During the first half of each year the waters off Newfoundland may become covered with floes of sea ice or "pack ice". While icebergs are composed of fresh water, pack ice is frozen sea water. The severity of ice varies considerably, depending on the strength and direction of the wind and air temperature. Most of the pack ice off Newfoundland's northern and eastern shores originates off Labrador. While some of the pack ice off the west coast also comes from the sea off Labrador via the Strait of Belle Isle, most of it originates in the Gulf of St. Lawrence. Beginning in January the pack ice begins to advance south, borne by the Labrador Current until (usually) in April the rate of melting overtakes the rate of advance and the ice retreats northward. The leading edge of the pack ice is known as "The Front" and is important to the annual seal hunt off Newfoundland's north coast.