| Page 7 | National Snow and Ice Data Center

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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.

Topic:

climatology and meteorology

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.

Topic:

depth of seasonal frost penetration

the maximum thickness of the seasonally frozen layer.

Topic:

frozen ground or permafrost

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.

Topic:

climatology and meteorology

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.

Topic:

frozen ground or permafrost

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.

Topic:

frozen ground or permafrost

desiccation crack

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

Topic:

frozen ground or permafrost

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.

Topic:

frozen ground or permafrost

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.

Topic:

frozen ground or permafrost

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.

Topic:

frozen ground or permafrost

climatology and meteorology

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.

Topic:

frozen ground or permafrost

climatology and meteorology

destructive metamorphism

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

Topic:

detachment failure

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

Topic:

frozen ground or permafrost

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.

Topic:

climatology and meteorology

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).

Topic:

climatology and meteorology

dielectric constant

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

Topic:

frozen ground or permafrost

dilation crack

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

Topic:

frozen ground or permafrost

dilation crack ice

ice that forms in dilation cracks.

Topic:

frozen ground or permafrost

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.

Topic:

climatology and meteorology

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.

Topic:

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.)

Topic:

discontinuous permafrost

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

Topic:

frozen ground or permafrost

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.

Topic:

frozen ground or permafrost

disequilibrium permafrost

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

Topic:

frozen ground or permafrost

draft

the thickness of sea ice extending below the water level. Draft and freeboard comprise total sea ice thickness.

sea ice thickness diagram

Freeboard and draft comprise sea ice thickness. Image courtesy Walt Meier.

Topic:

drain channel

preferred path for meltwater to flow from the surface through a snow cover.

Topic:

drift glacier

a semipermanent mass of firn formed by drifted snow behind obstructions or in the ground; also called a catchment glacier or a snowdrift glacier.

Topic:

drift ice

ice that moves from winds, currents, or other forces.

Topic:

drift of ice

displacement of a sea ice field from its place of origin under the effect of ocean currents and winds.

Topic:

climatology and meteorology

drifting buoy

floating (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.

Topic:

climatology and meteorology

drifting snow

snow 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.

Topic:

climatology and meteorology

drifting stations

research stations on the floes of the arctic ocean.

Topic:

climatology and meteorology

drizzle

very small, uniformly distributed water drops that appear to float while following air currents; unlike fog droplets, drizzle falls to the ground.

Topic:

climatology and meteorology

drumlin

remnant 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.

Drumlin field. Manitoba, Canada.

Topic:

drunken forest

trees leaning in random directions caused by melting permafrost.

Topic:

frozen ground or permafrost

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