The polar pack ice is interesting. Ice begins to form in the sea when temperatures are about 0 to -1.80 degrees Centigrade and the seas are calm. The first stage is the development of small needle-shaped crystals which coalesce and spread to form a film over the sea's surface. This slush ice may thicken to form an opaque skin of ice called nilas. When broken by wind action, it gathers into clumps and forms disks up to 3 meters wide. The raised rim and circular shape, due to the striking together of disks, gives the ice the appearance of lily pads from which is derived the name "paddy" or "pancake" ice.

If the temperature remains below freezing, 3 to 4 inches of pancake ice may coalesce. While generally developing a rough surface, a snow cover will give the appearance of a smooth white plain. By mid-winter, the ice thickens up to 10 to 20 feet and may last from 5 to 8 years.

Warming temperatures and wind can disintegrate an ice field even in mid-winter. Cracks may enlarge to form canals or "leads" of open water (as seen in this picture) contracting black with the white of the pack ice.

Pack ice drift may be produced by currents and wind. When wind blows a steady thirty knots for half a day or more, the wind force prevails over ocean currents. Smooth ice drifts with less speed than ridged ice; on the other hand, ridged ice fields have more inertia, but once it begins to move, it continues to move after the winds stop. Wind also sorts ice on the sea's surface, packing small floes to windward against larger floes.

One season's winter ice may begin as a flat, featureless area covered with snow, but due to ice movement is often heaped up into pressure ridges, that is one layer of ice overriding another. Polar ice, through weathering under successive summer temperatures, has a more undulating surface although pressure ridges may rise 30 or more feet in height, often showing up as colossal chunks.



copyright (c) 1985, 2000 Robert A. Antol

For more information, contact Bob Antol
URL: file - images/hazen011_5.html