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thin plate, sheet or layer; laminae (plural).

land sky

the relatively dark appearance of the underside of a cloud layer when it is over land that is not snow covered; this term is used largely in polar regions with reference to the sky map; land sky is brighter than water sky, but is much darker than ice blink or snow blink.

lapse rate

the rate of change of any meteorological element with height.

large-scale atmospheric processes

atmospheric processes with a representative scale (large-scale) of 10,000 kilometers (6,200 miles); in meteorology, it is a scale in which the curve of the earth is not negligible; the large-scale atmospheric flows are essentially nearly hydrostatic, nearly geostrophic and wave-like in appearance; they exist mainly in response to the latitudinal differences in radiative heating, to the particular value of the coriolis force and to the spatial distribution of the oceans and continents.

latent heat of fusion

the amount of heat required to cause a change of phase from solid to liquid, or the heat released when the phase change is from liquid to solid; in the case of melting snow, the phase change from ice to water requires a significant amount of heat—160 times that required to raise the temperature of the same amount of ice by just 1 degree Celsius; until the required amount of heat is supplied to completely melt all of the ice being considered, no further increase in temperature will occur.

latent heat polynya

a polynya that forms from strong winds in a persistent direction that push the ice away from a barrier (the coast, fast ice, a grounded iceberg, or an ice shelf).

lateral moraine

a ridge-shaped moraine deposited at the side of a glacier and composed of material eroded from the valley walls by the moving glacier.

lateral talik

a layer or body of unfrozen ground, overlain and underlain by perennially frozen ground.

latitudinal limit of permafrost

the southernmost (northernmost) latitude at which permafrost occurs in a lowland region in the northern (southern) hemisphere.

latitudinal zonation of permafrost

the subdivision of a permafrost region into permafrost zones, based on the percentage of the area that is underlain by permafrost.

layered cryostructure

the cryostructure of frozen silt or loam in which ice layers alternate with mineral layers that have a massive cryostructure.


long, linear areas of open water that range from a few meters to over a kilometer in width, and tens of kilometers long; they develop as ice diverges, or pulls apart.
Lead that developed through the middle of an Arctic camp site. (Photo courtesy of Don Perovich, U.S. Army Cold Regions Research and Engineering Laboratory.)

lead shore

a lead that forms between drift ice and the coast.

leeward/lee side

the downwind side.

lens ice

ground ice occurring as ice lenses.

lens-type cryostructure

the cryostructure of frozen silt or loam containing numerous ice lenses.

level ice

floating ice with a flat surface which has never been hummocked.


general term for dry atmospheric suspensoids, including dust, haze, smoke and sand.

little ice age

a period of cooling that started around the early 14th century, peaked in the 17th century, and ended in the mid-19th century, and was marked by expanding glaciers in Europe, North America, and Asia

long-term strength

the failure strength of a material after a long period of creep deformation.

longwave radiation

heat radiation with wavelengths greater than 4 millimeters.


area of low pressure in the atmosphere.

low-center polygon

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

low-level clouds

clouds in the lower region of the atmosphere, from ground level to 6500 feet (2000 meters); includes stratus, stratocumulus, the bases of cumulus and cumulonimbus, and sometimes nimbostratus clouds.

luminous pillars

white or sometimes reddish vertical streaks of light extending from above and below the sun; they are caused by light reflected off the mirror-like surfaces of ice; most commonly seen at sunrise and sunset.

macro-scale polygons

closed, multi-sided, roughly equidimensional, patterned ground features, typically 15 to 30 meters (16 to 33 yards) across; commonly resulting from thermal contraction cracking of the ground.

magnetic 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 dip pole.

marginal crevasse

a crevasse near the side of a glacier formed as the glacier moves past stationary valley walls; usually oriented about 45 degrees up-glacier from the side wall.

marginal ice zone

a part of the seasonal ice zone that varies in width (100 to 200 kilometers, 62 to 124 miles) that extends from the ice edge into the ice pack, where waves and swells affect the ice; often characterized by highly variable ice conditions; in general, it is wider in the Antarctic than the Arctic.
A scenic view within the marginal ice zone.

marine climate

a climate dominated by the ocean; because of the moderating effect of water, sites having this climate are considered relatively mild.

marine cryopeg

a layer or body of unfrozen ground, that is perennially cryotic (T < 0 degrees Celsius), forming part of the coastal or subsea permafrost.


of, relating to, or adjacent to the sea.

mass balance

the difference between accumulation and ablation on a glacier; usually calculated on an annual basis.

mass wasting

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

massive cryostructure

the cryostructure of frozen sand in which all mineral particles are bonded together with ice.

massive ice

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