Arctic Sea Ice
Sea ice is formed from frozen sea water, so it is salty, and the actual temperature it freezes at is -1.8C or 28.8F. Because the Arctic is an Ocean surrounded by land, there is a lot of sea ice on the Arctic Ocean. There is concern because in recent summers the sea ice has melted more than in previous summers, the summer of 2007 being the lowest area of ice since satellite measurments began in 1979.
Sea Ice Formation
Sea ice forms when the temperature falls well below freezing. Ice crystals form on the surface of the sea, when there is sufficient density of these crystals they form an ice type called frazil or grease ice, at this stage it is very plastic and the ripples in the water can be seen moving the ice, as the grease ice gets thicker it is called nilas and is transparent. Once this skin has formed, the ice then accumulates on the underside of it, this is called congelation growth. If the sea is rough enough to break up the nilas into plates, they can bump into each other until the edges become rounded and upturned, this is called pancake ice. By the end of the winter this ice could be 1.5 2 metres thick and is known as first-year ice.
Frost Flowersfrost flowers blooming on the Nilas. These delicate little bunches of ice crystals are very high in salt and at one time were believed to be forming around the salt, but now scientists from Cambridge University grew Frost flowers from Fresh water that was at 0C, by cooling the air above it to -20C. The important factor seems to be the temperature difference between the water and the air above it
Sea Ice Types
Fast ice is the ice that is attached to the coastlines, or to the bottomof the sea in shallow conditions, it is landfast ice and does not move around either with the wind or the tides. The area where it adheres to land is called Ice Foot. In areas of high tidal ranges, at low tide there is often a steep cliff between the fast ice that is moving and the ice foot, this takes place at the tide crack or lead. In winter months the fast ice can extend out to sea for many kilometers. The interface between fast ice and pack ice is the floe edge. Here the pack ice, composed of ice floes, can move in and out with the tides, winds and currents. The floe edge is part of the ring of life, the area where marine mammals can breathe air and haul out on the ice, where polar bears have the opportunity to hunt them for food.
The drifting or Pack ice is not tied into fast ice, but it may have been at one time, and broken off. These large blocks of ice are sometimes driven into each other by the winds and currents, forming pressure ridges, these can appear impassable on the surface but they also extend underwater, sometimes to a depth of nearly 50m. Large pressure ridges can be a considerable obstacle either for indigenous hunters with dogsleds or for parties skiing and man hauling sleds on the Arctic Ocean. In Winter the Arctic has 15,000,000 sq km of sea ice.
The frozen Arctic Ocean is the Polar Ice Pack, now very confusingly often referred to as the Arctic Ice Cap, a name previous used only for large bodies of freshwater glacial ice, as in the Greenland Ice Cap. The behaviour of the Polar Pack ice can influence climate behaviour because the white of the ice has the albedo effect and reflects the sunlight, while sea water absorbs the heat from the sun.
Text © B & C Alexander