National Snow and Ice Data Center

permafrost thickness

the vertical distance between the permafrost table and the permafrost base.

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permafrost zone

a major subdivision of a permafrost region.

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permanent ice zone

a region that is covered with sea ice year-round; most of the sea ice in the permanent ice zone is multiyear ice, but younger ice and open water may still be present; the permanent ice zone is what remains in summer after all melting has occurred (often called the summer minimum extent).

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sea ice

pH

power of hydrogen; a measure of the activity of hydrogen ions in solution, and therefore, its acidity or alkalinity.

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piedmont glacier

large ice lobe spread out over surrounding terrain, associated with the terminus of a large mountain valley glacier.

The massive lobe of Malaspina Glacier in Alaska is clearly visible in this photograph taken from a Space Shuttle flight in 1989. Agassiz Glacier is the smaller glacier to the left. The Malaspina Glacier is one of the most famous examples of this type of glacier, and is the largest piedmont glacier in the world. Spilling out of the Seward Ice Field (visible near the top of the photograph), it covers over 5,000 square kilometers as it spreads across the coastal plain. (Photo courtesy of SPACE.com and NASA.)

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pingo

an Inuvialuktun term for a perennial frost mound consisting of a core of massive ice with soil and vegetation cover; the size can range from a few meters to tens of meters, in both diameter and height; can be found in continuous and discontinuous permafrost zones.

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pingo ice

massive ice forming the core of a pingo.

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pingo remnant

a collapsed pingo.

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pingo scar

a pingo remnant in a contemporary non-permafrost environment.

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planetary permafrost

permafrost occurring on other planetary bodies (planets, moons, asteroids).

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plastic frozen ground

fine-grained soil in which only a portion of the pore water has turned into ice.

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Pleistocene

part of the geologic timescale, corresponding to the time period from 1.81 million to 11,550 years before the present.

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Poisson's ratio

the absolute value of the ratio between linear strain changes, perpendicular to and in the direction of a given uniaxial stress change.

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polar day

in polar regions, the portion of the year when the sun is continuously in the sky; its length changes from twenty hours at the arctic/antarctic circle (latitude 66 degrees, 34 minutes N or S) to 186 days at the north and south poles.

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polar explorer

a person working for a long time at one of the polar observing stations; Russian word is polyarnik.

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polar glacier

a glacier entirely below freezing, except possibly for a thin layer of melt near the surface during summer or near the bed; polar glaciers are found only in polar regions of the globe or at high altitudes.

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polar ice cap

a high-latitude region covered in ice; not a true ice cap, which are less than 50,000 square kilometers (12.4 million acres) and are always over land; more like an ice sheet; also called polar ice sheet.

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polar low

small, shallow depression which forms mainly in winter over some high-latitude seas within a polar or arctic air mass; its motion is approximately the same as the air stream in which it is embedded.

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polar night

in polar regions, the portion of the year when the sun does not rise above the horizon; its length changes from twenty hours at the arctic/antarctic circle (latitude 66 degrees, 34 minutes N or S) to 179 days at the North and South Poles.

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polar region

regions around the North and South Poles, north of the Arctic, or south of the Antarctic Circles, respectively; characterized by polar climate, very cold temperatures, heavy glaciation, and dramatic variations in daylight hours (24 hrs darkness in winter, 24 hrs daylight in summer).

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polar vortex

large-scale cyclonic circulation in the middle and upper troposphere centered generally in the polar regions; it is often called circumpolar vortex.

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pole of inaccessibility

in the northern hemisphere, the point in the Arctic Ocean farthest from land; in the southern hemisphere, the point on the Antarctic continent farthest from the Southern Ocean.

polyarnik

Russian word for a person working for a long time at one of the polar observing stations.

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polycrystal

a snowflake composed of many individual ice crystals.

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snow

polygon

literally means many angled; polygons are closed, multi-sided, roughly equidimensional shapes, bounded by more or less straight sides; some of the sides may be irregular; in cryospheric science, it refers to patterned ground formations.

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polygon trough

the narrow depression surrounding a high-centre polygon.

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polygonal pattern

a pattern consisting of numerous multi-sided, roughly equidimensional figures bounded by more or less straight sides.

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polygonal peat plateau

a peat plateau with ice-wedge polygons.

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polynya

irregularly shaped areas of persistent open water that are sustained by winds or ocean heat; they often occur near coasts, fast ice, or ice shelves.

Satellite view of polynyas (dark areas) near Oates Coast, Antarctica (solid white area at bottom of photo). (Photo courtesy of NASA.)

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sea ice

poorly-bonded permafrost

ice-bearing permafrost in which few of the soil particles are held together by ice.

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pore ice

ice occurring in the pores of soils and rocks.

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pore water

water occurring in the pores of soils and rocks.

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post-glacial rebound

Post-glacial rebound (sometimes called continental rebound, isostatic rebound or isostatic adjustment) is the rise of land masses that were depressed by the huge weight of ice sheets during the last ice age.

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ice sheets

pothole

a nearly vertical channel in ice that is formed by flowing water; usually found after a relatively flat section of glacier in a region of transverse crevasses; also called a moulin.

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