As per an estimate, sea ice covers about 10,000 square miles of the earth. Sea ice occurs in both the polar zones, Arctic as well as the Antarctic. In the northern hemisphere, it can currently exist as far south as Bohai Bay, China (approximately 380 north latitude), which is actually closer to the equator than it is to the north pole. In the southern hemisphere, sea ice only develops around Antarctica, occurring as far north as 550 south latitude. It therefore becomes interesting to know how ice is formed at sea; how it is sustained; how old the ice could be; wheral & how it travels; etc. Let us first talk about the basic water molecule itself.
Chemically, water molecule is a dipolar compound with covalent bond. The force keeping the molecules together is called Wanderwals Force. Higher the molecular weight higher the Wanderwal’s force. Higher the energy required to break the bond higher the temp required’. There is some definite energy required for breaking the molecules & even higher energy required for separating two molecules. The water is known to show up its anomalous behaviour in respect of its density.
In a simple explanation the anomalous expansion of water can be understood as follows: Hexagon shape is formed out of molecules of H2O in 3 dimensions. When temperature is lowered these hexagonal shapes tend to come closer causing density of water to increase. At 4°C the closeness is maximum. As if at 40 the shapes become watertight shapes to not allow any water particle to enter within. Thus, in the space that is created within hexagons no molecules can come in. At 0° these hexagons come sufficiently close to reach a steady state beyond which density does not change.
A note regarding above curve:
Vertical Axis is of temp from – 30C to +40C.
Horizontal Axis is Salinity from 0 to 350/00.
Max density curve is straight line curve starting from 40C.
Freezing Point curve is straight line curve starting from 00C.
The maximum density curve and freezing point curves meet at –1.330C at 24.70/00 salinity.
The diagram shows the relationship between temperature salinity and maximum density. It can be seen that in water with salinity of less than 24.7 the maximum density is reached before the freezing temperature and where the salinity is greater than 24.7 the freezing point is reached before the density attains its theoretical maximum value.
Let us observe the cooling process of a fresh water pond initially at 200C, as noticed in various situations.
Situation 1: The air on top is at 7oC: Upper water gets cooled to less than 200C & goes down. Warmer water from below rises.
Situation 2: The air on top is at 4oC: Temp of water is lowered to 4°C & due to convection it sinks below to displace warmer air. Convection will cease after the entire water mass is of 4°C.
Situation 3: The air on top is at 0°to 4oC: A layer of water with temp less than 4°C remains on top. Water at 4°C sinks or remains below. Since the water at 00 is lighter than that at 40, it cannot descend therefore cooling only of top layers takes place.
Situation 4: The air on top is at 0° or <0°: Ice is formed on top & acts as insulator. The lower water is at 4 °C.
Sequence: Ice spicules to Ice floes
Freezing of saline water does not occur in the same manner as fresh water. This is due to the dissolved salts in sea water. Sea water has a salinity of 350/00. The first indication of ice is the appearance of ice spicules or plates, with maximum dimensions up to 2.5cm, in the top few cm of water. These spicules known as frazil ice, coalesces to form grease ice, which has mat appearance. Under near freezing, but still the ice-free conditions, snow falling on the surface and forming slush may induce the sea surface to form a layer of ice. These forms may breakup under the action of wind and waves to form shuga. Frazil ice, slush, shuga and grease ice are classified as new ice. With further cooling sheets of ice rind or nilas are formed depending on the rate of cooling and on the salinity of water. The action of wind and waves may break ice rind and nilas into pancake ice which later freezes together and thickens into grey ice and grey white ice, the later attaining thickness up to 30cm. These forms of ice are called young ice. Rough weather may break this ice into cakes or floes.
Rafting: Rafting is most common in new and young ice. It is caused by pressure process. Finger rafting is a type of rafting whereby interlocking thrusts are formed, each floe thrusting ‘fingers’ alternately over and under each other. It is common in nilas and grey ice.
Floe: A floe is any contiguous piece of sea ice. Floes may be described in terms of several size based categories. A Giant floe can be over 10 km across. Vast floe is 2 to 10 km across. Big floes are up to 2000 m across. Then there are medium and small floes. The floes less than 20 m across are called cake ice.
Freezing & Brine drain
As the ice grows downward, brine is frozen into the ice crystals and thereafter drains downward. The salinity changes as the ice cover thickens. During the summer season, melted water from surface drains through the ice crystals, helping to flush out additional brine from the ice crystals. Ice which survives for more than one year forms a distinct horizontal layer, representing ice growth over an entire season. Old ice is normally thicker than the first-year ice. It is lower in salinity than upper layers. With low in salinities, older ice is much more stronger than the first-year ice. A heavy snow cover sometimes, depresses the underlying ice to below the water level. The lower layers of the snow cover along with water, freeze. This causes an ice layer to form. This happens over Great Lakes waters. The amount of brine trapped in the ice depends on the rate at which ice forms. Slow ice growth allows a large portion of the brine to drain away. The greater the brine content, weaker the ice would be.
The rate of freezing depends a lot on the severity and duration of cold air. At -300 to -400C, grey ice can form in open waters in 24 hours. However, the thickening ice also acts as an insulator against the cold air and the growth rate gradually diminishes. Even at these low temperatures, it would take a month for the ice to reach the thin first-year stage. Snow cover, has much greater insulating value than sea ice and thus, will contribute to lower growth rates.
Older ice is stronger
The age of ice is another important factor to consider. As air warms and the ice approaches its melting point, entrapped brine begins to drain away, lowering the overall salinity of the ice cover. Should temperatures drop back below the freezing point before the ice melts entirely, it will re-freeze as purer and stronger ice. For this reason, ice more than one year old will be stronger than first-year ice for a given thickness and temperature. This is an important factor to consider when navigating in regions where old ice may be found.
Movement of Ice
Ice, first forms near the coast and grows seawards. The fast ice is found more in shallow coastal areas, or ones with many islands, which are closely spread. Beyond the fast ice, the pack or drift ice will be found moving depending on wind and water current. It is estimated that pack ice will move at 300 to the right (northern hemisphere) of the wind direction at about 2% of the wind speed. The net movement of the ice is a complex result of both wind and water forces and therefore is difficult to forecast.
Thermal forces cause the deformation of Ice:
As temperatures drops, ice expands. For example, for a drop in ice temperature from -20 to -30C and a salinity of 10 parts per thousand, ice will expand by 30 cm for every 120 m of ice floe diameter.
Weather forces contribute additional energy to deform pack ice. As ice is subjected to pressure from winds or currents, it may fracture and buckle to produce a rough surface. In new and young ice, this results in rafting as one ice sheet overrides another.
First-year Ice: Sea ice is of not more than one winter’s growth. It may be 30 cm or more in thickness. It may be subdivided into thin first-year ice (sometimes referred to as white ice), medium first-year ice and thick first-year ice.
Second-year Ice: Old ice which has survived only one summer’s melt is called second-year ice. Thicker than first-year ice, it stands out of the water.
The component of the ice cover which is actually second-year ice is normally limited to the upper 50-100 cm, with the remainder being first-year ice. Thus, second-year ice may be recognised when pieces turn on their side, by the presence of a distinct, cloudy boundary between the two layers which is several centimeters thick. Below the boundary, the first-year ice will usually be apparent from its faint green colour of vertical columnar crystalline structure.
Multi-year Ice: The distinctive feature of the multiyear ice is the presence of hummocks and meltponds becoming increasingly pronounced. In addition, there is normally a well-established drainage pattern connecting the meltponds. The floes tend to have a higher freeboard than first-year ice. Where the ice is bare, the colour of multiyear ice may appear to be bluer than first-year ice. Multiyear ice floes vary considerably in size, thickness and roughness, depending on their growth and history. Even when the surface is hidden by rubble or snow, it is frequently possible to identify these very strong floes by the first-year ice ridging, which often forms around their perimeter. Multiyear ice is the strongest and hardest form of sea ice and represents a serious impediment to all ships, even to the most powerful icebreakers.
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