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a deposit of interlocking ice crystals (hoar crystals) formed by direct sublimation on objects, usually those of small diameter freely exposed to the air, such as tree branches, plant stems and leaf edges, wires, poles, etc.; the surfaces of these objects are sufficiently cooled, mostly by nocturnal radiation, to cause the direct sublimation of the water vapor contained in the ambient air.


a peak or pinnacle thinned and eroded by three or more glacial cirques.
The Matterhorn in Switzerland was carved away by glacial erosion. (Photo courtesy of the World Data Center for Glaciology, Boulder, CO.)

hostile ice

from the point of view of the submariner, an ice canopy containing no large ice skylights or other features which permit a submarine to surface.


(1) water vapor content of the air. (2) some measure of the water-content of air; see also absolute humidity, relative humidity, specific humidity, dew point.


(1) [sea ice] a smooth hill of ice that forms on the sea ice surface from eroding ridges, particularly during the summer melt; the formation of hummocks is similar to young mountain peaks with steep slopes that erode into smooth, rolling hills. (2) [frozen ground] Small lumps of soil pushed up by frost action, often found in uniformly spaced in large groups. Hummocks can form in areas of permafrost or seasonally frozen ground, and are one of the most common surface features of the Arctic.
Hummocks make the sea ice surface appear as rolling hills. (Photo courtesy of Ted Maksym, United States Naval Academy.)


[sea ice] pressure process by which floating ice becomes broken up into hummocks.

hydration shattering

a form of weathering that affects all rocks; water freezes in pores and cracks, which leads to an increase in specific volume (vol/unit mass) of the water, producing stress that is greater than the tensile strength of all common rocks; ultimately leads to shattering and fracturing of the rocks.

hydraulic conductivity

the volume of fluid passing through a unit cross section in unit time under the action of a unit hydraulic potential gradient.

hydraulic diffusivity

the ratio of the hydraulic conductivity and the storage capacity of a groundwater aquifer.

hydraulic thawing

artificial thawing (and removal) of frozen ground by the use of a stream or jet of water under high pressure.

hydrochemical talik

a layer or body of cryotic (but unfrozen) ground in a permafrost area, maintained by moving mineralized groundwater.


any product of condensation or sublimation of atmospheric vapor, whether formed in free atmosphere or at the earth's surface; also any water particles blown by the wind from the earth's surface.

hydrothermal talik

a layer or body of noncryotic unfrozen ground in a permafrost area, maintained by moving groundwater.


an instrument which measures the water vapor content in the atmosphere; there are several different means of transduction used in measuring this quantity and hence various types of hygrometers; these are: a) the psychrometer, which utilizes the thermodynamic method; b) the class of instruments which depends upon a change of physical dimension due to absorption of moisture (hair hygrometer, for example); c) those which depend upon condensation of moisture (dew point hygrometer); d) the class of instruments which depend upon the change of chemical or electrical properties due to absorption of moisture, and some others.


the solid crystalline form of water.

ice apron

a mass of ice adhering to a mountainside.

ice area

the region covered by sea ice, typically given in square kilometers. For a grid cell, it is the area of the portion of the cell covered by ice. For an entire hemisphere, it is the total area covered by ice, which corresponds to sum of the area of each cell multiplied by the fractional concentration for that cell.


ice blink

white glare on the underside of low clouds indicating presence of ice which may be beyond the range of vision.

ice cake

a floe smaller than 20 meters (66 feet) across.

ice canopy

pack ice from the point of view of the submariner.

ice cap

a dome-shaped mass of glacier ice that spreads out in all directions; an ice cap is usually larger than an icefield but less than 50,000 square-kilometers (12 million acres).
Ellesmere Island, Canada

ice cave

a natural bedrock cave that contains year-round ice.

Photograph of the Great Hall of Scarisoara Ice Cave in Romania

This photograph shows a 22-meter thick ice block along with ice stalactites and stalagmites in the Great Hall of Scarisoara Ice Cave, Romania.  (Photograph courtesy Aurel Persoiu)


ice cluster

a concentration of sea ice, covering 100's of square kilometers, which is found in the same region every summer.

ice concentration

the fraction of an area that is covered by sea ice. Ice concentration typically is reported as a percentage (0 to 100 percent ice), a fraction from 0 to 1, or sometimes in tenths (0/10 to 10/10).


ice content

the amount of ice contained in frozen or partially frozen soil or rock.

ice core

a core sample drilled from the accumulation of snow and ice over many years that have recrystallized and have trapped air bubbles from previous time periods, the composition of which can be used to reconstruct past climates and climate change; typically removed from an ice sheet (Antarctica and Greenland) or from high mountain glaciers elsewhere.

ice covered

land overlaid at present by a glacier is said to be covered; the alternative term glacierized has not found general favour.

ice divide

the boundary separating opposing flow directions of ice on a glacier or ice sheet.

ice edge

the boundary at any given time between open water and sea, river or lake ice of any kind, whether drifting or fast; may be termed compacted when it is clear-cut, or open when it forms the indefinite edge of an area of dispersed ice.
Aerial view of the sea ice edge. (Photo courtesy of Todd Arbetter, National Snow and Ice Data Center.)

ice extent

the total area covered by some amount of ice, including open water between ice floes; ice extent is typically reported in square kilometers. Extent defines a region as either "ice-covered" or "not ice-covered." A threshold determines this labeling. A typical threshold is 15 percent, meaning that if the data cell has greater than 15 percent ice concentration, the cell is labeled as "ice-covered." Sea Ice Index products have a threshold of 15 percent. A threshold can also be as high as 30 percent.

ice floe

a cohesive sheet of ice floating in the water; the sea ice cover is made up of conglomerates of floes; ice floes are not unique to sea ice, as they also occur in rivers and lakes.
Aerial view of ice floes.

ice fog

a suspension of numerous minute ice crystals in the air, reducing visibility at the earth's surface; the crystals often glitter in the sunshine; ice fog produces optical phenomena such as luminous pillars and small haloes.

ice fringe

a very narrow ice piedmont, extending less than about 1 km inland from the sea.

ice front

the vertical cliff forming the seaward face of an ice shelf or other floating glacier, varying in height from 2 to 50 meters (2.2 to 55 yards) above sea level.

Ice gland

A column of ice in the granular snow at the top of a glacier.

ice island

a form of tabular berg found in the Arctic Ocean, with a thickness of 30 - 50 meters (33 to 55 yards) and an area from a few thousand square meters to 500 square kilometers (123,550 acres); ice islands often have an undulating surface, which gives them a ribbed appearance from the air.