The Nature of Sea Ice Contributed by Sheila Gaquin, Point Hope, Alaska Lighter Than Water Did you know that sea ice is quite different from freshwater ice? As freshwater cools, its density increases. This means the molecules of water move closer together until the water reaches 4 degrees Celsius. Then the molecules of water begin to push apart. The coldest water (at 0 degrees C., the freezing point of fresh water) forms an ice layer that is lighter than water, so it floats. To picture this, think of people standing in a circle, shoulder to shoulder, and holding hands. As the temperature drops, 4-3-2-1-0 each person begins to move apart until everyone's arms are straight out to their sides. This creates a lot of space between each person, and this space fills with AIR. The air makes the ice lighter than water, so the ice floats on the surface. Good thing too! Imagine what the world would be like if ice SANK instead of floated! We'd have lakes and ponds that might never reach ice out! How Sea Ice is Different Strangely, when SALT water freezes it does NOT float--at first. It becomes denser than water and actually sinks just below the surface of the water until it reaches -2 degrees Celsius--the average freezing temperature of sea water. Then it begins to float. To picture this, think of the same people standing in the same circle, holding hands, but this time as the temperature drops, the people move CLOSER together, becoming heavier than water. Then suddenly, at -2 degrees Celsius, they move back with everyone's arms stretched out at their sides as in the fresh water example. There's another way sea ice differs from fresh water ice. It does not form into a sheet as it freezes. Instead it freezes into separate, floating crystals of ice. At this point, sea ice looks a bit like a milkshake. This is called frazil ice. The frazil ice undulates with the motion of the water, and yet it is solid enough for birds to land on. As the temperature continues to drop, the frazil ice finally forms a solid sheet of sea ice which floats on the surface of the water. Copyright 2003 Journey North. All Rights Reserved. Please send all questions, comments, and suggestions to our feedback form