Your Questions, Answered: Why Does the Ocean Freeze

But that wasn’t always the case. Maine’s bitter winter temperatures regularly caused the ocean to freeze, as early 1900’s photographs in the Penobscot Marine Museum collections illustrate. Large plates of sea ice would rise and fall with the tides, scouring coastal mud flats and the shore as they moved back and forth. Sometimes entire bays would freeze over as a result of long stretches of frigid weather, drawing people out onto the frozen ocean.

People visited the Monument off Belfast harbor, one cold winter. Penobscot Marine Museum photo.

How can the salty ocean turn to solid ice? Freshwater ice forms fairly quickly on lakes and rivers each winter. The ocean also can freeze, however it requires much colder temperatures over a much longer period.

Think of salt water as a bunch of blue and yellow marbles. The blue marbles are H2O molecules; the yellow are sodium (salt) ions. At room temperature, these marbles roam about chaotically, bumping into each other and bouncing away. To turn into sea ice, the blue marbles must slow down and become more organized.

Drop the temperature of salt water to 32o F., the temperature at which fresh water freezes and nothing much happens in salt water other than that the water molecules become a bit more sluggish.

Drop the temperature again, to 28o F. and below. At this point the blue marbles have stopped bouncing around and are beginning to draw together in a crystal or lattice formation. That formation is ice. The salt ions can’t fit into that lattice. They are forced out into the surrounding water or isolated in tiny brine pockets within the sea ice.

The Diane A., left, and the Janet M. sit in the ice at the Medomak Town Landing Jan., 1971. Penobscot Marine Museum photo.

But as the salt molecules are pushed into the water, the water itself becomes saltier. In order for that saltier water to freeze, the temperature has to drop even lower. Saltier water is denser, however, and so the water sinks deeper into the water column. For thick layers of sea ice, the top 300 to 450 feet of the ocean must be below 28 degrees.

There are different names for different stages of sea ice. As the ocean water first begins to freeze, small needle-like ice crystals called frazil form. If the ocean stays calm, frazil crystals turn into smooth, thin ice, called grease ice, which moves easily on the water surface. Grease ice will slowly develop into a continuous sheet of ice called nilas. Currents or light winds will push the nilas around so that they slide over each other. If not broken up, the nilas can thicken into a stable ice sheet with a smooth bottom surface.

According to the University of Maine Climate Change Institute, the upper Penobscot Bay typically froze once or twice a decade during the 1800s and the early 1900s. During the cold winter of 1904-1905, the sea ice was thick enough that people took their horse-drawn sleighs out across Belfast Harbor to the Monument, a nautical marker half a mile from shore. One notable and long-lasting cold snap in 1915 caused Penobscot Bay to freeze up as far south as Rockland and as far east as Deer Isle.