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precipitation of small balls or pieces of ice (hailstones) with a diameter ranging from 5 to 50 millimeters (0.2 to 2.0 inches), or sometimes bigger, falling either separately or agglomerated into irregular lumps; when the diameter is less than about 5 millimeters (0.2 inch), the balls are called ice pellets.


group of optical phenomena, in the form of rings, arcs, pillars or bright spots around the sun or moon, produced by the refraction or reflection of light by ice crystals suspended in the atmosphere (cirrus clouds, diamond dust, etc.).

hanging glacier

a glacier that terminates at or near the top of a cliff.
Mt. Kefton, Antarctica

hanging valley

a valley formed by a small glacier that has a valley bottom relatively higher than nearby valleys formed by larger glaciers.

hard frozen ground

frozen ground (soil or rock) which is firmly cemented by ice.


fine dust or salt particles dispersed through a portion of the atmosphere; the particles are so small that they cannot be felt or individually seen with the naked eye, but they diminish horizontal visibility and give the atmosphere a characteristic opalescent appearance that subdues all colors; a type of lithometeor.


a steep cliff, usually the uppermost part of a cirque.

heat balance

equilibrium between the gain and loss of heat at a specific place or for a specific system.

heat balance of the earth-atmosphere system

the equilibrium that exists between the radiation received by the earth and atmosphere from the sun and that emitted by the earth and atmosphere.

heat budget

relation between fluxes of heat into and out of a given region or body and the heat stored by the system; in general, this budget includes advective, evaporative, and other terms as well as a radiation term.

heat capacity

the amount of heat required to raise the temperature of a unit mass of a substance by one degree.

heat flux

the amount of heat transferred across a surface of unit area in a unit time.

heat sink

process, or region, in which energy is removed from the atmosphere in the form of heat.

heat source

process, or region, in which energy is added to the atmosphere in the form of heat.

heaving pressure

upward pressure developed during freezing of the ground.


area of high pressure in the atmosphere; used interchangeably with anticyclone.

high-center polygon

an ice-wedge polygon in which melting of the surrounding ice wedges has left the central area in a relatively elevated position.

high-level clouds

typically thin, white clouds above 6,000 meters (20,000 feet); at these altitudes, temperatures are so cold that clouds are composed primarily of ice crystals; includes cirrus, cirrocumulus and cirrostratus clouds.


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.