National Snow and Ice Data Center

dead ice

any part of a glacier which has ceased to flow; dead ice is usually covered with moraine.

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debris flow

a sudden and destructive variety of landslide, in which loose material on a slope, with more than 50% of particles larger than sand size, is mobilized by saturation and flows down a channel or canyon.

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deformability

the ability of a material to change its shape or size under the influence of an external or internal agency, such as stress, temperature, or pore pressure.

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degree of saturation

(1) the total degree of saturation of frozen soil is the ratio of the volume of ice and unfrozen water in the soil pores to the volume of the pores (2) the degree of saturation of frozen soil by ice is the ratio of the volume of ice in the soil pores to the volume of the pores.

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degree-day (C or F)

a derived unit of measurement used to express the departure of the mean temperature for a day from a given reference (or base) temperature.

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delayed strength

the failure strength of a material at a given strain rate or after a given period under deviatoric stress.

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dendrite

hexagonal ice crystals with complex and often fernlike branches.

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snow

density of frozen ground

the mass of a unit volume of frozen soil or rock.

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depression

in meteorology, an area of low pressure (i.e. a low); usually applies to a particular stage in the development of a cyclone.

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depth hoar

a kind of hoarfrost; ice crystals that develop by sublimation within a layer of dry snow; characterized by rapid recrystallization, usually caused by large temperature gradients; similar in physical origin to crevasse hoar; crystals are faceted, rather than rounded.

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snow

depth of seasonal frost penetration

the maximum thickness of the seasonally frozen layer.

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depth of snow

vertical interval between the top surface of a snow layer and the ground beneath; the layer is assumed to be evenly spread over the ground which it covers.

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depth of thaw

the minimum distance between the ground surface and frozen ground at any time during the thawing season in an area subject to seasonal freezing and thawing.

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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.

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desiccation crack

crack or fissure developed in fine-grained soil material as a result of shrinkage during drying.

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desiccation polygon

closed, multi-sided, patterned ground feature formed by desiccation cracks in fine-grained soil material, usually less than 2 meters (6.6 feet) in diameter.

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design depth of frost penetration

(1) (North American usage) the mean of the three largest depths of seasonal frost penetration measured during the past thirty years, or the largest depth of seasonal frost penetration beneath a snow-free soil surface measured during the past ten years (2) (Russian usage) the mean of the depths of seasonal frost penetration measured during at least the last ten years with the ground surface free of snow and the groundwater level below the depth of seasonal frost penetration.

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design freezing index

the cumulative number of degree-days below 0 degrees Celsius, calculated by taking the average of the seasonal freezing indices for the three coldest winters in the most recent 30 years of record.

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design thawing index

the cumulative number of degree-days above 0 degrees Celsius, calculated by taking the average of the seasonal thawing indices for the three warmest summers in the most recent 30 years of record.

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destructive metamorphism

snow metamorphism that rounds the comers and edges of an ice crystal.

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snow

detachment failure

a slope failure in which the thawed or thawing portion of the active layer detaches from the underlying frozen material.

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dew point

the temperature to which a given parcel of air must be cooled at constant pressure and constant water-vapor content in order for saturation to occur; when this temperature is below 0 degrees Celsius, it is called the frost point.

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diamond dust

a type of precipitation composed of slowly falling, very small, unbranched crystals of ice which often seem to float in the air; it may fall from a high cloud or from a cloudless sky, it usually occurs under frosty weather conditions (under very low air temperatures).

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dielectric constant

measure of the ability of the soil to store electrical energy in the presence of an electrostatic field.

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dilation crack

a tensile fracture in a frozen material due to surface extension caused by doming.

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dilation crack ice

ice that forms in dilation cracks.

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dip pole

either of the two points on the earth's surface at which magnetic meridians converge; the horizontal component of the magnetic field of the earth becomes zero at this point; also called the magnetic pole.

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dipole anomaly

An Arctic atmospheric pressure pattern characterized by high pressure centered over the northern Beaufort Sea and unusually low pressure centered over the Kara Sea. This pattern contributes to ice loss in summer, causing winds to blow ice away from the coast.

dirt cone

a cone-shaped formation of ice that is covered by dirt; a dirt cone is caused by a differential pattern of ablation between the dirt covered surface and bare ice.

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dirty ice

ice that contains sediments stirred up and tangled in the ice as it grows.

(Photo courtesy of Terry Whitledge, National Oceanic and Atmospheric Administration/Department of Commerce.)

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sea ice

discontinuous permafrost

permafrost occurring in some areas beneath the exposed land surface throughout a geographic region where other areas are free of permafrost.

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discontinuous permafrost zone

the major subdivision of a permafrost region in which permafrost occurs in some areas beneath the exposed land surface, whereas other areas are free of permafrost.

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disequilibrium permafrost

permafrost that is not in thermal equilibrium with the existing mean annual surface or sea-bottom temperature and the geothermal heat flux.

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