Ice at sea can be classified as following types:
- Sea Ice, formed by the freezing of sea water.
- Icebergs, huge masses of floating ice.
- River Ice, sometime encountered in harbours during spring break-up, generally presents only a temporary hindrance to shipping.
Sea Ice accounts for most of the ice met with at sea. Icebergs are important because they are dangerous to navigation but they are confined to limited areas.
Now let us understand the freezing of water
Fresh water and salt water do not freeze in same manner due to the presence of dissolved salts in sea water. Fresh water freezes at 0°C (32°F), due to dissolved salts in sea water it needs to be more colder - 2°C (28.4°F) to freeze. The greater the salinity, the lower the freezing temperature. Average seawater is of salinity 35 parts per thousand (PPT) though in some areas, where there is considerable discharge of river water, the salinity is much less.
The importance of salinity lies not only in lowering the freezing temperature, but also in its effect on the density of the water.
As the surface cools, the surface water becomes denser and sinks, creating convection currents. Water will not freeze until the entire body of water has cooled to freezing temperature. Hence, the surface of deep-sea waters does not freeze, even if the air temperature is extremely low.
The maximum density of fresh water occurs at 4°C and thus if further cooled, the density decrease and so the convection stops. Once this stable condition is achieved, cooling of surface water leads to a rapid drop in temperature and formation of ice begins when temperature falls to 0°C.
With salt present in water, it delays the lowering of temperature of the water to below its normal freezing point.
As seen in diagram, if the salinity is 24.7 PPT, the maximum density occurs at -1.3oC. Thus, once maximum density is reached, further cooling will result in formation of ice.
Formation of ICE at sea
The first indication of ice is the appearance of ice spicules or plates with max dimensions upto 2.5cms which occurs in the top few cms of water. These spicules, known as frazil ice. Frazil ice gives the sea surface an oily appearance. As cooling continues the frazil cystals of ice coalesces to form grease ice which has a matt appearance. Under near freezing, but yet ice free conditions, snow falling on the surface forms slush. These forms may break up, under the action of wind and waves to form shuga. Frazil Ice, Slush, Shuga and Grease Ice are classified as New Ice.
Further cooling, sheets of ice rind or nilas are formed depending on the rate of cooling and on the salinity of the water. Ice rind is formed when water of low salinity freezes slowly, resulting in a thin layer of ice which is almost free of salt. Whereas Nilas is formed if salinity of water is high then rapid cooling leaves pockets of salts in Ice which gives it elastic property. Nilas is Subdivided into Dark Nilas (less than 5 cms thick) and Light Nilas (5-10 cms thick).
The action of wind and waves may break up ice rind and nilas into pancake ice which later freezes together and thickness into grey ice and grey white ice, the latter attaining thickness upto 30 cm. These forms of ice are referred to as young ice. Rough weather may break this ice up into cakes or floes.
Further cooling results in First Year Ice (greater than 30 cm thick) which is subdivided into Thin First year (30 to 70 cm), Medium First year (70-120cm) and thick First year (greater than 120 cm thick). Thin First Medium first year ice has a range of thickness from 70-120 cms.
Ice that has survived one summer melt in the Artic is termed Old Ice. After 1stOctober this ice is classified as Second year ice. This ice is usually found in Polar Regions but some floes may drift south. Sea Ice that survives two summer's melt is called Multi-Year Ice. This type of ice averages 3 to 6 meters thick and is almost salt free. Therefore, it has a density almost the same as solid concrete. This ice will have a higher freeboard (floats higher in the water) and will have a stronger colour of blue. Multi Year ice floes should be given a safe wide berth by all shipping.
Under extreme conditions, when the air temperature may suddenly fall to between -30oC and -40oC, it is possible that a layer if ice can form and grow to a thickness of about 10 cm and in a day, 20 cm in 2 days and 30 cm in 4 days. However, the rate of growth decreases with the passage of time and it would take almost a month at such temperature to reach a thickness of 60 cm.
Different types of Sea Ice
Sea ice is divided into two main types according to mobility: Drift Ice, which is reasonably free to move under the action of wind and current, and fast ice, which does not move.
Fast Ice: Ice first forms near the coasts and spreads seaward. A certain width of fairly level ice, depending on the depth of water, becomes fast to the coastline and is immobile. The outer edge of the fast ice is often located in the vicinity of the 25 m depth contour.
Drift Ice: Beyond fast ice lies drift ice, formed, to a small but fundamental extent, from pieces of ice which have broken off from the fast ice. As these spread seaward they, together with any remaining old ice floes, facilitate the formation of new, and later young, ice in the open sea.
Deformation of ICE
Under action of wind, current and internal stress the pack ice is constantly in motion due to this pressure its surface gets deformed. In new and young ice, this may result in rafting and in thicker ice it leads to formation of ridges and Hummocks.
During ridging and hummocking, when large pieces of ice are piled up above the general ice level, vast quantities of ice are forced downward to support the weight of ice in the ridge or hummock. The downward extension of ice below a ridge is known as an ice keel, and that below a hummock is called a bummock.
Once pressure on the ice field reduces cracks, leads and polynyas may form, offshore wind drive the pack ice away from the coast line and open up a shore lead, which is a navigable passage between the. Main body of packed ice. Off shore winds may develop leads in fast ice called flaw. Flaw usually develops at boundary between fast ice and pack ice. This opening is also called flaw lead.
Pack ice is generally removed by direct action of wind or by current or by melting. During summer, as the temperature rises within the ice, the ice becomes riddled with brine pockets and becomes weak, wind and wave action then assist in melting of ice. Fast ice then breaks up into pack ice which eventually becomes ice floes at advance stage of decay and breaks into small pieces called Brash Ice at last stage before melting
Movement of Ice
Pack Ice moves under the influence of wind and current. Fast ice remains stationary. Wind stress causes pack ice to move downward which gets deflected to right in North Hemisphere and left in South hemisphere due to Coriolis force.
The speed of movement of pack ice will depend upon:
- Wind speed
- Concentration of Pack ice
- Degree of ridging
In open pack ice (1/10 ~ 3/10) the movement of ice will be more compared to close pack ice (7/10 ~ 8/10) because the under water volume is more in close pack ice. The movement of ice is more where degree of ridging is more than in comparison where the ice surface is smooth. The total movement of pack ice will be the resultant of wind drift component and current component.
As major portion of ice remains submerged the pack Ice may move all the full current rate. When the wind blows in same direction as current, the movement increases.