Rare sight in our island cove – freezing salt water

Published 9:00 pm Friday, January 19, 2007

Nature periodically reminds me that its wonders can be far more interesting than the objects that money can buy, as it did during the last snowstorm (although one could argue that I’m just easy to please).

We live on a cove off a saltwater bay, water that I’ve never seen freeze, even around the edges, despite occasional stretches of single-digit temperatures at night and teens and low 20s during the day.

So it was a little hard to process what I was seeing through the Jan. 10 snowstorm. It looked as if a stretch of ice (!) had broken into thousands of pieces, moving slowly as the tide began to change. Near our side of the cove, the pieces were closer together – an ice puzzle starting to come apart.

I had a pet theory involving a creek and wind, but rather than skate on thin ice, I called an expert, Knut Aagaard, professor in the University of Washington’s School of Oceanography.

“Water is a peculiar substance. It’s beautiful and it’s interesting,” Aagaard said.

Water is unusual in that it expands, and thus decreases in density, as it cools below 4 degrees Celsius (about 39 degrees Fahrenheit, its temperature of maximum density), Aagaard said.

“Once it hits (39 degrees), that’s as dense as the water gets, and cooled down below that, it gets lighter and lighter and stays on the surface. Further cooling gives a stable layer, and that doesn’t get mixed down if there’s no wind.”

As the temperature continues to fall, hexagonal-shaped freshwater ice crystals can start forming, and the cool thin layer on top can freeze.

Ice floats because it’s not as dense as water. A cup of water, if it freezes, takes up 8 to 9 percent more space, so a cup of ice is less dense than a cup of water. This lighter-than-water scenario is a good thing. A solidly frozen freshwater lake would probably kill everything in it; instead, the ice layer acts as an insulating blanket.

Saltwater freezing is a little different. Its freezing point is lower than that of fresh water because of the salt content. As it approaches the freezing point, some of the salt is “frozen out,” sinking and adding to the density below the surface. It cools and sinks as more heat is taken away, and the mix goes deeper and deeper, Aagaard said.

“With salt water of normal ocean salinities, you never reach the temperature of maximum density because it’s less than the freezing point.”

But at the surface, the water is nearly salt-free with a density closer to fresh water. But it’s not until “the entire water column is at freezing (temperatures) before a layer of ice can form on it. That’s why salt water is so hard to freeze.”

Under the right circumstances, it’s not impossible. Salinity can change with a lot of rain and runoff, especially if helped by an on-again off-again creek at the head of the cove.

“Salinity is the whole key in this business. You tweak the salinity by adding fresh water, and we’ve been adding fresh water since the record rainfalls in November; then the rainfall before the snow, and the creek spitting out fresh water.

“As the salt water gets diluted with fresh water and the salinity drops … then it takes the characteristic of fresh water and can reach the temperature of maximum density … and the same situation is set up as in a lake. You had some fresh water running out of the creek and then you had no wind.”

So a combination of lack of wind, a narrow cove, slack tide (thus little water movement), falling temperatures, record rains, and fresh water diluting the top layer of salt water was the perfect combination for a thin layer of ice to form, soon broken up with a change of tides and a slight breeze.

Columnist Sharon Wootton can be reached at 360-468-3964 or www.songandword.com.