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Glossary of Arctic Transportation, Permafrost, Snow and Hydrology Terms
Most of the definitions given below were taken from the Multi-Language Glossary of Permafrost and Related Ground-Ice Terms (van Everdingen, 1998). For a full list of permafrost-related terms, visit the US Permafrost Association website.

Active Layer

The layer of ground subject to annual thawing and freezing in areas underlain by permafrost

Active-layer Detachment Slide

Shallow landslides that develop in permafrost areas, involving reduction in effective stress and strength at the contact between a thawing overburden and underlying frozen material. Active-layer detachment slides can occur in response to high seasonal air temperature, summer rainfall events, rapid melting of snowcover, or surface disturbances See Lewkowicz (1992).

Active-layer Thickness

The thickness of the layer of ground subject to annual thawing and freezing in areas underlain by permafrost (cf. thaw depth) See Nelson and Hinkel (2003).


A large depression of the ground surface produced by thawing of a large area (e.g., > 1 ha) of very thick and exceedingly ice-rich permafrost.


  • A collective term used to describe all soil movements due to frost action.
  • Irregular structures formed in earth materials by frost penetration and frost action processes, and characterized by folded, broken, and dislocated beds and lenses of unconsolidated deposits, included organic horizons, or bedrock.

  • Degree-day

    A derived unit of measurement used to express the departure of the mean temperature for a day from a given reference temperature. Also see freezing index and thawing index.

    Depth Of Zero Annual Amplitude

    The distance from the ground surface downward to the level beneath which there is practically no annual fluctuation in ground temperature.

    Excess Ice

    The volume of ice in the ground that exceeds the total pore volume that the ground would have under natural unfrozen conditions.

    Freeze-thaw Cycle

    Freezing of material, followed by thawing. The two fundamental frequencies involved are diurnal and annual.

    Freezing Index

    The cumulative number of degree-days below 0°C for a given time period (usually seasonal).

    Frost Action

    The process of alternate freezing and thawing of moisture in soil, rock, and other materials, and the resulting effects on materials and on structures placed on or in the ground.

    Frost Creep

    The net downslope displacement that occurs when a soil, during a freeze-thaw cycle, expands perpendicular to the ground surface and settles in a nearly vertical direction.

    Frost Heave

    The upward or outward movement of the ground surface (or objects on or in the ground) caused by the formation of ice in the soil.

    Frost Mound

    Any mound-shaped landform produced by ground freezing, combined with accumulation of ground ice due to groundwater movement or migration of soil moisture. Also see Nelson et al. (1992).

    Frost Penetration

    The movement of a freezing front into the ground during freezing.

    Frost Susceptible Soil

    Subsurface earth materials in which segregated ice will form (causing frost heave) under the required conditions of moisture supply and temperature.

    Frozen Ground

    Soil or rock in which part or all of the pore water has turned into ice.

    Gas Hydrate

    A special form of solid clathrate compound in which crystal lattice cages or chambers, consisting of host molecules, enclose guest molecules.


    The slow downslope flow of unfrozen earth materials over a frozen substrate. Also see solifluction.


    The study of earth materials and processes involving temperatures of 0°C or below.

    Geothermal Gradient

    The rate of temperature increase with depth below the ground surface.

    Ground Ice

    A general term referring to all types of ice contained in freezing and frozen ground.

    High-center Polygon

    An ice-wedge polygon in which melting of the surrounding ice wedges has left the central area in a relatively elevated position.

    Ice Lens

    A dominantly horizontal, lens-shaped body of ice of any dimension.

    View Example Image
    Ice Segregation

    The formation of discrete layers or lenses of segregated ice in freezing mineral or organic soils, as a result of the migration and subsequent freezing of pore water.

    Ice Wedge

    A massive, generally wedge-shaped body with its apex pointing downward, composed of foliated or vertically banded, commonly white, ice.

    Ice-wedge Polygon

    A network of ice wedges defining the boundaries of a geometric polygon in plan view.

    Intrusive Ice

    Ice formed from water injected into soils or rocks.

    Latent Heat Of Fusion

    The amount of heat required to melt all ice (or freeze all pore water) in a unit mass of soil or rock. For pure water this quantity is 334 J g-1.

    Low-center Polygon

    An ice-wedge polygon in which thawing of ice-rich permafrost has left the central area in a relatively depressed position.

    Mass Wasting (mass Movement)

    Downslope movement of soil or rock on or near the earth's surface under the influence of gravity.

    Massive Ice

    A comprehensive term used to describe large masses of ground ice, including ice wedges, pingo ice, buried ice, and large ice lenses.

    Mean Annual Air Temperature (MAAT)

    Mean annual temperature of the air, measured at standard screen height above the ground surface.

    Mean Annual Ground -surface Temperature (MAGST)

    Mean annual temperature at the surface of the ground.

    Mean Annual Ground Temperature (MAGT)

    Mean annual temperature of the ground at a specified depth.


    The ratio of the freezing or thawing index at the ground surface to that derived from air temperature records.


    Permafrost mounds ranging from about 0.5 to about 10 m in height and exceeding about 2 m in average diameter, comprising (1) aggradation forms and (2) degradation forms. The term "palsa" is a descriptive term to which an adjectival modifier prefix can be attached to indicate formative processes. See Washburn (1983) and Nelson et al. (1992).

    Patterned Ground

    A general term for any ground surface exhibiting a discernibly ordered, more or less symmetrical, morphological pattern of ground and, where present, vegetation.

    View Example Image

    The conditions, processes, and landforms associated with cold, nonglacial environments, regardless of proximity to past or present glaciers.


    Earth materials that remains continuously at or below 0°C for at least two consecutive years.

    Permafrost, Continuous

    Regions in which permafrost occurs nearly everywhere beneath the exposed land surface. At the circumpolar scale the term continuous permafrost zone refers to a broad area, crudely conformable with latitude, in which permafrost is laterally continuous. See Nelson and Outcalt (1987).

    Permafrost, Discontinuous

    Regions in which permafrost is laterally discontinuous owing to heterogeneity of material properties, subsurface water, and surface cover.

    Permafrost, Dry

    Permafrost containing neither free water nor ice.

    Permafrost, Ice-rich

    Permafrost containing excess ice.


    A perennial frost mound consisting of a core of massive ice produced primarily by injection of water, and covered with soil and vegetation.

    Retrogressive Thaw Slump

    A slope failure resulting from thawing of ice-rich permafrost.

    Seasonally Frozen Ground

    Ground that freezes and thaws annually.


    Is a particular location where an established protocol is carried out to collect snow depths and snow densities.

    Snow Survey

    Is a field campaign designed to visit multiple snow measurement sites with the intent being to characterize snow conditions (in terms of snow depth and snow density) over a given study domain.

    Snow Transect

    Is where snow depths (quantity dependent on length of transect) are collected along a linear route. This is often done in an east-west and north-south direction covering an entire watershed, or along a road.

    Snow Pit

    A pit dug in snowpack to ground surface. On the vertical profile of the snowpack wall, snow layers are identified. A density snow pit involves taking density samples along the entire vertical profile of the snow wall.


    A general term referring to the slow downslope flow of saturated unfrozen earth materials over an impermeable substrate (cf. gelifluction).

    Thaw Consolidation

    Time-dependent compression resulting from thawing of ice-rich frozen ground and subsequent draining of excess water.

    Thaw Depth

    The instantaneous depth below the ground surface to which seasonal thaw has penetrated (cf. active-layer thickness). See Nelson and Hinkel (2003).

    Thaw Lake

    A lake whose basin was formed or enlarged by thawing of frozen ground. Hopkins (1949), Washburn (1980, 271).

    Thaw Settlement

    Compression of the ground due to loss of excess ground ice and attendant thaw consolidation.

    Thawing Index (DDT)

    The cumulative number of degree-days above 0°C for given time period (usually seasonal).

    Thermal Conductivity

    The quantity of heat that will flow through a unit area of a substance in unit time under a unit temperature gradient. In permafrost investigations thermal conductivity is usually expressed in W m-1 oC-1

    Thermal Erosion

    Erosion of ice-bearing permafrost by the combined thermal and mechanical action of moving water.

    Thermal Offset

    Temperature depression in the upper layer of permafrost, resulting from the combined effects of seasonal differences of thermal conductivity and the operation of nonconductive processes in the active layer. See Williams and Smith (1989) and Nelson et al. (1985). See Goodrich (1982).

    Thermokarst Terrain

    Irregular topography resulting from the melting of excess ground ice and subsequent thaw settlement.

    Zero Curtain Effect

    Persistence of a nearly constant temperature very close to the freezing point of water during annual freezing (and occasionally thawing) of the active layer. See Outcalt et al. (1990).