At 35 ppt sea water will begin to freeze at 28.6 degrees F. Shallow bodies of water freeze more rapidly than deep basins because there is less volume to be cooled. This is why the first ice of winter usually appears at river mouths, where the water is shallower and fresher. Sea ice first forms as salt-free crystals near the surface. The resulting formation of crystals is one of nature's better engineering jobs. The newly produced ice is flexible against the rolling action of the sea.
Pack ice consists of pieces of sea ice that drift under the influence of winds and ocean currents.
An individual piece of sea ice in the pack is called a floe and can vary from several feet to several hundred yards in length. The size of the pack depends upon temperature and surface wind. If wind pushes pack ice together or onto shore, it piles up until pressure causes the ice to buckle, forming mounds, hummocks, and ridges up to 25 feet or higher. Ice packs grow to their largest sizes in areas of extreme cold and survive longer during periods of light wind. The two largest areas of pack ice are in and near the Arctic Ocean and in the region of coastal Antarctica.