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(1) combined processes (such as sublimation, fusion or melting, evaporation) which remove snow or ice from the surface of a glacier or from a snow-field; also used to express the quantity lost by these processes (2) reduction of the water equivalent of a snow cover by melting, evaporation, wind and avalanches.

ablation area

the area of a glacier where more glacier mass is lost than gained.

ablation hollows

depressions in the snow surface caused by the sun or warm, gusty wind.

ablation moraine

mound or layer of moraine in the ablation zone of a glacier; the rock has been plucked from the mountainside by the moving glacier and is melting out on the ice surface.

ablation season

period during which glaciers lose more mass than they gain; usually coincides with summer.

ablation zone

area or zone of a glacier where snow and ice ablation exceed accumulation.

absolute difference

the difference, taken without regard to sign, between the values of two variables

absolute error

the difference between the measured or inferred value of a quantity and its actual value.

absolute humidity

(1) the ratio of the mass of water vapor to the volume occupied by a mixture of water vapor and dry air (2) mass of water contained in a unit volume of moist air.


growth of a cloud or precipitation particle by the collision and union of a frozen particle (ice crystal or snowflake) with a super-cooled liquid droplet which freezes on impact.


all processes by which snow or ice are added to a glacier, this is typically the accumulation of snow, which is slowly transformed into ice; other accumulation processes can include avalanches, wind-deposited snow, and the freezing of rain within the snow pack.

accumulation area

area of a glacier where more mass is gained than lost.

accumulation season

period during which a glacier gains more mass than it loses; usually coincides with winter.

accumulation zone

area of a glacier where more mass is gained than lost.


the extent to which the readings of an instrument approach the true value of the calculated or measured quantities, supposing that all possible corrections are applied.

acid precipitation

rain or snow containing acidic substances, resulting from the atmospheric pollution mainly with sulfur and nitrogen; acid precipitation has a lower pH than unpolluted rain.

active air-cooled thermal pile

a foundation pile on which a cold air refrigeration system has been installed to remove heat from the ground.

active construction methods in permafrost

special design and construction methods used for engineering works in permafrost areas where permafrost degradation cannot be prevented.

active ice wedge

an ice wedge that is growing as a result of repeated (but not necessarily annual) winter cracking.

active layer

the layer of ground that is subject to annual thawing and freezing in areas underlain by permafrost; also known as seasonal frost.

active liquid refrigerant pile

a foundation pile on which a liquid coolant refrigeration system has been installed to remove heat from the ground.

active rock glacier

a mass of rock fragments and finer material, on a slope, that contains either an ice core or interstitial ice, and shows evidence of present movement.

active thermokarst

the process by which characteristic landforms are currently developing as a result of thawing of ice-rich permafrost or melting of massive ice.

active-layer failure

a general term referring to several forms of slope failures or failure mechanisms commonly occurring in the active layer overlying permafrost.

active-layer thickness

the thickness of the layer of the ground that is subject to annual thawing and freeing in areas underlain by permafrost.

adfreeze strength

the tensile or shear strength which has to be overcome to separate two objects that are bonded together by ice.


the process by which two objects are bonded together by ice formed between them.


when a mountain glacier's terminus extends farther downvalley than before; glacial advance occurs when a glacier flows downvalley faster than the rate of ablation at its terminus.


the horizontal transfer of air mass properties by the velocity field of the atmosphere.

advection fog

fog which forms in the lower part of a warm moist air mass moving over a colder surface (land or water).


non-gaseous substances, divided into solid particles or liquid droplets, held in suspension in the atmosphere.

aggradational ice

the additional ground ice formed as a direct result of permafrost aggradation.

air freezing index

the cumulative number of degree-days below 0° C for the air temperature during a given time period.

air mass

an extensive body of the atmosphere in which physical properties, particularly temperature and humidity, exhibit only small and continuous differences in the horizontal plane; it may extend over an area of several million square kilometers and over a height of several kilometers.

air temperature

the ambient temperature indicated by a thermometer exposed to the air but sheltered from direct solar radiation, or placed in an instrument shelter 1.5 to two meters above ground; also called surface temperature.

air thawing index

the cumulative number of degree-days above 0°C for the air temperature during a given period.