Glossary_5

RUSSIAN GLOSSARY OF SEA ICE TERMINOLOGY

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Glossary Terms: A-D, E-H, I, J-R

S

SALINITY OF SEA ICE. A parameter providing an estimate of the chemical composition of seawater based on the available sum of the ions of chlorine, bromine, fluorine, sulfate, bicarbonate, sodium, potassium, magnesium and calcium in the water. The measurement is usually made by determining the electrical conductivity and temperature of the solution and is expressed as parts per thousand (‰) [grams of "salt" per kilogram of solution]. Near surface salinities in the polar oceans are commonly in the range of 32–34 ‰ while sea ice salinities are commonly in the range of 3–10‰. Salinity determinations on sea ice are performed on completely melted samples.

SALTING OF ICE. The increase in the salinity of sea ice resulting from the infiltration of sea water into the surface layers of an ice sheet due to distortions in the isostatic balance (e.g. depressions of the surface of the ice sheet to below sea level as the result of loading by a heavy snow pack on the surface of the ice). Salting can also occur in the desalinated lower layers of the ice after the termination of the summer melt season.

SCALE EFFECTS ON ICE SAMPLE STRENGTH. This effect is the result of the dependence of the strength of an ice samples upon their size with smaller samples generally being stronger than larger samples.

V.V. Lavrov has suggested a formula which allows one to calculate the flexural strength s 1 of an ice sample with given dimensions from the experimentally measured flexural strength s 2 of the geometrically similar sample of the same material but with different dimensions:

where is the length of the ice sample and h is its thickness (h is approximately equal to width). In the case where samples are not geometrically similar, their moduli of deformation Edef1 and

Edef2 are not equal and the above formula can be recast in the following form:

Comparison of the calculated values with the results of experiments gives satisfactory agreement. When dimensions of samples change from 5 ´ 5 to 100 ´ 100 square centimeters, the difference between the calculated and measured values of flexural strength does not exceed 25%.

SCATTERING OF ELASTIC ENERGY BY ICE. A measure of the internal friction y as defined as the relation between the energy scattered by an oscillating ice body during a single oscillation period to the maximum elastic energy of this body:

SCIENCE OF SEA ICE. A branch of knowledge which studies the formation, structure, composition, properties, evolution and destruction of sea ice as it occurs naturally in the world’s oceans. The science of sea ice is a branch of oceanography.

SCIENTIFIC & OPERATIVE GROUP. A group of hydrometeorological profile experts possessing both high qualifications and extensive on-the-job experience, established for the period of ice navigation.

SCIENTIFIC & OPERATIVE MAINTENANCE. Work implemented by hydrometeorological experts directed toward facilitating the reception of information about actual and forecast hydrometeorological and ice conditions and the transmission of this information to Arctic sea operation managers and to captains of ice-breakers and transportation vessels.

SCOUR. An elongated hole, resulting from the local acceleration of currents, in river ice (often called a "pool") or in fast sea ice.

SEA ICE COVERAGE. A parameter characterizing the distribution of ice in the sea (or a part of the sea) and defined as the ratio of the area covered with ice to the total area of the sea (or its part). Parameters such as compactness and age are not taken into account. The relative area of ice is expressed as a percentage.

SECULAR BEHAVIOR OF ICE PARAMETERS. The natural changes in ice parameters observed over tens to hundreds of years. Typically these values are determined on the basis of data averaged over many years.

SEISMOACOUSTIC MODULUS. See acoustic modulus (Young’s Modulus).

SHEAR MODULUS (SHEAR RIGIDITY). The ratio of the shear stress t at some point of the body to the shear angle g, whose value determines determines the distortion of the initially right angle between the planes in which tangential forces are acting ( Figure 8): G = g/t The modulus G characterizes the ability of ice to resist a change in its shape at constant volume. The shear modulus is smaller than Young’s modulus by a factor of 2.6 - 2.7. In the literature, authors sometimes use the terms distortion modulus or the modulus of transverse deformation instead of the term shear modulus.

SHELF GLACIER. A floating or partially grounded glacier extending off-shore in the shape of a shelf which gradually thins toward its outer edge where it ultimately ends in an ice cliff ( Photograph 37). In the near-shore part (measuring away from the coast) the thickness of a shelf glacier changes from 1,300 m or more to 300 m and near its seaward edge from 400 m to 50 m where the ice cliff is called a glacier barrier. The glacier’s speed increases from 300-800 m/year near the coast to values up to 1,800 m/year near the ice edge.

SHELF HARBOR. A bend in an ice barrier forming a bay where vessels can moor and unload directly upon the ice shelf.

SHELF ICE. Stagnant ice of a shelf glacier that is floating. Such ice typically has thicknesses of 40-60 m. About 2/3 of its thickness from the bottom consists of saline ice, the upper one third of the ice is both fresh and layered with various inter-layers of mineral deposits. The upper ice surface is commonly wavy. The calving of arctic ice shelves results in the formation of ice islands.

SHUGA (Russ.)See natural ice, intrawater ice. A type of new ice appearing as an accumulation of spongy white lumps a few centimeters across. Shuga can form from slush, grease ice and occasionally from anchor ice rising to the surface.

SIZE DISTRIBUTION FUNCTION FOR ICE FLOES. A probabilistic measure taking into account the relative number of ice-floes in each size gradation. In practice, we often use the differential distribution function F(Si) - the ratio of the area comprised of ice-floes of size Si to the total area occupied by all the ice-floes. If the probabilities of ice-floe distribution by size are given discretely, then:

where pi = pi (Si) is the frequency of ice-floes of a certain area Si and Sav is the average area covered with ice.

SMALL-SCALE DRIFT OF ICE. The drift of ice at distance scales of from several meters to several kilometers and lasting from fractions of a second to tens of minutes.

SNEZHURA. (Russ). A mixture of water and snow with no rigidity existing as a thin surface layer on the ocean or on the surface of an ice cover. Snezhura does not contain frazil crystals. [There is no equivalent English expression.]

SNOW CONCRETE. A mixture of snow and water at a 4 : 1 or 3 : 1 ratio that is first compacted and then frozen.

SNOW-COVERED ICE. Sea or fresh-water ice with various degrees of snow coverage.

SNOW CRUST. A crust of fine-grained strongly bonded snow on the surface of a snow cover consisting of closely-packed snow particles compacted by wind. Snow crusts usually develops on windward slopes of packed snow drifts.

SNOW-DRIFTS. Wind-drifted accumulations of snow which have been deposited on the lee sides of obstacles.

SNOW ICE. An ice type produced by freezing of water -saturateed snow. The water may be either fresh or saline.

SNOW RIDGES. Large snow-drifts that are elongated to the windward and that have lengths of up to several meters and heights of up to 1.5 meter (normally, 20 - 30 centimeters). The snow ridges alternate with wind furrows. In English these are often referred to as longitudinal snow dunes.

SOUND VELOCITY IN ICE. The velocity of elastic perturbations (deviations of the water molecules from their equilibrium positions) that depends on the compressibility and the density of the ice. If not specified otherwise, the term "sound velocity" refers to the phase velocity of the propagation of elastic oscillations in ice. The speed with which the phase of a monochromatic infinite sinusoidal wave moves in space is defined as:

,

where T is the period, l is the wavelength, and j = 2p (t/T - c/l) is the wave phase. A constant phase moves at the velocity C = dx/dt = l/T , (the phase velocity).

SPACE FACTOR OF A HUMMOCK. The ratio of the volume of the ice in a hummock (excluding the holes and air layers between the ice blocks forming the hummock) to the total volume of the hummock where the latter is defined on the basis of the external geometry of the hummock.

SPACE SURVEY. The investigation of snow and ice formations using satellite based instruments operating at various different spectral intervals.

This method is very promising for monitoring ice boundaries, ice movements and for studies related to ice navigation.

SPATIAL HETEROGENEITY OF AN ICE COVER. The spatial variability per unit area in the observed numerical values of an element or indicator of an ice sheet (e.g., the distribution of the horizontal dimensions of ice-floes, the ice thickness distribution).

SPATIAL STRUCTURE OF THE ICE SHEET. The spatial disposition of ice formations, their aggregates, and open water as conditioned by differences in interactions that depend on the nature of the forces affecting the ice sheet and changing with both short-term and seasonal rhythms. All this forms a series of mosaic combinations alternating in space.

ARCH STRUCTURES. A structure formed by a system of bow-shaped cracks and channels. Such structures are commonly observed in areas that are close to large straits, e.g. Fram Strait, Long Strait, Bering Strait, etc. ( Photograph 52)

DENDRITIC STRUCTURES. A spatial structure formed by systems of branched cracks, channels, and open water spaces that is typical of ice sheets during the autumn time period.

LAYERED STRUCTURES. An ice sheet structure formed by a system of parallel, linear oriented cracks, leads, and open water areas, or a system of narrow parallel zones of chaotic ice formations. Such features are typically observed during the winter. As a rule, the ice in the zones formed between such main fractures is so highly deformed and compacted that it can only be crossed with difficulty even by ice-breakers.

POLYGONAL STRUCTURES. The structure of an ice sheet formed by systems of crossing cracks, channels, and open water areas. Such structures typically occur in spring ice sheets and in near-barrier regions. The prevailing shapes of ice formations of this structure are rectangular and double-wedged ice blocks .

SHEAF STRUCTURES. A structure formed by a system of one-sided directed cracks, channels, and fractures. Such structures are typical of locations where protruding capes or islands are observed (e.g. regions of the Cape of Desire, Rudolf Island). Trapezoidal blocks of ice are typically formed.

SPOTTED STRUCTURES. The spatial structure of an ice sheet where various combinations of ice fields, ice fragments and broken ice-floes form spots of either increased or decreased compactness. Spotted structures are typical of ice sheets in the summer.

VORTEX STRUCTURES. The spatial structure formed by ice sheet elements involved in cyclonic and anticyclonic vortices. It may occur in the form of ice spirals.

SPECIALIZED FORECASTS. Recommendations published by operative bodies of Roshydromet (Russian Hydrometeorological Service) based on the analysis and forecast of hydrometeorological conditions and designed to support the implementation of specific production tasks of separate industries in the national economy.

SPECIALIZED HYDROLOGICAL FORECASTS FOR NAVIGATION. Recommendations that include information about the most advantageous shipping routes in seas and oceans, or forecasts of the expected start and the end of transit periods by non-self-propelled vessels (lighters or barges).

SPECIALIZED ICE FORECASTS. Ice forecasts that contain information about the expected types of navigation conditions occurring on different sections of a route, the length of ice zones of varying age and compactness, and the estimated navigation times and charges for specific convoys along different routes.

SPECIALIZED FORECASTS FOR THE FISHING INDUSTRY. Recommendations that contain information about the estimated location of marketable schools of fish and the expected time for the start of the fishing season.

SPECIALIZED FORECASTS FOR OVER-ICE TRANSPORT. Recommendations for the use of specific types of transport on ice roads depending on their bearing capacity.

SPECIFIC HEAT OF ICE MELTING. The amount of heat required to change a unit mass of ice which is at the melting temperature from the solid state into the liquid state.

At 0°C and normal atmospheric pressure the specific heat for clear bubble-free ice is 333.5 kJ/kg. This value is identical with the specific crystallization heat of fresh water.

For sea ice, the term effective heat of melting is used which is the heat needed to melt a unit mass of sea ice. The effective heat of melting of sea ice is appreciably less than that of pure ice as the result of its higher salinity.

SPECIFIC HEAT OF ICE SUBLIMATION. The amount of heat needed to cause a unit mass of ice to transfer into the vapor phase at the same temperature.

SPECIFIC HEAT OF WATER VAPOR SUBLIMATION. The amount of heat needed to cause a unit mass of water vapor to change into ice at the same temperature.

SPREAD OF AN ICE EVENT. The process of enlarging a sea ice sheet or area, or the area where an ice formation event occurs.

NON-ZONAL SPREADING. The spreading of an ice event or process without a correlation with a specific zonal feature (circle of latitude) in a given ocean area. It is one of the major physical and geographic mechanisms. For example, the spreading of ice in the Greenland Sea is of this type.

ZONAL SPREADING. The spreading of an ice event or process in the sea when the speed and direction of the event or process is essentially the same at all the points along a circle of latitude.

MAXIMUM SPREADING. The lowest latitude reached at maximum ice extent. For example, the location of the maximum winter border of sea ice in the world’s oceans is identified by two characteristics: the critical depth of the vertical convective circulation in the upper ocean and the conditional maximum depth of convective cooling as the result of the heat exchange between the ocean and the atmosphere at that site.

STABILITY OF THE ICE DRIFT DIRECTION. The degree to which the direction of the ice drift system is preserved. Its value is determined by the ratio of the vector modulus of the average speed W to the average scalar speed W*.

.

The values of q change from 0 to 1. The more stable the ice drift, the closer to 1 is the stability coefficient. Ice drift may be considered to be stable at q > 0.85, and unstable at q < 0.60.

STABLE ICE FLOWS. The movement of ice in a single-direction during an extended time period. An example of such a drift system is the transarctic drift of ice from the seas of the Siberian shelf towards Fram Strait . This is well reflected on charts showing ice drifts in the Arctic Basin averaged for annual and even winter time periods. However, for shorter averaging periods and particularly during the summer season, ice drift in any direction may be observed in the same region.

STAGNANT SEA ICE. A field of sea ice that remains relatively fixed in position as the result of the fact that it is frozen to the shore, ice wall, ice barrier, or shoal.

FAST ICE. Sea ice fixed to the shore by an ice wall or an ice barrier that is subject to small vertical tidal fluctuations as well as to minor horizontal displacements. Fast ice forms either as the result of the natural extension of an ice sheet out from the coast or by the freezing of a region of pack ice to the shore. In some regions in the Arctic a fast ice area may remain stable for two and more years while transforming from winter ice into old ice; and in Antarctic seas - into shelf ice. The width of fast ice may vary from a few tens of meters to several hundred kilometers (Photograph 11 and 53).

Fast Ice Foot. A narrow zone of fast ice that is fixed to the shore and is not affected by tidal fluctuations. This ice typically remains in place for some time after the fast ice breaks up.

Stable Fast Ice. The part of the fast ice located between the ice foot and the ice barrier, hummock, or a chain of hummocks grounded on a shoal (stamukhi).

Submerged Off-Shore Ice. The initial formation stage of fast ice represented by a strip of stagnant thin ice 200 m wide and frozen to the shore.

Unstable Fast Ice. An area of fast ice formed offshore of the hummock barrier and subject to fracturing and conversion into pack ice during any season of the year.

COASTAL RIDGE (OFF-SHORE ICE BAR). An ice pile occurring on a gently sloping shore.

GROUNDED ICE FLOE. A level ice floe or a slightly hummocky ice floe which has temporarily become shoaled.

ON-SHORE ICE. Small, broken ice-floes or ice-blocks found on the shore during periods of low tide or when pushed onto the shore by strong, on-shore movements of the offshore pack ice.

SHOALED ICE. An ice formation that has temporarily become stagnant due to its running aground.

STAMUKHA Russ. (Grounded Hummock). A large hummocky pile of ice grounded in water depths of 20 meters or more. The height of such stranded ice can be 10 m or more. The average void volume of such stranded ice is 0.35. Chains of stamukhi often form along coastal shoals (Photographs 53, 54).

STATE OF ICE SHEET. A set of qualitative and quantitative changes in ice sheet features and events occurring between two consequent surveys (observations) of the ice conditions in the same region. The two states of an ice sheet will be considered different if the numerical values of at least one parameter or indicator significantly differ from each other.

SPREADING ICE. The initial stage of ice rarefaction.

BROKEN ICE. An ice sheet that is in a state of mechanical destruction; i.e., external forces have broken it into fragments of different sizes. One possible measuring unit for breakage during the winter will be the scale of the distances between cracks and leads and in the summer, the relative number of ice fields, fragments and broken ice. When estimating the degree of breakage of an ice sheet, one should note the direction and width of cracks, leads, and open water areas (rasvodije) and the age of the ice that has formed these features.

RAREFIED ICE. The ice that is in a state of rarefaction with the distances between the ice-floes increasing. The compactness of an ice sheet is measured using a 10 point compactness scale.

DESTRUCTING ICE. An ice sheet that is in a state of thermal destruction as the result of processes associated with melting.

Dried Ice. The ice after the surface melt water has flowed into the sea through cracks and the formation of thaw holes.

Lacy Ice. Ice on the surface of old floes which has been so completely dissected by melt processes that it now resembles lace.

Rotten (honey-comb) Ice. The ice in the final stages of melting that is friable and saturated with water. It has a dark-gray color and looks like a honey-comb ( Photograph 55).

Submerged Ice. Ice covered with a continuous layer of melt or river water.

COMPACT ICE. The ice which is in a state of compactness as indicated by a decrease in the distances between the ice-floes.

PRESSED or PRESSURED ICE. Ice which is under compression.

STATIC ELASTIC CHARACTERISTICS. Elastic characteristics defined in accordance with the results of tests performed without the observance of the conditions of elastic behavior in ice, e.g. by the relatively slow bending of beams and consoles. As a rule, elastic characteristics as determined by using static methods show a significant spread in their measured values that may differ from the dynamic characteristics by an order of magnitude or more.

The results of rapidly performed static tests made to define the elastic modulus are sometimes called the conditionally-instant, or quasi-elastic modulus.

STRENGTH CHARACTERISTICS OF ICE. The values that characterize the strength of ice as a solid.

Characteristics determined from sample loading times that do not exceed several seconds (from the start of the loading to the moment of failure) are called short-term (conditionally instant) strength. (see also Brittle strength).

PROPORTIONALITY LIMIT sH. The stress at which the deviations from a linear dependency between stress and strain reach the value as established by technical considerations. The criterion sH describes the adequacy of Hook’s Law in fitting a given set of data. In practical calculations sH= syield is taken as the basis ( Figure 12).

STRENGTH LIMIT (TEMPORARY RESISTANCE) OF ICE sbr . The conditional stress (defined by the ratio of the effective force to the initial cross-sectional area of the ice sample), corresponding to the peak load preceding the failure of the sample. It is the primary characteristic of materials that fail at low plastic deformations.

The scientific and technical literature includes the following terms for this class of failure: tensile (fracture), flexural, compressive (under either uniaxial or complex loading) and shear strengths.

When evaluating these types of characteristics, the total time of loading should rarely exceed a few seconds from the start of loading to the moment of destruction. Because of this reason these characteristics are also referred to as the conditionally-instant or brittle short term strengths or the ultimate strengths under tensile, bending, compressive and shear stresses.

Limits of the Tensile and Flexural Strength of Ice st and sf. Mechanical characteristics determined through experiments in which ice samples are subjected to critical tensile and flexural deformations.

In accordance with recently agreed international recommendations, the following optimum methods should be used to determine ice strengths. To obtain the tensile strength st, dumb-bell shaped samples whose dimensions of the cross-sections where the fracture occurs significantly exceed the sizes of the ice crystals in the sample, should be used. If the bending or flexural strength sf is required, then floating cantilever beams that are cut using the entire thickness of the ice sheet should be used to establish the failure moments.

These calculations are performed using the following formulae:

where Pt and Pf are the loads at failure, Sf is the cross-sectional area at the location of the fracture, l is the length of cantilever, b is the cantilever width, and h is the ice sheet thickness.

There are a number of empirical dependencies relating st and sf to factors such as the temperature and the salinity of the ice, the loading rate, the specific structural features of the sample, and its geometric dimensions.

It is important to note that when ice fails in flexure (bending), as in the case of a fixed cantilever whose free end is pulled upward, the sample is subjected to a complex set of stresses: i.e., the upper portion of the sample is subjected to compression while the lower portion of the sample is subjected to tensile stresses. As a result st is, on the average, 30%-40% lower than sf.

Limits of the Compressive and Shear Strength of Ice. scom and ssh Mechanical characteristics determined through experiments in which ice samples are subjected to critical compressive and shearing deformations.

The compression strength of ice scom is obtained by axially loading either prismatic or cylindrical samples to failure. In performing such tests, great care should be taken to assure that a plane uniform stress state is achieved in the portion of the sample that fails. This is usually achieved by minimizing the tangential stresses (perpendicular to the axial load) at the places where the ends of the sample contacts the loading platens.

Here Scom is the cross-sectional area of the sample as measured perpendicular to the direction of loading and Pcom is the compressive stress at failure

The shear strength of ice is calculated using:

where Psh is the destructive load directed along the area of shear Ssh.

It has been determined that scom increases with decreasing temperature. Also its values when the load is applied perpendicular to the direction of crystal growth (typically in the vertical) are larger than when the ice is loaded perpendicular to that direction (parallel to the surface of the ice sheet). Shear strengths are smaller if the plane of shear is perpendicular to the long axes of the crystals.

LIMIT OF YIELD sy. The stress that corresponds to the lower position of the yield platform as shown in Figure 12. In ice, such a platform is observed at moderate deformation rates when testing using uniaxial compression. Physically it corresponds with the start of internal crack formation. In general, the yield limit fixes the boundary between the elastic and elastic-plastic zones of deformation and is the major characteristic of the strength behavior of plastic materials.

LIMIT OF ELASTICITY sel. The stress at which the residual deformation initially reaches a value that is arbitrarily fixed by technical considerations (e.g. 0.001; 0.003; 0.03%). This criterion limits the region where Hooke’s law is assumed to hold. In practical calculations, sel= sy is usually assumed (see Figure 12).

STRUCTURE OF AN ICE DRIFT FIELD. The aggregate of interrelated elements that characterize the spatial changes in an ice drift field. These elements include stable ice floes as well as their rotations, vortices, and spirals.

STUCK ICE. Ice formed through atmospheric icing that grows on the surface of a project due to the freezing of either frazil crystals or snow mixed with sea water.

STUFFED ICE. A condensed mass of primary or young ice fragments formed as a strip in coastal regions, shoals, and close to fast ice due to the strong effect of wave motion. The vertical thickness of stuffed ice can reach 20 m or more.

SUBLIMATION OF ICE (SNOW). The direct transition of ice (snow) from the solid to the vapor phase.

SUBSIDENT ICE. An area of an ice sheet in a shoal close to the shore that has subsided due to a decrease in the local water level.

SUPPORTING POWER (BEARING CAPACITY) OF AN ICE SHEET. The ability of an ice sheet to support a variety of different loads without failing or undergoing undue damage.

The supporting power of an ice sheet is usually expressed in terms of the maximum load that can be supported by the sheet for a specified period of time without resulting in the failure of the sheet. The supporting power of an ice sheet is significantly affected by both the length of time that the load is applied and how the load is distributed. Generally, three typical ice loading regimes are distinguished:

1) dynamic, when the stresses vary widely but within the elastic range;

2) static, when the inertia forces may be ignored;

3) durable load regime, when the plastic properties of ice are fully utilized.

For many types of logistical tasks, the loading is only applied for a short period of time and the stresses always remain less than the elastic limit. In such cases an ice sheet can be considered to be an elastic plate resting on an elastic foundation. Clearly the bearing capacity of a specific ice sheet should be less than the loading that would result in the occurrence of cracks that completely penetrate the ice sheet, in that such cracks are known to significantly decrease the bearing capacity. The supporting power of an undamaged ice sheet at locations near an open lead is also significantly reduced.

The analysis of ice sheet behavior based on elastic plate theory permits only an approximate analytical description of the behavior of the ice under loading, especially under loads of long duration. In such cases an accurate calculation of the maximum stress considering creep and an estimate of the influence of cracks on the load capacity of the ice should be included as should the effects of the vertical temperature gradient across the thickness of the ice.

SURFACE BRINE. A concentrated salt solution formed on the surface of an ice sheet during fast ice formation.

SWELLING HILLS. Small mounds of ice with heights of several dozen centimeters caused by the volume expansion which accompanies the freezing of water lenses located in water pools that were deeply thawed ( Photograph 56). These features primarily occur in old ice.

SYSTEMS OF UNIFORMITY VIOLATIONS. The spatial distribution of sets of cracks, leads, and open water areas (rasvodije) in a certain area of ice at a specific moment of time. Straight, crossing, arcuate, branching, as well as other fracture systems can be distinguished.

T

TANGENT OF THE MECHANICAL LOSSES IN ICE. A mechanical characteristic of ice that is a measure of its internal friction:

tan j = Q-1,

where j corresponds to the displacement between the stress s(t) = so exp (iwt) and the deformation e(t) = eo exp (iwt) as determined using quasi-elastic vibrations with a frequency w ; and Q-1 is the reciprocal of the mechanical quality factor (by analogy with the Q-factor of an oscillatory electric circuit).

THAWED PATCH. A vertical hole where an ice floe has melted completely through during the summer thaw period.

THEORETICAL ICE STRENGTH. A hypothetical property of ice s calc that is calculated by assuming that the strength of a failure surface is given by the simultaneous breakage of all interatomic linkages located on the surface. As with other solids s calc for ice is estimated by the value 0.1 E, where E is Young’s modulus.

Actual strength values are usually several orders of magnitude lower than the theoretical values. The reason for low strengths of real materials is the uneven distribution of internal stresses causing the interatomic linkages to be loaded unequally. Also real materials invariably contain flaws (dislocations) which appreciably lower their strength.

When both external and internal stresses are considered, local deformations may occur that may reach the theoretical strength values, leading to breaks in the interatomic linkages. The growth and linking of such failures form microscopic cracks whose development results in the destruction of the body. The theoretical strength is also called the ideal strength or the cohesion force density (i.e. the force of molecular interaction of parts of the same body) or simply cohesion, that may be characterized by the heat of evaporation.

THERMAL CONDUCTIVITY OF ICE (THERMAL CONDUCTIVITY COEFFICIENT). A parameter l characterizing the effectiveness of thermal energy transfer in ice. The thermal conductivity is the proportionality coefficient between the thermal flux density q and temperature gradient as given by the well-known equation:

q = –l grad T

Strictly speaking, the thermal conductivity of ice is numerically equal to the thermal flux density observed at a temperature difference of 1°K per unit distance. If the temperature decreases, the thermal conductivity increases. In accordance with theoretical calculations and laboratory measurements, the thermal conductivity of freshwater ice is equal to approximately 2.22 W/(m K) at a temperature of approximately 0°C.

THERMAL CRACKING OF ICEBERGS. A process of thermal iceberg destruction resulting from the release of the energy contained in the compressed air contained in the bubbles in the ice. The pressure in such bubbles ranges from 0.5 to 2.0 MPa. The air released from such bubbles may cause cavitation or resonance excitation.

THERMAL DIFFUSIVITY OF ICE. A parameter characterizing the rate of change of the ice temperature during non-stationary thermal processes. The coefficient of thermal diffusivity a is given by

,

where Cp is the specific heat of ice at constant pressure, r is ice density and l. is the thermal conductivity.

THERMIC DESTRUCTION. Destruction of an ice sheet as a result of its melting usually resulting from increases in the air temperature to values above 0°C. Thermic destruction decreases ice strength, changes its structure and texture, and diminishes the horizontal and vertical dimensions of floes, etc.

External evidence of thermic destruction is the breaking and splitting of ice sheets as fixed by the following times: the day when part of fast ice splits off; the day when the first signs of melting and decreasing strength occur; the day when the visible area of fast ice (excluding its bottom) is dissected by a great number of cracks resulting from horizontal shifts in the ice but not resulting in a measurable decrease in compactness; and finally the day when the fast ice has broken into ice blocks that have shifted in relation to each other, appreciably decreasing the compactness of the ice.

THERMODYNAMIC MODELS. Models describing processes of growth and melting of sea ice in terms of sets of equations based on the thermal conductivity and the heat balance equations.

THERMOPHYSICAL (THERMIC) PROPERTIES OF ICE. Ice properties that define the conditions of heat transfer and ice temperature change.

TIDAL ICE DRIFT. Ice drift resulting from or affected by tidal currents. Depending on the character of the tides, the drift may be semi-diurnal or diurnal.

TIME OF SAFE LOAD PARKING ON ICE. The period between the time when a known load is placed on an ice sheet and the time when the ice sheet starts to fail under the load (i.e. when there is a loss of bearing capacity).

Based on the experimentally determined observations of breaking loads, one can derive the following expression for calculating the time tp that a load may safely remain on an ice sheet:

.

Here Pp(0) is the load causing plate to fail immediately after its application, i.e. at time

tp = 0; Pp(tp) is the load causing the ice plate to fail after time tp where time is expressed in hours. At tp > 0, Pp(tp) < Pp(0).

It is important that one correctly determine the value of Pp(0). Probably, the most accurate results to date have been obtained by D.F. Panfilov. In his experiments on fresh water ice the static load was created with the help of a die (stamp) and parameters were chosen so as to result in failure within a 5 - 20 second interval. By using averaged experimental data, one can construct a curve, which is characterized by a small scatter of points and which can be approximated by the following equation:

where b is the diameter of the loading area ; l is the so-called action radius given by l = Ö D/r g where D = (Eh3)/[12(1 - n2)] is the flexural rigidity of a plate and r is the density of the water.

For technical applications one must also know the loads under which an object can park on the ice or move slowly over its surface without failure occurring. One must also know the loads which will cause the ice to fail instantly (values useful in designing icebreakers). These loads correspond to the upper (U) and lower (L) envelopes limiting the experimental data. The area under the lower curve delimits permissible loads while that area above the upper curve corresponds to failure loads. These curves can be described with the help of the following formulas:

Therefore, in accordance with the results obtained by Panfilov, a permissible load for an unbounded ice plate at –10°C when the load is concentrated at b< h can be determined as follows

Here sp can be considered to be equal to the value of the flexural strength of a floating cantilever beam.

According to Panfilov’s results, the permissible load applied to the edges of a lengthy crack in an ice sheet (e.g., in the case of a bridge between two semi-infinite plates) can be determined from the condition

For a semi-infinite plate the permissible load is given by

Panfilov’s experiments were performed at 0.1 < b/l < 1.0 . He also showed that (Pp)L 2Pcr, i.e., that cracks in ice develop at loading values equal to half the load corresponding to the lower limit as described above.

TIME RESISTANCE. See strength characteristics of ice; ultimate strength.

TORTUOSITY COEFFICIENT OF ICE CRYSTAL FACES. A parameter that characterizes the degree of tortuosity (twisting or crookedness) of ice crystal faces as compared to an ideal crystal. The coefficient of tortuosity characterizes the degree of ice crystal ideomorphism (a term applied to crystals bounded by their own rational crystal faces).

TORTUOSITY OF ICE DRIFT. Frequent changes in the direction of ice drift with time result in tortuous drift trajectories. The degree of tortuousity is measured in terms of the tortuousity index which is defined by the ratio of the actual length of ice-floe drift track, including all turns, to the displacement of an ice-floe during a specified period of time ( Figure 16). For instance, a daily tortuosity index is equal to the ratio of the sum of the absolute values of the drift rate measured every hour, to the great circle distance between the starting and ending points.

TOTAL ICEBERG DISCHARGE. The mass of fresh continental ice discharged into the sea from some specified location.

For example, for the Northern hemisphere, the total iceberg discharge is approximately 4.7´1017 g/year. Of this value the Greenland icecap contributes 4.6´1017 g/year; Spitsbergen archipelago - 0.95´1017 g/year; Franz Josef Land - 0.026´1017 g/year; Novaya Zemlya - 0.02´1017 g/year; and all the other arctic islands - 0.004´10 17 g/year;

TROUGHS. This term refers to furrows (also referred to as gouges) on the sea floor caused by the ploughing action of grounded icebergs. Troughs may occur at depths of up to 380 m. At the present, extreme dimensions of furrows are known to be up to 3 km long, 30 m wide and 6.5 m deep.

TYNDALL FIGURES (FLOWERS). Melt figures that occur inside ice crystals produced by internal melting resulting from absorbed radiant energy. Tyndall figures are located parallel to each other in the basal planes and resemble snowflakes in their shape. X-ray studies have shown that the directions of the arms of the figures indicate the directions of the a crystallographic axes. The figures are named after John Tyndall who first described them in 1858.

TYPE OF ICE. A taxonomic subdivision of ice classification used as an addition to generic types of ice. Types of ice include classifications by structure, composition, dimensions, an genetic origin, etc.

U

ULTRASONIC MODULUS. See acoustic modulus of ice elasticity (Young’s modulus).

UNDERHUMMOCK. The underwater part of a hummock whose draft usually, depending on its age, exceeds the hummock’s above water height by 3.5 to 5 times. If the ice feature is a hummocky ridge, its underwater portion would be commonly be referred to in English as a ridge keel.

UNDERWATER ICE DOME. A concavity in the ceiling (lower surface) of old ice. Underwater ice domes serve as traps for fresh melt water that flows under the ice in the spring and in the summer as well as for potential gas accumulations, oil products and other substances spilled under the ice and having a lower density than sea water. [In the non-Russian submarine community, these features are called skylights].

V

VELOCITY OF WAVE FRONT. See group velocity of wave spreading in the ice sheet.

VERIFICATION OF FORECASTS. The assessment of reliability or validity of a set of forecasts.

VESSEL (CONVOY) DRIFT. The passive movement of a vessel (convoy) under the influence of wind and currents. In cases where the drifting vessel (convoy) is beset, the movement is the same as the surrounding drifting ice.

VESSEL ICING. The formation of an ice layer on the body of a vessel or on its main deck and superstructure due to the freezing of water or spray ( Photograph 44). Vessel icing is observed when the vessel moves through open water at air temperatures below 0°C.

VESSEL MOVEMENT BY RECIPROCATING MOTION. Reciprocating motion of a ship with stops due to complicated ice conditions followed by acceleration and the breaking of ice. In English this procedure is called backing and ramming.

VISCOUS ICE FLOW. During certain types of loading situations the deformation of ice can be well described as behaving in a viscous manner; i.e., as a Newtonian viscous fluid which is a material whose strain rate is linearly proportional to stress :

In the above relation A is a yield coefficient which is a function of temperature, of grain size, and of parameters associated with diffusive processes in ice crystals. The coefficient A = 1/h is numerically equal to the reciprocal of the viscosity coefficient. The variations in A that are observed when investigating subjects such as glacier flow are the result of the increase in stresses with time and of the fact that ice properties deviate from those a perfectly viscous fluid. The terms "viscous or quasiviscous ice flow" are best applied to ice at near melting temperatures when deformation occurs as the result of small loads. It is known that creep processes in sea ice are facilitated by the fact that it contains liquid brine.

VOLUME MODULUS OF ELASTICITY. See Ice Compressibility.

W

WAKE BEHIND AN ICEBERG. A band of freshened and cooled water occurring behind a drifting iceberg. The influence of an iceberg on the temperature and the salt content of nearby sea water depends on the dimensions of the iceberg as well as its rate of melting.

WATER ICE. See congelation ice

"WATER" SHADOW. An area of open water located on the leeward side of an individual iceberg located among pack ice. "Water" shadows of many large-sized icebergs can merge into ice-free water areas that extend for up to dozens of kilometers.

WATER SKY. A comparatively dark sky observed near the horizon over areas of open water located in a region of unbroken ice.

WAVY ICE. An area of young ice (sometimes one-year thin ice) with a wavy surface that has resulted from prolonged compression ( Photograph 15). This ice feature is characteristically observed in recently refrozen leads, channels and polynyas. In some places in the Antarctic the horizontal stresses necessary to buckle such thin ice sheets are provided by the movements of near-by glaciers and icebergs.

WAVES IN THE ICE. The occurrence of periodic deflections in an ice sheet (or an ice sample of limited size). The statistical characteristics of these oscillations (amplitude, wavelength, period, velocity) depend on the visco-elastic characteristics of the ice (density, elasticity modulus, bulk modulus, coefficient of mechanical losses) for mechanical processes and on the electric characteristics of ice (dielectric constant, loss tangent, refractive index) for situations where the propagation of electromagnetic waves in ice are being considered.

AIR-COUPLED FLEXURAL WAVES. Waves originated by the interaction between flexural waves propagating in the ice and sound waves propagating in the air, under the condition that the two velocities are equal.

ELASTIC WAVES. Periodic mechanical deformations, the propagation of which in ice causes sign-variable mechanical stresses having amplitudes that do not exceed the elasticity limits of the ice.

FLEXURAL-GRAVITATIONAL WAVES. A wave process in ice fields caused by elastic forces in the ice field (considered as an elastic plate) and by gravitation. Frequencies of flexural-gravitational waves vary between 0.1 and 10 Hz depending on the ice thickness while the average wave amplitude ranges between 200 - 250 micrometers.

Flexural-gravitational waves can cause breaking of the edges of an ice sheet even during the winter. This process is particularly striking in the Southern Ocean.

GRAVITATIONAL WAVES. A wave-like motion of an ice sheet caused by the propagation of gravitational water waves (surface and internal) ( Photograph 57). In the Arctic and Antarctic, the gravitational waves observed in the ice are the result of storms in the open ocean. The period of gravitational waves ranges between 5 to 200 s for wind-excited waves and 3 - 30 minutes for slow waves [15].

LONGITUDINAL WAVE (VOLUME WAVE OR EXPANSION WAVE). A wave that propagates in ice with a velocity given by

where E is Young’s modulus, n is Poisson’s ratio and r is the ice density. This type of wave is connected with changes in the ice volume (compression or expansion).

In longitudinal waves, the particles are displaced parallel to the direction of wave propagation. The velocity of longitudinal waves in a sheet of sea ice depends on the ice density and ranges from the velocity of sound in snow to the velocity of sound in water.

RALEIGH WAVES. Elastic surface vibrations propagating along either free or lightly-loaded boundaries in ice. Their amplitude drops off rapidly with depth into the ice sheet. The velocity of Raleigh waves is determined by the formula:

where n is Poisson’s ratio and Ct is the velocity of propagation of the transverse wave.

SLOW WAVES. The waves that develop in an ice sheet when it is subjected to the impact of short-period ocean internal waves [15]. Observed periods range from 3 to 30 minutes for ice up to 10 m thick. The phase velocity of slow waves varies from 0.5 to 2.0 meters per second.

TRANSVERSE WAVES (DISTORTIONAL WAVES). A wave that propagates in ice with a velocity Ct without causing a change in volume. In transverse waves, points are displaced in the direction perpendicular to the direction of wave propagation. The velocity of a transverse wave can be calculated using

where G is the shear modulus and r is the ice density.

The expressions for velocities of longitudinal and transverse waves are valid for an unbounded continuous isotropic medium whose dimensions in all directions are significantly larger than the wavelength.

WIND DRIVEN DRIFT OF ICE. The drift of ice under the effect of wind.

WIND FURROWS. A deflation form of snow surface relief caused by wind erosion of dense snow. They occur as elongated furrows located between wind-weathered ridges of snow (sastrugi) ( Photograph 58).

X

Y

YOUNG ICE See age of an icesheet

Z

ZONE OF ISLAND INFLUENCE ON ICE DRIFT. The area around an island where the island’s presence decreases the drift rate of pack ice and distorts its direction.

REFERENCES

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> PHOTOGRAPH CREDITS

> AARI Files:

> > V. Borodachev: Photographs 10, 14, 35, 36, 48

> > V. Gavrilo: Photograph 42

> > V. Grischenko Photographs 18, 46a, b, 50

> > Mironov Photograph 51

> Arctic Submarine Laboratory, U.S. Navy

> > Photographs 6, 11a, 16, 17, 19, 20, 21, 22, 23, 26, 28, 37, 43, 44, 47, 52, 53, 55, 57

> G. F. N. Cox

> > Photograph 30

> V. Ryabinin

> > Photograph 31

> W. F. Weeks

> > Photographs 1, 2, 3, 4, 5, 7, 8, 9, 11b, 12, 13, 15, 24, 25, 27, 29, 32, 33, 34, 38, 39, 40, 41, 45, 49, 54, 56, 58, 59

Return to Glossary Index

Glossary Terms: A-D, E-H, I, J-R