National Snow and Ice Data Center

nilas

a thin sheet of smooth, level ice less than 10 centimeters (4 inches) thick; appear darkest when thin.

Nilas are the dark sheets of ice near the bottom of the photo. (Photo courtesy of Ted Maksym, United States Naval Academy.)

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nimbostratus

a principal cloud type (cloud genus); gray and often dark; rendered diffuse by more or less continuously falling rain, snow, sleet, etc. of the ordinary varieties and not accompanied by lightning, thunder, or hail; precipitation in most cases reaches the ground; may or may not merge with low, ragged clouds that frequently occur below.

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nip

ice is said to nip when it forcibly presses against a ship which is beset; a vessel so caught, though undamaged, is said to have been nipped.

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noncryotic ground

soil or rock at temperatures above 0 degrees Celsius.

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nonsorted circle

a patterned ground form that is equidimensional in several directions, with a dominantly circular outline which lacks a border of stones.

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nonsorted net

a patterned ground with cells that are equidimensional in several directions, neither dominantly circular nor polygonal, and lacking borders of stones.

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nonsorted polygon

a patterned ground form that is equidimensional in several directions, with a dominantly polygonal outline which lacks a border of stones.

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nonsorted step

a patterned ground feature with a step-like form and a downslope border of vegetation embanking an area of relatively bare ground upslope.

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nonsorted stripe

form patterned ground with a striped and nonsorted appearance, due to parallel strips of vegetation-covered ground and intervening strips of relatively bare ground, oriented down the steepest available slope.

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North American high

the relatively weak general area of high pressure which, as shown on mean charts of sea-level pressure, covers most of North America during winter; this pressure system is not nearly as well-defined as the analogous Siberian high.

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North Atlantic Oscillation

an oscillation in the strength of the Icelandic Low and Azores High, the two dominant surface pressure features in the North Atlantic. When both are unusually strong, the North Atlantic Oscillation (NAO) is in its positive phase; when both are unusually weak, it is in its negative phase. The NAO has climate impacts not just in the Arctic, but in North America and Europe. The NAO, identified by Sir Gilbert Walker in the 1920s, is similar to the Arctic Oscillation.

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north pole

90° N latitude; one of the two points where the Earth's axis of rotation meets the Earth's surface (the other being the south pole, diametrically opposite).

nunatak

a rocky crag or small mountain projecting from and surrounded by a glacier or ice sheet.

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observation

a weather or meteorological observation is an evaluation of one or more meteorological elements that describes the state of the atmosphere either at the earth's surface or aloft.

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observational network

a group of stations (surface meteorological, upper-air, or other) spread over a given area for making regular observations.

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ogives

alternate bands of light and dark ice seen on a glacier surface.

Ogives, Juno Icefield, Alaska

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old ice

sea ice more than 2-years-old, up to 3 meters (10 feet) or more thick; hummocks on old ice are even smoother than in second-year ice, and the ice is almost salt-free; when old ice is bare of snow, it is blue and lacks the greenish tint of second-year ice.

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old snow

deposited snow whose transformation into firn is so far advanced that the original form of the ice crystals can no longer be recognized.

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snow

onshore permafrost

permafrost occurring beneath exposed land surfaces.

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open lead

a lead that connects two open bodies of water; ships can traverse between them through this lead; it also refers to a lead where open water is found, or a lead that has not completely frozen.

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open pack ice

composed of floes seldom in contact and with many leads; ice cover 4/10ths to 6/10ths.

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open talik

a body of unfrozen ground that penetrates the permafrost completely, connecting suprapermafrost and subpermafrost water.

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open water

a large area of freely navigable water in which floes may be present in concentration under 1/10th; if there is no sea ice present, the area may be termed open water, even though icebergs are present.

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open-cavity ice

ice formed in an open cavity or crack in the ground by reverse sublimation of water vapour.

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open-system freezing

freezing that occurs under conditions that allow gain or loss of water by the system.

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open-system pingo

a pingo formed by doming of frozen ground; caused by groundwater that moves downslope through taliks and onto the pingo surface, where it freezes.

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orbiculic cryogenic fabric

a distinct soil micromorphology, resulting from the effects of freezing and thawing processes, in which coarser soil particles form circular to ellipsoidal patterns.

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organic cryosol

an organic soil having a surface layer containing more than 17% organic carbon by weight, with permafrost within 1 meter (3.3 feet) below the surface.

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oriented lake

one of a group of lakes possessing a common, preferred, long-axis orientation.

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outburst flood

any catastrophic flooding from a glacier; may originate from trapped water in cavities inside a glacier or at the margins of glaciers or from lakes that are dammed by flowing glaciers.

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outlet glacier

a valley glacier which drains an inland ice sheet or ice cap and flows through a gap in peripheral mountains.

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ozone

a nearly colorless (but faintly blue) gaseous form of oxygen, with a characteristic odor like chlorine; has a formula of O3 and a molecular weight of 48; found in trace quantities in the earth's atmosphere at all times, primarily in the stratosphere between heights of about 10 to 50 kilometers (6 to 31 miles; the ozonosphere or ozone shield) where its production results from photochemical processes involving ultraviolet radiation; its maximum concentration occurs between 20 to 25 kilometers (12 to 16 miles); in the lower atmosphere, ozone is commonly formed as a product of electrical discharges through the air.

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ozone shield

stratospheric ozone layer, giving protection to the earth's surface due to intense absorption of harmful solar ultraviolet radiation by the gas.

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ozonosphere

stratospheric ozone layer, giving protection to the earth's surface due to intense absorption of harmful solar ultraviolet radiation by the gas.

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pack ice

ice that is not attached to the shoreline and drifts in response to winds, currents, and other forces; some prefer the generic term drift ice, and reserve pack ice to mean drift ice that is closely packed.

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