Cryosphere glossary
thaw unconformity
a boundary sometimes identified in perennially frozen ground, representing the base of a relict active layer, as well as the corresponding earlier permafrost table.
thaw weakening
the reduction in shear strength due to the decrease in effective stresses resulting from the generation and slow dissipation of excess pore pressures when frozen soils containing ice are thawing.
thaw-sensitive permafrost
perennially frozen ground which, upon thawing, will experience significant thaw settlement and suffer loss of strength to a value significantly lower than that for similar material in an unfrozen condition.
thaw-stable permafrost
perennially frozen ground which, upon thawing, will not experience either significant thaw settlement or loss of strength.
thawed ground
previously frozen ground in which all ice has melted.
thawing (of frozen ground)
melting of the ice in frozen ground, usually as a result of a rise in temperature.
thawing front
the advancing boundary between thawed ground and frozen ground.
thawing index
the cumulative number of degree-days above 0 degrees Celsius for a given time period.
theodolite
an optical instrument used to visually track a radiosonde balloon and determine its bearing and elevation while in flight.
thermal conductivity
the quantity of heat that will flow through a unit area of a substance in unit time under a unit temperature gradient.
thermal diffusivity
the ratio of the thermal conductivity to the volumetric heat capacity.
thermal erosion
the erosion of ice-bearing permafrost by the combined thermal and mechanical action of moving water.
thermal expansion (or contraction) coefficient
the volume change per unit volume of a substance due to a one degree change in its temperature.
thermal pile
a foundation pile on which natural convection or forced circulation cooling systems or devices have been installed to remove heat from the ground.
thermal properties of frozen ground
the properties of the ground governing the flow of heat through it, and its freezing and thawing conditions.
thermal regime of the ground
a general term encompassing the temperature distribution and heat flows in the ground and their time-dependence.
thermal talik
a layer or body of unfrozen ground (in a permafrost area) in which the temperature is above 0 degrees Celsius due to the local thermal regime of the ground.
thermal-contraction crack
a tensile fracture resulting from thermal stresses in frozen ground.
thermal-contraction-crack ice
ice formed in thermal contraction cracks in the ground.
thermo-erosional cirque
the usually steep, horseshoe-shaped headwall of a retrogressive thaw slump.