Sea Ice Formation
Ice formation in the Arctic begins with frazil ice and continues as pockets of trapped brine migrate downwards.
Salt lowers the freezing temperature of water, so that seawater freezes at about -2° C, whereas pure water freezes at 0°C. When the air temperature drops in the Arctic, a thick layer of seawater becomes ready to freeze all at once. The first signs of freezing are changes in colour and texture as millions of tiny floating ice needles form, creating a dense “soup” called frazil ice. As the temperature falls further, the frazil ice thickens and traps pockets of salty seawater, or brine, within its layers creating a slushy mixture called grease ice. Eventually the grease ice accumulates, reaching a thickness of up to 1 metre. In response to gravity, the pockets of brine move downward, and within a few weeks accumulate in the lower layers. As a result the ice at the surface contains little salt. Their removal from the upper layers increases its freezing point, aiding the change from grease ice to solid ice. Sea ice thus freezes solid from the top layer down. Eventually, all of the salt water is drained and the ice layer is frozen solid into pack ice. During the winter, the area of the Arctic Ocean covered with pack ice expands to form a continuous vast sheet. When melting occurs in the spring and summer, the pack ice retreats and breaks up into smaller pieces.
Grease ice is the next stage of formation Finally, pack ice forms a solid sheet that covers parts of the Arctic ocean