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When Saltwater Freezes

Vol. II, No. 85

Frozen saltwater along the Westport shoreline. Photo by John Kantor.

Salt melts ice. Any veteran snow-shoveler, trying to clear an icy sidewalk or driveway, knows that.  Every highway snow-plow crew knows it too. Salt can prevent ice from forming, and it can help melt it too. Salt is like anti-freeze. So, what’s the deal with saltwater ice?

Fresh water freezes at 32 degrees Fahrenheit. That’s common knowledge. Saltwater freezes too, but at a lower temperature. How low? It depends on the salinity. It’s an inverse relationship. The saltier the water, the lower the temperature at which it freezes. The saltwater of open ocean, or of Long Island Sound, has a lower freeze point than the brackish water typically found right along the coast, say, in marshes and estuaries fed by fresh water tributaries. Its lower salinity means a higher freeze point, so it ices up earlier when the deep freeze temperatures set in.

But salinity isn’t the only thing which affects saltwater ice formation. Wind strength makes a difference too. The recent wintry temperatures froze certain areas of Long Island Sound. The parts which froze first were those in the lee of the strong northwest wind. Wind agitates the water and creates waves. The motion deters ice crystals from forming. Water needs to be relatively still for ice to transform from its liquid to its solid state. Taking advantage of this property, many marinas use a bubbler system (underwater air-blowing pipes) to keep the water moving and to prevent ice formation around their floating docks and boats stored in-water for the winter.

Ice can be very damaging to things left in the water. Water expands when it freezes. It expands about 9% or so in physical size. When water needs room to expand to become ice, it makes its own space and is not easily denied. The expansion and contraction cycles of freezing and thawing are forceful enough to pry open pavement cracks and cause potholes in streets.  They also can crush boats, floating docks, and mooring balls.

Frozen saltwater does not usually sit still. Coastal ice floats up and down and in and out with each tide cycle. Its power is immense.  I have seen it slowly lift and drag along whole mooring fields. Much like a glacier, a moving ice sheet of sufficient thickness can snap exposed 500 lb. steel mushroom anchors like toothpicks.

But take heart. The spring thaw is just around the corner, so scenes like this will soon be distant memories… at least until next winter.