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Glossary
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the removal of ice accumulation on aircraft, ships and other objects by mechanical, thermal or chemical devices.
dead iceany part of a glacier which has ceased to flow; dead ice is usually covered with moraine.
debris flowa 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.
deformabilitythe 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.
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.
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.
delayed strengththe failure strength of a material at a given strain rate or after a given period under deviatoric stress.
dendritehexagonal ice crystals with complex and often fernlike branches.
density of frozen groundthe mass of a unit volume of frozen soil or rock.
depressionin meteorology, an area of low pressure (i.e. a low); usually applies to a particular stage in the development of a cyclone.
depth hoara 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.
depth of seasonal frost penetrationthe maximum thickness of the seasonally frozen layer.
depth of snowvertical 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.
depth of thawthe 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.
depth of zero annual amplitudethe distance from the ground surface downward to the level beneath which there is practically no annual fluctuation in ground temperature.
desiccation crackcrack or fissure developed in fine-grained soil material as a result of shrinkage during drying.
desiccation polygonclosed, multi-sided, patterned ground feature formed by desiccation cracks in fine-grained soil material, usually less than 2 meters (6.6 feet) in diameter.
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.
design freezing indexthe 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.
design thawing indexthe 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.
destructive metamorphismsnow metamorphism that rounds the comers and edges of an ice crystal.
detachment failurea slope failure in which the thawed or thawing portion of the active layer detaches from the underlying frozen material.
dew pointthe 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.
diamond dusta 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).
dielectric constantmeasure of the ability of the soil to store electrical energy in the presence of an electrostatic field.
dilation cracka tensile fracture in a frozen material due to surface extension caused by doming.
dilation crack iceice that forms in dilation cracks.
dip poleeither 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.
dirt conea 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.
dirty iceice that contains sediments stirred up and tangled in the ice as it grows.
permafrost occurring in some areas beneath the exposed land surface throughout a geographic region where other areas are free of permafrost.
discontinuous permafrost zonethe 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.
disequilibrium permafrostpermafrost that is not in thermal equilibrium with the existing mean annual surface or sea-bottom temperature and the geothermal heat flux.
drain channelpreferred path for meltwater to flow from the surface through a snow cover.
drift glaciera semipermanent mass of firn formed by drifted snow behind obstructions or in the ground; also called a catchment glacier or a snowdrift glacier.
drift iceice that moves from winds, currents, or other forces.
drift of icedisplacement of a sea ice field from its place of origin under the effect of ocean currents and winds.
drifting buoyfloating (or drifting on ice) ocean buoy equipped with meteorological and/or oceanographic sensing instruments linked to transmitting equipment for sending the observed data to collecting centers.
drifting snowsnow raised from the surface of the earth by the wind to a height of less than 1.5 to 2.0 meters (5 to 6.6 feet) above the surface; it dose not restrict horizontal visibility at 2 meters (6.6 feet) or more above the surface.
drifting stationsresearch stations on the floes of the arctic ocean.
drizzlevery small, uniformly distributed water drops that appear to float while following air currents; unlike fog droplets, drizzle falls to the ground.
drumlinremnant elongated hills formed by historical glacial action; it is not clear exactly how they are formed and why they form only in some glaciated regions.
trees leaning in random directions caused by melting permafrost.
dry densitythe mass of a unit volume of dried material (e.g. soil).
dry frozen groundfrozen ground with a very low total water content consisting almost completely of interfacial water, and not cemented by ice.
dry permafrostpermafrost containing neither free water nor ice.
dump morainea mound or layer of moraine formed along the edge of a glacier by rock that falls off the ice; sometimes called a ground moraine.
dynamic modulus of elasticitythe ratio of stress to strain for a material under dynamic loading conditions.
dynamic Poisson's ratiothe absolute value of the ratio between the linear strain changes, perpendicular to and in the direction of a given uniaxial stress change, respectively, under dynamic loading conditions.
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