Sea ice. Properties and classification of sea ice. Sea ice. General information Types of ice in the sea

About −1.8 °C.

An assessment of the amount (density) of sea ice is given in points - from 0 (clear water) to 10 (solid ice).

Properties

The most important properties of sea ice are porosity and salinity, which determine its density (from 0.85 to 0.94 g/cm³). Due to the low density of ice, ice floes rise above the surface of the water by 1/7 - 1/10 of their thickness. Sea ice begins to melt at temperatures above −2.3°C. Compared to freshwater, it is more difficult to break into pieces and is more elastic.

Salinity

Density

Sea ice is a complex physical body consisting of fresh ice crystals, brine, air bubbles and various impurities. The ratio of the components depends on the conditions of ice formation and subsequent ice processes and affects the average density of ice. Thus, the presence of air bubbles ( porosity) significantly reduces the density of ice. Ice salinity has less of an effect on density than porosity. With an ice salinity of 2 ppm and zero porosity, the ice density is 922 kilograms per cubic meter, and with a porosity of 6 percent it decreases to 867. At the same time, with zero porosity, an increase in salinity from 2 to 6 ppm leads to an increase in ice density only from 922 to 928 kilograms per cubic meter.

Nilas (foreground) in the Arctic

Thermophysical properties

The color of sea ice in large massifs varies from white to brown.

White ice formed from snow and has many air bubbles or brine cells.

Young sea ice of a granular structure with significant amounts of air and brine often has green color.

Multi-year hummocky ice, from which impurities have been squeezed out, and young ice, which froze under calm conditions, often have light blue or blue color. Glacier ice and icebergs are also blue. The needle-like structure of the crystals is clearly visible in blue ice.

Brown or yellowish ice is of river or coastal origin, it contains admixtures of clay or humic acids.

Initial types of ice (ice lard, slush) have dark grey color, sometimes with a steely tint. As the thickness of the ice increases, its color becomes lighter, gradually turning white. When melting, thin pieces of ice turn gray again.

If the ice contains a large amount of mineral or organic impurities (plankton, aeolian suspensions, bacteria), its color may change to red, pink, yellow, up to black.

Due to the property of ice to retain long-wave radiation, it is capable of creating a greenhouse effect, which leads to heating of the water underneath it.

Mechanical properties

The mechanical properties of ice mean its ability to resist deformation.

Typical types of ice deformation: tension, compression, shear, bending. There are three stages of ice deformation: elastic, elastic-plastic, and destruction stage. Taking into account the mechanical properties of ice is important when determining the optimal course of icebreakers, as well as when placing cargo on ice floes, polar stations, and when calculating the strength of a ship’s hull.

Conditions of education

When sea ice forms, small drops of salt water appear between entirely fresh ice crystals, which gradually flow down. The freezing point and the temperature of greatest density of sea water depend on its salinity. Sea water, the salinity of which is below 24.695 ppm (so-called brackish water), when cooled, first reaches the highest density, like fresh water, and with further cooling and without stirring it quickly reaches its freezing point. If the salinity of the water is above 24.695 ppm (salt water), it cools to the freezing point with a constant increase in density with continuous mixing (exchange between the upper cold and lower warmer layers of water), which does not create conditions for rapid cooling and freezing of water, that is, when Under the same weather conditions, salty ocean water freezes later than brackish water.

Classifications

Sea ice in its own way location and mobility divided into three types:

• floating (drifting) ice,

Forecast of changes in ice thickness by 2050

By stages of ice development There are several so-called initial types of ice (in order of formation time):

• intra-water (including bottom or anchor), formed at a certain depth and objects located in the water under conditions of turbulent mixing of water.

Further types of ice in time of formation - nilas ice:

• nilas, formed on a calm sea surface from fat and snow (dark nilas up to 5 cm thick, light nilas up to 10 cm thick) - a thin elastic crust of ice that easily bends on water or swell and forms jagged layers when compressed;
• flasks formed in desalinated water in a calm sea (mainly in bays, near river mouths) - a fragile shiny crust of ice that easily breaks under the influence of waves and wind;
• pancake ice formed during weak waves from icy fat, snow or slush, or as a result of a break as a result of waves of a flask, nilas or so-called young ice. They are round-shaped ice plates from 30 cm to 3 m in diameter and 10-15 cm thick with raised edges due to rubbing and impacts of ice floes.

The further stage of development of ice formation is young ice, which are divided into gray (10-15 cm thick) and gray-white (15-30 cm thick) ice.

Sea ice that develops from young ice and is no more than one winter old is called first-year ice. This first-year ice can be:

• thin first-year ice - white ice 30-70 cm thick,
• average thickness - 70-120 cm,
• thick first-year ice - more than 120 cm thick.

If sea ice has been subject to melting for at least one year, it is classified as old ice. Old ice is divided into:

• residual first-year ice - ice that has not melted in summer and is again in the freezing stage,
• two-year-old - lasted more than one year (thickness reaches 2 m),
• multi-year - old ice 3 m thick or more, which has survived melting for at least two years. The surface of such ice is covered with numerous irregularities and mounds formed as a result of repeated melting. The lower surface of perennial ice is also highly uneven and varied in shape.

Research of sea ice at the North Pole

The thickness of perennial ice in the Arctic Ocean reaches 4 m in some areas.

Antarctic waters mainly contain first-year ice up to 1.5 m thick, which disappears in the summer.

Features of navigation in ice conditions depend on the navigation area and its inherent ice regime, which in turn depends on many factors: the geographical location of the area, the nature of the currents, salinity and temperature of the water, winds, tidal phenomena, the presence of rivers flowing into the seas in this area.

Information about ice regimes is given in hydrometeorological sketches of the navigation route, consisting of characteristics of the meteorological, hydrological and ice regime.

Illustrative materials for such essays include atlases of physiographic data, ice maps and hydrometeorological maps, and special appendices to navigation directions.

Having the indicated manuals, as well as data from ice patrol, meteorological stations, aerial reconnaissance and other sources, the navigator can, in most cases, obtain a fairly accurate idea of ​​​​the distribution of ice and the navigation characteristics of the upcoming route. Data on the distribution of ice, indicating their edges and varieties, are recommended to be plotted on blank maps or on tracing paper taken from navigation charts.

Asymmetric icebreaker

During the passage of the ship, an important role is played by receiving additional information and corrections from radio stations performing a special service, as well as from icebreakers and individual ships located in the same area. In addition, it is necessary to have information about the weather conditions during the transition and ice forecasts.

To correctly assess the information received about ice, it is necessary to know its classification, and, if possible, the navigation characteristics that determine the degree of ice passability.

Navigation in ice places increased demands on ship crews, navigators and sailors. Steering a ship in ice places a number of specific demands on sailors at the helm. In addition to following the commands of the watch officer, the helmsman must be able to navigate independently when moving among the ice.

Ice classification

Sea floating ice is not connected to the shore or bottom and is in constant motion (drifting) under the influence of wind and current. Floating ice is the predominant category of ice in the seas and oceans. Floating ice in the sea is formed independently or as a result of a break in fast ice (shore ice).

Floating ice varies in shape, size, age, concentration and other characteristics.

By age they are distinguished:

• initial ice formations (ice needles, ice lard, snowflake, slush, pancake ice, flask, dark nilas);
• young ice (light nilas, gray ice) 5 - 15 cm thick;
• winter ice (gray-white, white ice) 15 - 200 cm thick.

According to their shape, ice is divided into:

• motionless (ice bank, fast ice, riser, stamukha);
• drifting, or floating (extensive large and small ice fields, coarse and small broken ice, pieces of ice, ice porridge).

Based on the structure of ice and the state of its surface, they are distinguished:

• smooth ice;
• layered;
• hummocky;
• snowless;
• snowy ice and frosting.

Based on size, floating ice is divided into the following types:

• large ice fields consisting of ice floes over 10 km in size;
• ice fields consisting of ice floes with a diameter of 2 - 10 km;
• small ice fields - 0.5 - 2.0 km across;
• fragments of fields - 100 - 500 m in diameter;
• coarse ice - ice floes with a diameter of 20 - 100 m;
• small broken ice - ice floes measuring 2 - 20 m in diameter;
• grated ice - broken ice less than 2 m in diameter;
• nesyak - a large hummock or a group of hummocks frozen together and representing a separate ice floe, up to 5 m high;
• large nesyak - a medium-sized heavily hummocked ice floe, rising 5 m above the water;
• small nesyak - a small piece of ice of a greenish tint, barely rising above the water;
• ice porridge - an accumulation of ice consisting of fragments no more than 2 m in diameter;
• iceberg - a monolithic piece of ice that has broken off from a glacier, protruding more than 5 m above sea level and floating (or aground); According to their shape, icebergs are divided into table-shaped, dome-shaped, inclined, pointed-topped, rounded or pyramidal;
• ropak - a separate ice floe standing vertically or obliquely and surrounded by relatively smooth ice.

Terminology

Average ice extent limits are the average position of the ice edge for a given month or season, derived from long-term observations.

Rare ice— various types of floating ice, mostly broken, evenly distributed and occupying up to 30% of the visible sea surface (concentration 1 - 3 points).

Thin Ice- various types of broken drifting ice, occupying more than half of the visible surface (concentration 4 - 6 points). Ice thinning is caused by two reasons:

• tidal currents, periodically compressing and thinning the ice, and
• melting ice.

Solid Ice- accumulation of floating ice covering about 80% of the visible surface (concentration 7 - 9 points).

Solid ice- a continuous mass covering the entire visible space of the sea (cohesion 10 points).

Nuclear-powered icebreaker Russia moving in the ice

Ice can be light, heavy, or deformed.

Light ice up to 60 cm thick can be easily overcome by icebreakers, and under favorable conditions - by ships with reinforced hull reinforcement.

Heavy ice more than 60 cm thick with hummocks more than one year old is difficult to overcome only by powerful icebreakers.

Deformed ice, layered with a depth of layers up to 20 m. This ice is hummocky and can be impassable even for the most powerful icebreakers.

Hummocking- a type of formation of ice obstacles when faults, collisions and compression of ice form hummocks.

Hummocks- a pile of ice floes, usually frozen; can be located in separate formations and groups, often in ridges.

Depending on their location, hummocks can be coastal or offshore. They are formed from breaking, crushing and advancing ice.

In slush, ships move easily, but the dense elastic cover of snow makes movement difficult, since it does not prick the stem, but only compresses; thin ice or crust, vessels pass through with some difficulty.

Ice compression- compaction under the influence of winds and currents. Ice compression is also observed during changes in tidal currents, regardless of the winds. Winds can only strengthen or weaken, delay or accelerate tidal compression. This phenomenon is the biggest difficulty for swimming.

The concentration of floating ice is determined on a ten-point scale:

Drifting Ice Concentration Scale
Points	Area size	Characteristic
0	No ice	Pure water
1	The area occupied by drifting ice is 9 times less than the area of ​​the water gaps between them	Rare ice
2	The area occupied by drifting ice is 4 times less than the area of ​​the water gaps between them	Rare ice
3	The area occupied by drifting ice is 2 - 2.5 times less than the area of ​​the water gaps between them	Rare ice
4	The area occupied by drifting ice is 1.5 times less than the area of ​​the water gaps between them	Thin Ice
5	The area occupied by drifting ice is equal to the area of ​​the water gaps between them	Thin Ice
6	The area occupied by drifting ice is 1.5 times larger than the area of ​​the water gaps between them	Thin Ice
7	The area occupied by drifting ice is 2 - 2.5 times larger than the area of ​​water gaps between them	Solid Ice
8	The area occupied by drifting ice is 4 times larger than the area of ​​the water gaps between them	Solid Ice
9	The area occupied by drifting ice is 9 times larger than the area of ​​the water gaps between them	Very compact ice
10	Ice floes completely cover the visible surface of the sea	Solid ice

Signs of approaching ice

To ensure navigation safety, it is very important to detect the approach of ice in advance, especially in poor visibility or fog, in order to promptly reduce the speed, strengthen surveillance, and check the location of the vessel. Signs of approaching ice are:

• “Ice reflection” or “ice sky” is a characteristic whitish reflection on clouds above individual accumulations of ice. The reflection is especially clear when the air is well transparent, when the ice is covered with snow;
• “water sky” - dark spots on low clouds over areas of clear water located among ice; dark spots on clouds are sometimes a reflection of dirty ice. Under cloudless skies, clear water or ice can sometimes be detected by refraction;
• a decrease in sea water temperature, sometimes sharp, indicating an almost extreme approach to ice;
• a decrease in air temperature observed when approaching vast fields of ice, especially with wind from the ice.
• change in wave character; a short wave, sometimes pushing when approaching the ice from the windward side and weakening when approaching from the leeward;
• the appearance of small pieces of ice and “ice porridge”;
• the appearance of fog over the horizon;
• noise, crackling and rustling heard when approaching hummocky ice;
• echoes from whistles or gunshots, reflected from close, high hummocky ice masses and from large icebergs;
• the appearance of walruses, seals and flocks of birds.

Ice maps

An ice map gives a general idea of ​​the distribution of ice in the navigation area. Information about the state of ice is obtained using artificial Earth satellites, ice reconnaissance aircraft and helicopters, ship observations, coastal observation points, and automatic drifting ice stations. Using all this information, coastal services prepare ice charts that are transmitted to ships.

The decision about the movement of a vessel in ice is made based on the analysis of ice maps, on which the characteristics of the ice cover are displayed in the form of symbols. The main symbol in this symbol system is an oval, which indicates the main navigation characteristics of ice (Fig. 1), where the letter C indicates the total ice concentration in points.

Rice. 1 Oval Sea Ice Symbol

• Ca, Cb, Cs— ice concentration of the thickest (Ca), less thick (Cb) and third thickest (Cc), points;
• Sa, Sb, Sc— age of ice, the concentration of which is, respectively, Ca, Cb, Cc;
• Fa, Fb, Fc- the predominant forms of ice, the age of which is, respectively, Sa, Sb, Sc.

The following basic numeric symbols are used for ice age:

• 1 - initial types of ice;
• 2 - nilas, up to 10 cm thick;
• 3 - young ice, 10 - 30 cm thick;
• 4 - young ice, 10 - 15 cm thick;
• 5 - young ice, 15 - 30 cm thick;
• 6 - first-year ice, 30-250 cm thick;
• 7 - old ice, more than 250 cm thick;
• Δ—continental ice;
• X—age unknown.

The following digital symbols are used to indicate the shape of ice formations:

• 1 - grated ice or ice porridge;
• 2 - crushed ice;
• 3 — coarse ice;
• 4 — fragments of ice fields;
• 5 - large ice fields;
• 6 - extensive ice fields;
• 7 - giant ice fields;
• 8—fast ice;
• 9 - icebergs;
• X - shape unknown.

An example of the use of the oval sea ice symbol shown in Fig. 1 means that in this area there is ice with a total concentration of 6 points. Of these, 2 points are fragments of old ice fields, 1 point is large broken young ice, 3 points are nilas, the shape of which is not determined.

Along with the main symbol - the oval, other symbols are used on the ice map, complementing and specifying the overall picture of ice distribution:

Additional Ice Symbols
	ice hummockiness, in points;
	ice destruction, in points;
	snow cover of ice (C – area of ​​snow-covered ice in tenths of the total area; S – snow cover in points ← direction of sastrugi);
	ice compression in points;
	recommended routes.

On the ice map, each ice zone with approximately the same characteristics is distinguished along its border by isolines (Fig. 2). For clarity, different areas may be shaded.

Rice. 2 Ice map

Legend
Coloring of overview maps by age (stages of development) of ice: used during formation, formation and partial destruction of ice “winter coloring by age”
Age characteristics of ice:
conditional coloring by color:		use of graphic symbols:
* * *	initial types of ice
⊛	nilas, flask (thickness up to 10 cm)
	gray ice (10-15 cm)
	gray-white ice (15-30 cm)
	thin first-year (white) ice (30-70 cm)
	medium-thick first-year ice (70-120 cm)
	thick first-year ice (more than 120 cm)
	residual first-year ice
	two-year ice (up to 2.5 m or more)
	multi-year ice (about 3 m or more)
Forms of floating ice:		Legend, age:
	crushed ice		Nilas
	crushed ice		grey
	debris from ice fields		gray-white
	large fields		thin
	vast ice fields		average
	giant ice fields		thick
	ice porridge		old
	pancake ice		fast ice
Age characteristics of fixed ice (fast ice) in cm:		Generalized characteristics of ice:
	nilas ice (5-10 cm)		age composition of drifting ice
	young ice (10-30 cm)		ice hummockiness (in points)
	thin first-year ice (30-70 cm)		compression index (in points)
	first-year ice of medium thickness (70-120 cm)		ice layering
	thick first-year ice (>120 cm)		ice breakdown

Coloring of overview maps by cohesion: used during the period of destruction and melting of ice "summer coloring by cohesion"
Ice concentration:		Forms of floating ice:
	solid, frozen solid. and very spl. drifting. ice (9-10/10)		crushed ice
	solid ice (7-8/10)		crushed ice
	discharged ice (4-6/10)		debris from ice fields
	rare ice (1-3/10)		large fields
	individual ice floes (<1/10)		vast ice fields
	pure water		giant ice fields
	iceberg waters		ice porridge
			pancake ice
Legend
	purely
	1-3
	4-6
	7-8
	9-10
	10
	fast ice

Modern means of delivering and displaying hydrometeorological information on ships

In 2006, on the basis of the Arctic and Antarctic Institute (AARI), a system for monitoring and forecasting the state of the atmosphere and hydrosphere was created to support maritime activities in the Arctic and freezing seas of the Russian Federation.

The main sources of initial information are:

• artificial earth satellites;
• ground network of coastal and island polar stations;
• automatic drifting buoys;
• domestic and foreign centers of hydrometeorological information.

Problems to be solved:

• ice control;
• long-term planning of operations;
• choosing the optimal sailing route.

As a result, an “ice terminal” was developed that allows the following data to be displayed on the ship’s computer monitor in the form of opaque and transparent layers, combined with the navigation map:

• surface images obtained from satellites;
• actual ice charts;
• forecast ice maps;
• synoptic maps and weather forecasts;
• navigation recommendations.

Information is received through communication channels provided by Inmarsat, Globalstar, Iridium or Internet systems. Below are examples of the use of “ice terminals” on ships (Fig. 3 – 6).

Rice. 3 Ice map Rice. 4 Ice forecast in the Strait of Tartary Rice. 5 Recommended route in the Strait of Tartary Rice. 6 Vessel route when traveling in ice

Suggested reading:

Sea ice is classified:

by origin,

according to shapes and sizes,

according to the condition of the ice surface (flat, hummocky),

by age (stage of development and destruction),

according to navigation criteria (ice passability by ships),

according to dynamic characteristics (fixed and floating ice).

By origin Ice is divided into sea, river and glacier ice.

Marine ice is formed from sea water and has a greenish or whitish (in the presence of air bubbles or snow) tint.

Freshwater Ice is carried out from rivers in spring and summer and has a grayish or brownish tint due to inclusions of suspended matter.

Glacier ice (of continental origin) is formed as a result of the calving of glaciers descending into the sea - icebergs, drifting ice islands.

By appearance and shape ices are divided into:

ice needles, formed on the surface or in the water column,

ice lard– accumulation of frozen ice needles in the form of spots or a thin layer of grayish lead color,

snowflake– a viscous mushy mass formed during heavy snowfall on chilled water,

sludge– accumulation of lumps of ice, snow and bottom ice,

Nilas– thin elastic ice crust up to 10 cm thick,

bottle– thin transparent ice up to 5 cm thick, formed from ice crystals or fat in a calm sea,

pancake ice– ice, usually round in shape with a diameter of 30 cm to 3 m and a thickness of up to 10 cm.

According to the age ice happens:

young ice is 15-30 cm thick, has a gray or gray-white tint,

annual ice - ice that has existed for no more than one winter, with a thickness of 30 cm to 2 m.

two-year– ice that reached a thickness of more than 2 m by the end of the second winter,

perennial pack ice is ice that has existed for more than 2 years, more than 3 m thick, blue in color.

By navigation feature ice permeability is assessed on a 10-point scale cohesion ice. Ice concentration (thickness) is the ratio of the area of ​​ice floes and the spaces of water between them in a given area. The practice of ice navigation has shown that independent navigation of an ordinary sea vessel is possible when the concentration of drifting ice is 5-6 points.

According to dynamic characteristics Ice is divided into fixed and floating.

Fixed ice exist in the form fast ice off the coast. The thickness of perennial fast ice off the coast of Greenland is more than 3 m, and off the coast of Antarctica there are tens and even hundreds of meters. The thickness of one-year fast ice in the Arctic Ocean is about 2–3 m, the width is up to 500 km (Laptev Sea).

floating Ice is formed either by freezing of floating ice or as a result of breaking off fast ice.

The term used to refer to any type of floating sea ice drifting ice.

The sizes of drifting ice are different: when the size is more than 500 m in diameter, they are called icyfields, for sizes 100…500m - ice fragmentsfields, with sizes 200...100m - large ice, for sizes less than 20m - , crushed ice.

The movement of ice occurs under the influence of wind or currents, under the influence of which they change their compactness. When the wind blows onshore, the concentration of drifting ice increases; when the wind blows from the shore, the ice thins out. As the speed of currents increases, the ice thins out, and as the speed decreases, the ice accumulates. The accumulation (compression) of ice occurs during the change of tidal currents, and lasts 1-2 hours, after which thinning of the ice is observed. When the water level rises, the ice thins out, and when it falls, it consolidates.

Glacier ice – icebergs(ice mountains) form in areas of the Arctic Ocean and off the coast of Antarctica. Currents carry them to moderate latitudes of both hemispheres. Icebergs sometimes reach enormous sizes. In 1854, in the area of ​​44°S. 28°W. An iceberg 120 km long and 90 m high was encountered. Only a tenth of the iceberg rises above the water.

When the sea surface cools to freezing point temperature, a large number of disks or plates of pure ice, called slush, appear in the upper layer of water (a few centimeters thick) . The thickness of these ice floes is very small, the average size is approximately 2.5 cm * 0.5 mm, and the shape can be extremely varied - from squares (or almost squares) to hexagonal formations. The optical axis of such a plate is always perpendicular to the plane of its surface. These elemental ice crystals float on the surface of the water, forming what is called ice grease, which gives the surface of the sea a somewhat oily appearance. In calm water, the plates float in a horizontal position and are With- the axes are directed vertically. Wind and waves cause the plates to collide, turn over and take different positions as a result; Gradually freezing, they form a permanent ice cover, in which individual crystals are randomly oriented. In the first stage of formation, young ice is surprisingly flexible; under the influence of waves coming from the open sea or caused by a moving ship, it bends without breaking, and the amplitude of vibrations of the ice surface can reach several centimeters.

Subsequently, if the temperature does not increase, individual plates play the role of seed crystals. The mechanism of this process has not yet been fully studied. As can be seen from Fig. 4, ice consists of individual crystals, each of which has purely individual properties, for example, the degree of transmission of polarized light (the same for the entire given crystal, “but different from others). In some cases, a structural cell of ice is called a grain rather than a separate crystal, since it is clear that it has a complex substructure and consists of many parallel plates. The relationship between this substructure and the primary sludge mentioned above is quite obvious. There is no doubt that some of the grain is formed from frozen sludge plates, which are then preserved as separate layers of crystal. However, apparently, there is some other process, since in some cases crystals begin to grow on the lower surface of a fairly thick ice cover, and they also have a plate-like structure. Whatever the mechanism of crystal formation, all of them - both in sea ice and in freshwater - consist of a large number of plates, exactly parallel to each other. The optical axis of the crystal is located perpendicular to these plates.

Interesting results are obtained from studying the distribution of crystals according to the orientation of their optical axes depending on the depth of their occurrence in the ice thickness. Orientation can be characterized by two angles - polar, which is the angle between c-axis both vertical and azimuthal, i.e. an angle measured from some arbitrary direction, for example from the north-south line. The magnitudes of azimuthal angles usually do not obey any law; rare exceptions to this rule may be caused by unusual tidal phenomena. Polar angles exhibit a certain pattern. As mentioned above, the orientation of crystals near the ice surface is quite variable, since it depends on the influence of wind during ice formation. But as you go deeper into the ice, the polar angles increase, and at a depth of about 20 cm The optical axes of almost all crystals are oriented horizontally. A laboratory study of the freezing of distilled water (Perey and Pounder, 1958), provided that it was cooled from only one direction and the water was in a calm state, gave the results shown in Table. Horizontal sections were taken from the ice surface and from depths 5 and 13 cm. Each section was examined using a universal polariscope. At the same time, the ratio of areas (in percentage) occupied by crystals with the same - within 10-degree intervals - orientation of the optical axes was determined.

Orientation of crystals in ice sheets (Pounder, 1967)

A similar situation is observed in natural sea ice that has reached a certain “age”. Exceptions occur in cases where, during the growth of the ice cover, movements occur that cause compression and fracture of the ice. Thus, the bulk of sea ice that has existed for a year or more consists of crystals, the optical axes of which are directed horizontally and oriented chaotically in azimuth. The length (vertical height) of such crystals reaches 1 m and more, with a diameter from 1 to 5 cm. The reasons for the predominance of crystals with horizontal optical axes in ice help to understand Fig. 4. Since an ice crystal has one main axis of symmetry, it can grow primarily in two directions. Ice molecules attach to the crystal lattice either in planes (of the crystal) perpendicular to c-axis and called basal planes , or in the direction of the c-axis, which in turn leads to an increase in the area of ​​the basal planes. Based on the laws of thermodynamics, we can come to the conclusion that the first type of crystal growth should be more intense than the second, which is confirmed by experiments.

Rice. 5 The predominance of crystal growth with inclined optical axes, causing the gradual disappearance of a crystal with a vertical With-axis. (Pounder, 1967)

Ice Interface -water

Studying the undersurface of growing sea ice helps understand how water freezes. Lower 1-2 cm Ice strata consist of plates of pure (fresh) ice with layers of brine between them. The plates that make up part of a separate crystal are parallel to each other and are usually located vertically. This is the so-called skeletal (or frame) layer. The mechanical strength of this layer is usually extremely low. With further freezing, the plates thicken somewhat, ice bridges appear between them and solid ice gradually forms, in which the brine is contained in the form of drops or cells between the plates. A decrease in ice temperature leads to a decrease in the size of the cells filled with brine, which take the form of long vertical cylinders of almost microscopic dimensions in cross section. Such cells can be found in Fig. 4 in the form of rows of black dots located along the lines between the plates. A certain number of brine cells are also present at the boundaries between the crystals, but the bulk of the brine is contained inside individual grains. In Fig. Table 5 shows the results of a statistical study of the thickness of the plates in a sample of annual sea ice. It can be seen that the plates have a uniform thickness, on average in the range of 0.5-0.6 mm. The diameter of the nests containing brine is usually about 0.05 mm.

Rice. 6

Sufficient data on the length of such nests is still not available; it is only known that it fluctuates within much wider limits than the diameter. Approximately we can assume that the length of the nests is about 3 cm.

Thus, we see that in most cases sea ice consists of macroscopic crystals with a complex internal structure - it contains plates of pure ice and a large number of cells containing brine. In addition, ice usually contains many small spherical air bubbles formed from air dissolved in water, released during the freezing process. The portion of sea ice volume occupied by liquid brine is an extremely important parameter called brine content v (Fig. 6). It can be calculated by knowing the salinity, temperature and density of sea ice. Based on the knowledge of the phase relationships of salt solutions contained in sea water at low temperatures, (Assur, 1958) calculated v for those values ​​of salinity and ice temperature that are found on the globe. The results obtained by Assur do not take into account the presence of air bubbles in the ice, but the effect of the latter on the value of v can be determined experimentally by comparing the density of a sample of sea ice with the density of freshwater ice at the same temperature. (Pounder, 1967)

Rice. 7 Brine migration along a temperature gradient (Pounder, 1967)