National Snow and Ice Data Center

pressure melting

melting that occurs in ice at temperatures colder than normal melting temperature because of added pressure.

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pressure ridge

process that occurs when wind, ocean currents, and other forces push sea ice around into piles that rise and form small mountains above the level sea ice surface; ridges are initially thin and transparent with very sharp edges from blocks of ice piling up; also see keels.

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pressure tendency

the character and amount of atmospheric pressure change for a three-hour or other specified period ending at the time of observation.

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pressure-melting

lowering the melting point of ice by applying pressure.

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prime meridian

the meridian (line of longitude) defined to be 0 degrees and passing through the Royal Greenwich Observatory in London; also known as the International Meridian or Greenwich Meridian; the Prime Meridian and the opposite 180th meridian (at 180 degrees longitude) separate the Eastern and Western Hemispheres.

pseudomorph

in geology, a mineral compound resulting from a process by which the primary mineral component is replaced by another, although the compound maintains constant appearance and dimensions.

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psychrometer

an instrument used for measuring the water vapor content of the atmosphere; a type of hygrometer; it consists of two thermometers, one of which (the dry bulb) is an ordinary glass thermometer, while the other (wet bulb) has its bulb covered with a jacket of clean muslin which is saturated with distilled water prior to an observation; when the bulbs are suitably ventilated, they indicate the thermodynamic wet- and dry-bulb temperatures of the atmosphere; one variety is the assman psychrometer (a special form of aspiration psychrometer for which the ventilation is provided by a suction fan).

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psychrometric tables

tables prepared from psychrometric data and used to obtain vapor pressure, relative humidity, and dew point from wet- and dry-bulb temperatures.

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puddle

an accumulation of melt water on an ice surface, mainly due to melting snow, but in later stages also to the melting of ice; the initial stage consists of patches of slush.

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snow

push moraine

moraine built out ahead of an advancing glacier.

The Columbia Glacier surged (advanced rapidly) earlier this century, part of it flowing into a forest. The push moraine in this photograph from 1914 shows Columbia Glacier literally pushing up trees and dirt as it advanced. (Photo courtesy of D.K. Handy, archived at the World Data Center for Glaciology, Boulder, CO.)

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radar

RAdio Detection And Ranging was an original active microwave sensor that became widely operational with the onset of World War II.

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radiation

(1) emission or transfer of energy in the form of electromagnetic waves (2) the process by which electromagnetic radiation is propagated through free space by virtue of joint undulatory variations in the electric and magnetic fields in space; this concept is to be distinguished from conduction and convection.

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radiosonde

instrument intended to be carried by a balloon up through the atmosphere, equipped with sensors to measure one or several meteorological variables (pressure, temperature, humidity, etc.), and provided with a radio transmitter for sending this information to the observing station.

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radius-dependent metamorphism

snow metamorphism that occurs when there are large differences in convex and concave portions of a crystal.

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snow

rafted ice

deformed sea ice in which one piece has overridden another; also called telescoped ice.

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rafting

a process by which currents or winds push around thin ice so they slide over each other; also called finger rafting.

Nilas showing finger rafting. (Photo courtesy of the Antarctic Sea-Ice Processes and Climate program (ASPeCt).)

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ram

a horizontal underwater projection of ice from an ice front, ice wall, iceberg or floe.

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randkluft

a fissure that separates a moving glacier from its headwall rock; like a bergschrund.

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rawinsonde

radiosonde which is tracked by radar or radio-theodolite to measure the wind aloft.

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reconstituted glacier

a glacier that is reconstructed or reconstituted out of other glacier material; usually formed by seracs falling from a hanging glacier then re-adhering; also called reconstructed glacier, regenerated glacier, or glacier remainie.

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reconstructed glacier

a glacier that is reconstructed or reconstituted out of other glacier material; usually formed by seracs falling from a hanging glacier then re-adhering; also called reconstituted glacier, regenerated glacier, or glacier remainie.

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recurring 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.

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red algae

algae common on temperate glaciers and perennial snow; its red color sometimes prompts people to call it watermelon snow.

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snow

reformatting

Provides the user the ability to convert the data from its native format to a different format.

regelation

motion of an object through ice by melting and freezing that is caused by pressure differences; this process allows a glacier to slide past small obstacles on its bed.

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regenerated glacier

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relative error

The ratio of the absolute error in a measurement to the size of the measurement.

relative humidity

the (dimensionless) ratio of the actual vapor pressure of the air to the saturation vapor pressure; usually expressed as a percent, and can be computed from psychrometric data.

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relative permittivity

the relative permitivity of a soil is the ratio of the permitivity of the soil to the permitivity of a vacuum.

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relict active layer

a layer of ground, now perennially frozen, lying immediately below the modern active layer; its thickness indicates the greater annual depth of thaw that occurred during a previous period.

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relict ice

ice formed in, and remaining from, the geologically recent past.

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relict permafrost

permafrost existing in areas where permafrost can not form under present climatic conditions; reflects past climatic conditions that were colder.

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reprojection

Provides the user the ability to convert the data from its native reference projection to a different reference projection.

residual stress

the effective stress generated in a thawing soil if no volume change is permitted during thaw.

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residual thaw layer

a layer of thawed ground between the seasonally frozen ground and the permafrost table.

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