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freezing front

the advancing boundary between frozen (or partially frozen) ground and unfrozen ground.

freezing index

the cumulative number of degree-days below 0 degrees Celsius for a given time period.

freezing point

(1) the temperature at which a pure liquid solidifies under atmospheric pressure (2) the temperature at which a ground material starts to freeze.

freezing pressure

the positive pressure developed at ice-water interfaces in soil as it freezes.

freezing rain

rain, the drops of which freeze on impact with the ground or with objects on the earth's surface or with aircraft in flight.

freezing-point depression

the number of degrees by which the freezing point of an earth material is depressed below 0 degrees Celsius.

friable permafrost

permafrost in which the soil particles are not held together by ice.

friendly ice

from the point of view of the submariner, an icey canopy containing many large ice skylights or other features which permit a submarine to surface; there must be more than ten such features per 37 kilometers (30 nautical miles) along the submarine's track.


in meteorology, generally, the interface or transition zone between two air masses of different density; since the temperature distribution is the most important regulator of the atmosphere density, a front almost invariably separates air masses of different temperature; when warmer air replaces the colder, it is a warm front, and vice-versa.


the condition which exists when the temperature near the earth's surface and earth-bound objects falls below freezing (0 degrees Celsius or 32 degrees Fahrenheit).

frost action

the process of alternate freezing and thawing of moisture in soil, rock and other materials, and the resulting effects on materials and on structures placed on, or in, the ground.

frost blister

a seasonal frost mound produced through doming of seasonally frozen ground by a subsurface accumulation of water under elevated hydraulic potential during progressive freezing of the active layer.

frost boil

a small mound of soil material, presumed to have been formed by frost action.

frost bulb

a more or less symmetrical zone of frozen ground formed around a buried chilled pipeline or beneath or around a structure maintained at temperatures below 0 degrees Celsius.

frost creep

the net downslope displacement that occurs when a soil, during a freeze-thaw cycle, expands normal (perpendicular) to the ground surface and settles in a nearly vertical direction.

frost flowers

crystals of ice that form when water vapor becomes a solid (bypassing the liquid phase) and deposits itself on the sea ice surface; frost flowers roughen the surface and dramatically affect its electromagnetic signal.
Close-up view of frost flowers. (Photo courtesy of Don Perovich, U.S. Army Cold Regions Research and Engineering Laboratory.)

frost heave

the upward or outward movement of the ground surface (or objects on, or in, the ground) caused by the formation of ice in the soil.

frost heave extent

the difference between the elevations of the ground surface before and after the occurrence of frost heave.

frost jacking

cumulative upward displacement of objects embedded in the ground, caused by frost action.

frost mound

any mound-shaped landform produced by ground freezing combined with accumulation of ground ice due to groundwater movement or the migration of soil moisture.

frost penetration

the movement of the freezing front into the ground during freezing.

frost phenomena

effects of frost action on earth materials and on structures placed in or on the ground.

frost point

the highest temperature at which atmospheric moisture will sublimate in the form of hoarfrost on a cooled surface; it is analogous to the dew point.

frost shattering

the mechanical disintegration of rock by the pressure of water freezing in pores and along grain boundaries.

frost smoke

fog-like clouds, due to the contact of cold air with relatively warm water, which appear over newly-formed leads, or leeward of the ice edge, and which may persist while new ice is forming.

frost sorting

the differential movement of soil particles of different sizes as a result of frost action.

frost weathering

the disintegration and break-up of soil or rock by the combined action of frost shattering, frost wedging and hydration shattering.

frost wedging

the mechanical disintegration, splitting or break-up of rock by the pressure of water freezing in cracks, crevices, pores, joints or bedding planes.

frost-stable ground

ground (soil or rock) in which little or no segregated ice forms during seasonal freezing.

frost-stable soil

soil in which little or no segregated ice forms during seasonal freezing.

frost-susceptible ground

ground (soil or rock) in which segregated ice will form (causing frost heave) under the required conditions of moisture supply and temperature.

frost-susceptible soil

soil in which segregated ice will form (causing frost heave) under the required conditions of moisture supply and temperature.

frozen fringe

the zone in a freezing, frost-susceptible soil between the warmest isotherm at which ice exists in pores and the isotherm at which the warmest ice lens is growing.

frozen ground

soil or rock in which part or all of the pore water has turned into ice.


the phase transition of a substance passing from the solid to the liquid state, melting; in meteorology, fusion is understood to refer to the melting of ice, which, if the ice is pure and subjected to one standard atmosphere of pressure, takes place at the ice point of 0 degrees Celsius (32 degrees Fahrenheit).

gas hydrate

a special form of solid clathrate compound in which crystal lattice cages or chambers, consisting of host molecules, enclose guest molecules.