2 degrees Celsius per kilometer or less, provided that the average lapse rate between this level and all higher levels within 2 kilometers does not exceed 2 degrees Celsius per kilometer; occasionally, a second tropopause may be found if the lapse rate above the first tropopause exceeds 3 degrees Celsius per kilometer.

troposphere

lower part of the atmosphere, extending from the surface up to a height varying from about 7 to 9 kilometers (4.3 to 5.6 miles) at polar regions to approximately 17 kilometers (10.6 miles) in tropics; characterized by decreasing temperature with height, appreciable vertical wind motion, appreciable water vapor content, and weather.

trough

in meteorology, an elongated area of relatively low atmospheric pressure; the opposite of a ridge.

tundra

treeless terrain, with a continuous cover of vegetation, found at both high latitudes and high altitudes.

turbic cryosol

a mineral soil showing marked evidence of cryoturbation, as indicated by broken horizons and displaced material.

turf hummock

a hummock consisting of vegetation and organic matter with or without a core of mineral soil or stones.

turf-banked (solifluction) lobe

a solifluction lobe with its front covered by a vegetation mat.

turf-banked (solifluction) terrace

a solifluction terrace with its front covered by a vegetation mat.

two-layer permafrost

ground in which two layers of permafrost are separated by a layer of unfrozen ground.

two-phase thermosyphon

a passive heat transfer device, filled with a temperature-dependent liquid/vapour combination, installed to remove heat from the ground.

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.

valley glacier

a mountain glacier whose flow is confined by valley walls.
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vane

instrument used to measure wind direction; also called wind vane.

vapor pressure

the pressure exerted by water vapor molecules in a given volume of air.

vein ice

a comprehensive term for ice of any origin occupying cracks in permafrost.

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