RUSSIAN GLOSSARY OF SEA ICE TERMINOLOGY
Glossary Terms: E-H, I, J-R, S-Z
ABLATION OF SNOW AND ICE A decrease in the mass of snow and ice due to melting and evaporation.
ABSOLUTE ELONGATION OF ICE. See deformation of ice; absolute deformation.
ACCESSIBLE SPEED DURING VESSEL NAVIGATION IN ICE. The maximum speed that a vessel can achieve under specific ice conditions. Its value is primarily determined by the dimensions and shape of the vessel and the power of vessel’s engine.
ACCURACY OF ICE FORECASTS. An averaged error of forecasts, drawn up using a given forecasting method.
ACTIVE ICE LAYER. The surface layer of an ice sheet that is subjected to periodic melting and refreezing. At medium latitudes during the spring melt season, the development of such a layer depends on the temperature change during the day while in old ice at high latitudes it depends on the intensity of seasonal melting and refreezing.
ACTIVE RADIO DETECTION OF ICE. The remote sensing of an ice sheet with the help of electromagnetic waves. Radio detection is typically carried out by irradiating sea ice with electromagnetic waves and then analyzing the nature of the return.
At present, pulse and phase-frequency modulation methods are commonly used in the investigation of sea ice by radio detection. Radio probing of ice is based on the ability of electromagnetic waves to penetrate ice and be reflected at boundaries where there is a change in electromagnetic properties.
ACTUAL ICE NAVIGATION TIME. The period of time between the date when the first cargo vessel arrives on a given route and the date when the last vessel leaves it.
ADHESION OF ICE. The freezing of ice to foreign bodies due to the forces of intermolecular attraction. The adhesive strength (per unit area) is measured in terms of the force applied to, or the work performed on, an area of ice necessary to detach it from the associated (typically flat) surface of the other material.
ADIABATIC MODULUS OF ELASTICITY. The modulus of elasticity measured under the condition of adiabatic deformation. For instance, adiabatic conditions occur during the propagation of elastic waves when the oscillatory displacements of particles are rapid enough so that the heat exchange with the environment during the period of one oscillation is negligible. The adiabatic modulus of elasticity can be calculated with reasonable accuracy with the help of the following formula:
where E is the normal elastic modulus, EAd is the adiabatic modulus of elasticity, a is the thermal expansion coefficient, Cp is the specific heat capacity at constant pressure, and T is the ice temperature.
The difference between the adiabatic and the isothermal elastic moduli of ice is very small (only 0.1% in the case of fresh-water polycrystalline ice at a temperature of -10°C). Therefore the above term is not frequently used.
ADVECTION OF ICE. The movement of ice in a horizontal direction without any essential changes in the compactness or other properties of the ice.
AGE OF AN ICE SHEET. The period of time between the initial formation of a relatively homogeneous level continuous part of an ice cover and the time of the current observations. Qualitative terms describing the relative age of ice as indicated by its thickness are initial, black, young, one-year old and old ice.
INITIAL ICE. A primary, usually not compact layer of ice needles and plates, ice and snow crystals or ice-snow porous clumps that are lightly frozen together. The age of initial ice is expressed in hours and the thickness of initial ice does not exceed 5 cm.
Grease ice. A layer of ice needles and plates that has thickened somewhat in the course of freezing and is typically distributed over the water surface in the shape of strips or small spots. Because of its matte surface, grease ice usually appears to have a dead grayish color (Photograph 1).
New snow ice. A layer consisting of a viscous mass of water saturated snow (Photograph 2).
Slush ice (interwater ice). An agglomeration of primary needle-shaped and lamellar crystals formed within the surface water layer as the result of supercooling.
BLACK ICE (NILAS). A thin (up to 5 cm), elastic layer of ice showing a dark color. It is easily flexed by water waves. When compressed, it commonly rafts forming toothed or keyboard-like overlapping layers referred to as finger rafts (Photograph 3).
Dark ice. A rotten, snow-free ice cover (less than 5 cm thick) showing a wet brine-covered surface. This ice type develops from initial ice.
Light ice. Opaque-grayish snow-free ice (5-10 cm thick) showing numerous air-holes of various shapes.
Pancake ice. Young ice accumulations with a round or oval shape in the horizontal plane and typical diameters of 0.3 to 3.0 m. Pancakes typically have raised edges in the form of accumulations of brash ice and ice crystals which are produced as a result of oscillations between adjoining pancakes caused by waves and swell (Photograph 4). Frequently the formation of pancakes is preceded by the formation of glass ice which is then broken by swell and rounded into pancakes by intra-floe contacts.
Glass ice. A transparent crust of breakable fresh-water ice (up to 5 cm thick). Glass ice is typically rigid and brittle.
YOUNG ICE. Gray, thin ice (10 - 30 cm thick), which is easily flexed by waves. Under compression its layers commonly raft over one another ultimately forming stable unbroken multi-layered ice sheets (Photograph 5). The age of young ice varies from several hours to 25 - 30 days depending on the geographic location and the local climatic conditions during a given year.
Gray ice. Young ice with thicknesses up to 10 - 15 cm that commonly shows a gray-colored surface and contains a few round-shaped air holes. Gray ice is both less wet and less brittle than glass ice. Upon compression, its layers raft over each other, though they are also capable of hummocking. If hummocks form, they can reach heights of 10 - 15 cm.
Gray-white ice. Young gray ice with thicknesses between 15 - 30 cm that presents a grayish-white, comparatively level surface which may retain a thin snow layer. When compressed it typically forms hummocks rather than rafts. The average heights of such hummocks are 16 - 32 cm (45-50 cm at maximum).
FIRST-YEAR ICE. This is a general expression referring to white ice during its first year of existence (Photograph 6) . Ice thicknesses by the end of the winter can vary over a wide range with values between 30 - 250 cm; the exact value depends on the time of initial ice formation and on the climatic conditions in that region during the applicable time period. Commonly the snow cover on first-year ice is comparatively stable showing specific patterns of dunes and snow ridges. Fractures and cracks which may develop into leads are also common. Under compression first-year ice can develop a variety of deformation features (ridges, hummocks, ice barriers and belts). During the melt period, its surface is covered with snow melt pools, small thaw lakes and channels, which exhibit specific patterns for each first-year ice type.
First-year thin ice. Gray-white ice which is 30 - 70 cm thick. Fresh fractures are a white color. Dunes and snow ridges are elongated in the predominant wind direction. As a result, thawing sets up an elongated pattern of melt pools and small thaw lakes. Hummocks are normally 30 - 75 cm high with maximum values reaching 100 cm. In most cases hummock ridges are linear, whereas fresh fractures extend along zigzag lines. At some locations two stages of first-year thin ice can be distinguished with the first stage being 30 - 50 cm thick while the second stage is 50 - 70 cm thick.
Medium thick first-year ice. First-year ice with thicknesses from 70 to 120 cm. In non-Arctic seas, this type of first-year ice represents the limit of ice growth for extremely severe winters. The surfaces of fresh fractures are light-greenish in color. Snow ridges and dunes may be elongated in several directions. Many hummock ridges intersect, with hummock heights reaching 175 cm. This ice-type usually melts completely during the Arctic summer.
First-year thick ice. First-year ice with thicknesses ranging from 120 to 250 cm. The surfaces of fresh fractures have a greenish tinge. This ice is quite strong and only hummocks under very intense compressive stresses. Hummocks are typically 1.5 – 2.5 m high, and in some cases 3.2 – 3.5 m. A stable snow cover characteristically develops and displays a complicated pattern of dunes and snow ridges. During the thaw period, large numbers of small thaw lakes and water patches develop near the edges of hummocks.
OLD ICE. Ice that has survived melting during at least one summer. Such ice is usually covered with a thick layer of snow. Melt-smoothed hummocks, which can survive for many years, are gradually transformed into hills and hillocks. The thaw pools that normally develop from snow melt are usually rounded in outline. The color of old ice varies from greenish to light blue.
Residual first-year ice. The initial stage of the two-year ice cycle. In the northern hemisphere this stage lasts from the moment of new stable ice formation until the 1st of January or until the 1st of July in the Southern hemisphere. Residual first-year ice is similar in appearance to first-year thick ice during the period of maximum melting.
Second year ice. Sea ice during its second-year, i.e. sea ice that has survived one summer but not two. Fresh fractures are greenish - light-blue in color. The near surface layer is mostly desalinated (unlike more saline first-year ice). This ice type is distinguished by the following morphometric characteristics: higher freeboards, more rounded and uneven upper surfaces, and hummocks with typically smaller horizontal dimensions. Hummocking takes place only under very strong compression. Floes are normally edged with hummocky formations which have developed from young or first-year thin ice (Photograph 7). Thawing produces rounded, thaw pools (Photograph 8) and a few thaw patches.
Multiyear ice. Sea ice that is over two years in age. Such ice is typically made up of rounded floes that have hilly upper surfaces that originate by irregular melting during multiple thaw cycles (Photograph 9). The height of the surface topography is 1.5 - 2.5 meters. This ice type is usually covered with compact snow and is normally light blue in color. This color is typical for the so-called paleocrystic ice which occurs in the Canadian parts of the Arctic. During summer, the surface of paleocrystic ice develops rounded melt pools. Its surface drainage systems are typically well developed (Photograph 10).
Pack ice (pack). This term used to be applied to multiyear ice (sometimes it is still used that way). For instance, Canadian pack ice is in fact multiyear ice which originated in the Canadian Arctic archipelago. However, current usage of the term pack ice does not refer to the age of the ice but only to the fact that it is not fast but is drifting as the result of atmospheric and oceanic stresses applied to its upper and lower surfaces (Photograph 11).
AGE OF HUMMOCKY FORMATIONS. Qualitative estimates of the relative age of hummocky formations as determined by the overall shapes of their above-water portions, the degree of bonding between and the amount of rounding of the blocks within the hummocks, and the nature of the associated snow cover.
FRESH HUMMOCKY FORMATIONS. Newly developed, snow-free hummocky formations with sharp tops and ridges whose above-water slopes are typically 35 - 40°. Ice fragments and ice debris both on the surface and under water retain their initial angular shapes. Widths in new hummocky formations commonly exceed heights by 7-8 times. The ratio of the draft of hummock ridges to their sail height as measured relative to sea level is 5 : 1 (Photograph 12).
SMOOTHED HUMMOCKY FORMATIONS. Hummocky formations with the ridges rounded by ablation and the above-water slopes leveled. Average slope angles are 25 - 30°.
OLD HUMMOCKY FORMATIONS. Hummocky formations resembling chains of hills, considerably smoothed and solidified (Photograph 13).
AGGREGATE OF ICE CRYSTALS. A group of parallel-fibre crystals that show similar crystallographic orientations relative to the arrangements of the basal (0001) plane. The occurrence of such aggregates is typical of sea ice that has formed in the absence of appreciable wave action and under conditions where a stable, presumably steady directional current exists (Figure 1).
AIR CONTENT IN AN ICEBERG. The volume content of air bubbles in an iceberg expressed as a percent of total volume. The air content of icebergs ranges from 1 to 15% by volume.
AIR HOLE. A hole in an ice sheet that has formed as a result of either natural or artificial processes. Air holes of natural origin mostly occur in black and young ice as the result of convection in the underlying seawater (Photograph 14). Artificial air holes are made by sawing or cutting out pieces of ice and removing the fragments until a hole results.
AMORPHOUS ICE. Ice possessing no organized crystal structure. Amorphous ice is formed when vapor condenses on surfaces maintained at temperatures below -160°C.
ANISOTROPY OF MECHANICAL PROPERTIES OF ICE. Such anisotropy results from the fact that several types of crystal structures which naturally occur in sea ice can result in directionally dependent physical and mechanical properties.
Pure polycrystalline ice is distinguished by two types of "initial" anisotropy: textural anisotropy, determined by the form and arrangement of the crystal boundaries and structural anisotropy, determined by preferred C-axes crystallographic orientations. In contrast to "initial" anisotropy, which exists from the initial time of ice formation, ice may also develop induced anisotropy through deformation processes which result in recrystallization which manifests itself through shifts and rotations of individual crystals.
Sea ice sheets are also characterized by so-called transverse anisotropy in their properties (i.e., the upper part of the ice has properties that are different from that of the lower part of the ice). For instance, the fact that the upper part of the ice is usually colder than the lower part explains the differences in the in-situ cantilever beam strength as measured when the load is applied upwards causing the initial tensile failure to occur in the weaker (warmer) ice at the bottom of the sheet or is applied downwards causing the initial failure to occur in the stronger (colder) ice near the top of the sheet (Figure 2).
ANNUAL BEHAVIOR OF ICE COVER ELEMENTS. The changes in the value of an ice sheet element during one year. The annual behavior of an ice sheet element is determined either on the basis of data averaged over many years of observations or on the basis of data obtained during a given year.
ATMOSPHERIC ICE. Ice formed on a surface of an object from atmospheric water (sometimes, from the fallen snow).
AUTHOR’S LAY-OUT OF AN ICE MAP. The geographical lay-out (rough) of an ice map designed by the author without complying with the requirements of accuracy and design.
AUTONOMOUS NAVIGATION OF A VESSEL (ICEBREAKER) IN ICE. The navigation of a vessel (icebreaker) through an ice-covered region without assistance from other vessels (icebreakers).
AUXILIARY ICEBREAKING VESSEL. A specialized towed vessel designed for icebreaking operations. Such vessels may be used in combination with either a cargo vessel or an icebreaker.
BARCHAN. A snow dune having a crescentic shape in plan view with both the concave side and the steeper slope facing downwind. This type of dune is occasionally seen occurring on the surface of flat ice.
BARE ICE. Ice with a snow-free surface.
BASAL PLANES OF AN ICE CRYSTAL. Planes oriented perpendicular to the optical or C-axis of an ice crystal (Figure 1).
BASAL PLATE. A conventional name for the elementary plates (Figure 1) which constitute an ice crystal and are arranged perpendicular to the optic or C-axis.
BEARING CAPACITY OF ICE SHEET. See Supporting power of ice sheet.
BLACK ICE. (NILAS).
BORDER OF ICEBERG SPREADING. A boundary (usually on a map) separating the maximum observed equatorward locations where icebergs have been sighted from more southerly locations where icebergs have never been sighted.
BRASH ICE ZONE. Parts of an ice sheet consisting of crushed and ground-up fragments of ice either surrounding large ice-floes or located between them. Brash ice zones may comprise 30 - 35% of the total area of an ice field.
BRINE IN ICE. The liquid solution of sea salts contained in the ice at temperatures below 0°C. This brine typically occurs in the form of liquid inclusions of different sizes and shapes which are commonly elongated in the vertical direction.
BRITTLE STRENGTH OF ICE. See Strength characteristics of ice.
"BROWN" ZONES. Changes in the color of sea water in the vicinity of icebergs and along the front of shore glaciers in the Antarctic, usually attributed to intense biological activity.
CARGO VESSEL ICE CERTIFICATE. A document specifying a vessel’s navigation speed through ice based on qualities such as its ice-crossing capacity, ice-resisting strength, maneuverability in ice, and availability of special equipment and systems. The particular ice conditions that will be encountered will also be considered.
CATACLASIS OF ICE. Severe deformation from crushing and grandulation and usually accompanied by intense shearing. Cataclasis is commonly produced by grinding between floes and is frequently seen in hummock belts and ridges. .
CHANNEL See also Lead.. A widened main crack. Channels can be subdivided into narrow (50 - 200 meters wide), medium (200 - 500 meters wide), and wide (widths of over 500 meters). Traditionally, channel definitions also include the strip of open water (in some cases filled with ice fragments) left by the transit of an icebreaker or a vessel through ice (Photograph 16 ). [In English the term channel is used primarily to refer to the open water areas produced by or utilized by shipping while cracks that have become sufficiently wide that they cannot be jumped are called leads].
CLASSIFICATION OF ICE FORECASTS. Ice forecasts are classified according to their objectives, term, stages, content, as well as other features.
CLASSIFICATION OF FORECASTS BY PURPOSE
SYSTEMATIC ICE FORECASTS. Ice forecasts that are prepared on a regular basis and within fixed time periods by operative divisions using approved forecasting methods.
SPECIAL ICE FORECASTS. Ice forecasts prepared by operative divisions at the request of specific consumers of forecasting information. Special forecasts commonly are focused on a particular problem or on a region or time period where forecasts are not routinely provided.
CLASSIFICATION OF FORECASTS BY FORECASTING OBJECTIVES
RIVER ICE FORECASTS. Ice forecasts prepared for rivers, their estuaries and for coastal areas affected by river water.
SEA ICE FORECASTS. Ice forecasts for seas or their parts (regions) and for the Arctic Ocean and Antarctic seas.
CLASSIFICATION OF FORECASTS BY FORECASTING STAGES.
PRELIMINARY ICE FORECAST. An ice forecast prepared prior to the main ice forecast. A preliminary ice forecast is prepared on the basis of limited information and prognostic dependencies that give bigger errors. Therefore such forecasts have a somewhat tentative character.
MAIN ICE FORECAST. The first ice forecast for every parameter or characteristic of the structure, state or composition of an ice sheet. It also characterizes the evolution of an ice sheet. A main ice forecast is prepared in accordance with the ice forecast schedule. The term of forecast preparation, as well as its form and content are established on the basis of a special agreement concluded between the forecasting organization and the customer.
ICE FORECAST CORRECTION. An ice forecast adjusting an earlier main ice forecast. Ice forecast corrections may be prepared in accordance with the forecast preparation schedule or whenever necessary depending on the rate or intensity of ice process development and the need to adjust earlier forecasts regarding some phenomena or processes.
EMERGENCY (EPISODIC) ICE FORECAST. An ice forecast prepared in the case of a customer’s emergency request.
CLASSIFICATION OF FORECASTS BY TERM
SHORT-TERM ICE FORECASTS. Detailed ice forecasts with a term of several hours to 3 days.
MEDIUM-TERM ICE FORECASTS. An ice forecast with a term of 3 to 15 days.
LONG-TERM ICE FORECASTS. Ice forecasts with a term of 15 days to 6 months.
SUPER-LONG-TERM ICE FORECASTS. Ice forecasts with a term of 6 months to several years.
CLASSIFICATION OF FORECASTS BY THEIR CONTENT
FORECAST OF THE START OF ICE FORMATION. An ice forecast that contains information on the expected time of initial ice formation. This forecast is prepared for separate regions, localities, or for the whole area of a water body. In the latter case, the ice forecast is prepared in the form of a prognostic map showing the isochrones indicating the start of stable ice formation.
FORECAST OF THE ACHIEVEMENT OF A SPECIFIED THICKNESS. An ice forecast that contains information on the expected times when the thickness of young ice reaches 5 - 10 and 20 - 25 centimeters. Beginning from this moment, navigation requires the use of icebreakers.
ICE THICKNESS FORECAST. An ice forecast that contains information on the expected thickness of ice on a specified date.
FORECAST OF ICE THICKNESS AT THE START OF MELTING. An ice forecast that contains information on the expected maximum thickness of ice in a specified locality.
FORECAST OF THE TIME OF FAST ICE BREAK-UP AND OF THE FINAL DESTRUCTION OF THE ICE. An ice forecast that contains estimates of the expected time of initial break-up of the fast ice as well as the time of complete break-up of fast ice and the formation of drifting ice.
FORECAST OF ICE COVERAGE. An ice forecast containing information on the expected (for a given moment of time or averaged over some period of time) amount of ice, including all types of ice occurring in a given part of the sea or water area irrespective of the ice’s age, compactness, and other parameters.
FORECAST OF ICE MASSIF AREA. An ice forecast containing information on the expected (for a given moment of time or averaged over a specified period of time) area of ice fields having a compactness of 7 - 10 points.
ICE DISTRIBUTION FORECAST. An ice forecast containing information on the expected location of an ice field’s edge as well as boundaries separating ice fields with different values of compactness and age. Such forecasts also commonly contain information on the character of development and the state of large discontinuities in an ice sheet such as air-holes and clearings.
FORECAST OF THE DEGREE OF BREAK-UP. An ice forecast containing information on the expected time when a specified degree of break-up will occur in the ice in a given locality or region.
ICE DRIFT FORECAST. An ice forecast providing information on the projected direction, speed, and total distance of expected ice drift during a specified time period. Such forecasts are prepared on the basis of prognostic maps of air pressure, isobaric coefficients, and the values of angles of an ice drift deviation from the isobars. An ice drift forecast can also be prepared on the basis of information on the direction and velocity of wind, data on wind coefficients, and representative angles between drift and wind directions typical for a given region and time of year. Preparation of such forecasts also requires a knowledge of the ocean currents in the forecast area.
ICE COMPRESSION FORECAST. An ice forecast providing information on the wind-driven compression of ice in a given area of a sea or along a sea route. This forecast also takes into account the results of calculations of tidal compression and decompression.
COEFFICIENT OF DYNAMIC VISCOSITY OF ICE. The coefficient of dynamic viscosity of ice h is defined by the decrement d in the mechanical oscillations of an ice sample when measured at a frequency f and with the elastic modulus E:
h = d ´ E / (2 p ¦ 2 )
Values of h calculated using the parameters of damped oscillations are 5 to 6 orders larger than static viscosity coefficients corresponding to the plastic flow of ice.
COEFFICIENT OF ICE THERMAL CONDUCTIVITY. See thermal conductivity of ice.
COEFFICIENT OF LIGHT REFLECTION BY ICE. The ratio of the reflected intensity to the intensity of the incident light wave.
In meteorology, an ice sheet is characterized by an albedo - a photometric parameter that characterizes the ability of a matte (dull) ice surface to reflect (scatter) the incident light flux. The albedo of an ice surface is defined by the ratio of reflected radiation to that of incident radiation.
COEFFICIENT OF WIND-DRIVEN DRIFT OF ICE. The ratio of the drift velocity of the ice to that of the wind causing the drift. Average values of this parameter for the Antarctic basin are equal to 0.02.
COMPACT ICE EDGE. A comparatively narrow transition zone comprised of compact ice located between open water and a region of pack ice having a different compactness (Photograph 24).
CONDITIONAL-INSTANT ICE STRENGTH. See brittle strength of ice.
CONGELATION. This term refers to the formation of ice by the direct freezing of bulk water. More specifically the expression congelation ice refers to ice that has grown from bulk water or seawater as the result of the latent heat of freezing being conducted upward through the overlying ice to the atmosphere.
CONTINENTAL ICE. Ice formed on the land as a result of accumulation and transformation of different types of solid atmospheric precipitation.
CONTINUOUS NAVIGATION OF A VESSEL. The forward movement of a vessel through specified ice conditions without stopping.
CONVERGENCE OF ICE. See divergence of ice.
CONVOY NAVIGATION WITH RARE (FREQUENT) ICE BREAKING. Convoy navigation under complicated ice conditions that result in the occasional (frequent) complete stopping of the vessels and that require the utilization of ice breaking services by the ice-breaker in order for the convoy to continue further movement.
COVE IN ICE An area of open water that deeply penetrates into an ice field resulting in the formation of a concavity along the ice edge.
COVERAGE OF VESSELS WITH ICE. The formation of a snow/ice cushion in the area where the vessel’s body directly contacts the ice. The formation of such a snow/ice cushion results either in a decrease in the vessel’s speed or in a full stop.
CRACKS (IN AN ICE SHEET). Any fracture in fast ice, consolidated ice or in a single floe that has a width of less than 1 m (Photograph 25). [In field usage, if the fracture is narrow enough for one to jump across, it is a crack. If it cannot be jumped, it is a lead.] Cracks are formed in ice sheets as a result of the accumulation of stresses that exceed the strength of the ice. Such cracks can be subdivided in accordance with their genetic and morphologic features:
The shape along their lateral extent - straight (linear, wedge, slot-type), bent (arc, link, round-type), fractured (zigzag, sinusoidal, cycloid-type).
The shape of crack edge - smooth, uneven, notched.
The length - intrablock (up to 5 km long), interblock (up to 100 km long), main (some hundreds of kilometers long).
The penetration depth - gaping, unopened.
The age - fresh (cracks containing either clear water or primary ice types) , young (cracks covered with young ice), old (cracks covered with winter ice).
ISOSTATIC CRACKS. Cracks that form as the result of unbalanced loads from hummocks, their ridges or barriers, as well as from artificial loads.
PEELING CRACKS. The cracks that form on the border of the divide between winter ice and either old or black or young ice when the tensile strength of these latter ice types is higher than the cohesive strength of the bond attaching them to the stronger winter ice.
SHIFT CRACKS. Cracks that can develop in black ice and in other young ice types that have formed in existing leads and channels, open water areas and pools as the result of relative shifts in the positions of adjacent ice floes.
SLIDE-APART CRACKS. Cracks formed in the ice sheet as a result of bilateral compression.
SLIP CRACKS. Cracks formed in winter ice in zones where shearing is occurring.
SPLIT CRACKS. Cracks formed in an ice sheet due to compression in locations where sharp-edged protuberances of hummock ridges or extending capes of stronger ice types are encountered.
TEAR-OFF CRACKS. Cracks occurring due to the elongation of black ice or young ice, and also in ice sheets that show significant spatial heterogeneity.
THERMAL CRACKS. Cracks formed in the upper layers of an ice sheet as the result of thermal stresses produced by rapid changes in ice temperature usually resulting from rapid changes in air temperature.
TIDAL CRACKS. Cracks that form in fast ice as a result of flexural deformation that occurs associated with the rise and fall of the tides. Tidal cracks form along coast lines and also around stranded ice features such as stamukhi and other grounded obstacles.
WAVE CRACKS. These cracks form near the edges of ice sheets as the result of the effect of waves and swell. They consist of stretched longitudinal and short crossing cracks that form a polygonal pattern on the ice sheet .
CROSS-ICE CAPACITY OF A VESSEL. The ability of a vessel to move under specific ice conditions. The cross-ice capacity of a vessel under given ice conditions is expressed in terms of its speed.
CRYOHYDRATE. A mechanical mixture of ice crystals and solid salt crystals in which the latter was precipitated at temperatures below 0°C.
CRYSTALLINE HOARFROST (RIME). Fresh-water atmospheric ice which forms from water vapor in the air on the surface of an object (Photograph 26).
CRYSTALLIZATION NUCLEI. Small (10-7 – 10-3 cm) particles of mineral and organic origin whose surfaces assist in the nucleation of ice crystals from supercooled water.
CRYSTALLIZATION OF WATER. The formation of ice crystals as a result of the ordering of water molecules into the structure of the hexagonal phase Ice I. The details of the resulting crystals (shapes, sizes, orientations, etc.) depend on the cooling rate and the thermodynamic conditions of the water body.
CURVE OF LONG-TERM ICE STRENGTH. A dependence between the stress ss and the time to failure ts which can be developed from a set of creep curves (Figure 4, see also Limit of Long-Term Ice Strength). This dependence is a convenient method for describing the longevity of an ice sample placed under a given load.
For some types of fresh-water and sea ice, the following relations between ts and ss are known:
where A, n, l, and s¥ are rheologic parameters derived from data obtained through experiments on the uniaxial compression of ice or from experiments on the in situ compression of ice with the help of a down-hole expansometer. (Figure 5).
DEFORMATION MODULUS OF ICE. The ratio of stress to the relative change in the size of a sample in the direction of the force. This term was introduced to ice science by V.V. Lavrov in order to more clearly distinguish the true modulus of elasticity of ice from its quasi-properties observed during prolonged deformation in static compression or tension experiments. The apparent elasticity moduli obtained in these experiments are calculated with the help of formulae from the strength of materials theory relating ice deformation to the imposed stress. Apparent moduli are called the flexural deformation modulus, the compression deformation modulus (modulus of dilatation) and the tensile deformation modulus. These parameters strongly depend on many factors, including the experimental method used and the dimensions of the sample.
Because the behavior of ice under tension and compression is different, the flexural deformation modulus is a reduced (effective) modulus and can be expressed in terms of the elasticity modulus for compression Ecomp and the elasticity modulus for tension Etens:
It is obvious that when Ecomp = Etens, Ered = Ecomp = Etens.
Therefore, the reduced modulus of elasticity is obtained if one assumes that the material behaves in the same way whether it is subjected to compression or tension. In this connection, we recommend that experimental data on deformation moduli available in the literature not be used unless the exact experimental conditions are described.
DEGREE OF ICE COVERAGE WITH SNOW. A parameter characterizing the degree of ice surface coverage with snow. It is defined by the ratio of the area of snow-covered ice to the total area of an ice sheet. It is typically assumed that the distribution pattern of snow-covered areas is uniform.
DEGREE OF ICE SHEET POLLUTION . A parameter characterizing the degree that an ice sheet is polluted. It is defined as the ratio of the area of polluted ice to that of ice characterized by a uniform distribution of pollutants and it is measured in points.
DE-ICING OF A WATER AREA. The disappearance of ice from a part or the whole of a water area due either to the complete melting of the ice or to its drifting away from the area under observation leaving open water.
External evidence that the de-icing process is underway is the thinning of the ice sheet, a process that can continue until the sheet disappears as the thickness becomes zero. However, in most cases, thinning is associated with decreases in concentration and with associated lateral melting as well as with increased movement of the ice. The sea ice in the foreground of Photograph 19 is in the last stages of the de-icing process.
DESALTING (DESALINATION) OF SEA ICE. The process or processes by which the amount of sea salts in the ice decreases.
DESTRUCTION OF THE ICE SHEET. The process by which a uniform ice sheet is broken up under the effect of external forces resulting in a decrease in the horizontal dimensions of compact ice formations (see Photograph 27).
DESTRUCTIVE ICE LAYER. A layer of an ice sheet with a structure resulting from selective internal melting caused by penetrating solar radiation. Typically such layers are in the upper part of the ice sheet.
DIRTY ICE. Any ice type which contains a sufficient amount of mineral or organic inclusions on either its surface or internally to cause the ice to have a dirty appearance (Photograph 28).
DISCHARGE OF ICE STRESS. A sharp decrease of stress in a local ice volume with no obvious change in its continuity near the sites where the detectors are installed.
In the case of drifting ice fields, dynamic cracking will commonly be the source of such stress discharges, and in the case of thermal effects - thermal cracks. Stress discharges may be measured by a stress detector or a strain gauge installed in the ice. In the latter case, the discharged stress Ds is defined by the equation:
where e is the relative discharge deformation and E is the modulus of elasticity of the ice.
DIVERGENCE OF ICE. The term divergence refers to a differential characteristic of a vector field that is used to describe the relative drift velocities of ice. The divergence characterizes the variation of the displacement vector in the vicinity of an observation point. It belongs to the class of volume derivatives of vector fields:
,
where vx, vy, and vz are the vector projections of the drift velocity V onto the coordinate frame axes.
A positive divergence signifies an increase of an ice area due a decrease in compactness as the floes move further apart. This is usually associated with a lowering of the in-ice stress. A negative divergence (convergence) indicates that the area of an ice field is decreasing as the floes move closer together. Convergence is typically accompanied by deformation such as ridging and rafting and/or an increase in the degree of compactness and in the in-ice stress..
DRIFT DIVIDE. A linear, elongated area containing finely fragmented ice-floes (Photograph 53). The drift divide area separates two ice fields or an ice field and fast ice and is characterized by a rapidly changing drift velocity gradient. Due to a decrease in compression, the drift divide may develop cracks, pools of open water and, in some cases, areas of rarefied ice. Drift divides can be areas of intense deformation (Photograph 59)
DYNAMIC SEA ICE. Sea ice of hydrospheric origin which accumulates on objects due to the freezing of splashes and droplets of sea-water having a salt content by weight of more than 24.7 pro mille (‰). This ice type characteristically occurs as the result of the collision of waves with the marine structures.
DYNAMO-ELASTIC PARAMETERS OF ICE. A set of experimental characteristics measured by observing the elastic behavior of ice (i.e. when the load is applied for a time that is sufficiently short so that the time of loading does not exceed the stress relaxation time). Dynamo-elastic parameters of ice are obtained on the basis of experimental values of velocities of longitudinal and transverse waves where the velocities are measured by high-frequency acoustic methods.