National Snow and Ice Data Center

snow

ablation area

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

Topic:

ablation hollows

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

Topic:

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.

Topic:

ablation season

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

Topic:

ablation zone

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

Topic:

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.

Topic:

accretion

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.

Topic:

accumulation

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.

Topic:

snow

accumulation area

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

Topic:

accumulation season

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

Topic:

accumulation zone

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

Topic:

accuracy

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.

Topic:

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.

Topic:

active air-cooled thermal pile

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

Topic:

active construction methods in permafrost

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

Topic:

active ice wedge

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

Topic:

active layer

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

Topic:

active liquid refrigerant pile

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

Topic:

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.

Topic:

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.

Topic:

active-layer failure

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

Topic:

active-layer thickness

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

Topic:

adfreeze strength

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

Topic:

adfreeze/adfreezing

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

Topic:

advance

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.

Topic:

advection

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

Topic:

advection fog

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

Topic:

aerosol

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

Topic:

aggradational ice

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

Topic:

air freezing index

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

Topic:

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.

Topic:

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.

Topic:

air thawing index

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

Topic: