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
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.
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