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The Freezing of Lakes
Water – 36oF
six inches down
In the
beginning of September it was a bit chilly crossing the lake in the
mornings before sunrise, so we started wearing our winter coats. One of
my friends at work, asked if the lake was freezing yet. He was surprised
that it wasn’t doing so, not even a skim of ice. I thought
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strange that he didn’t know how lakes freeze because
he is a chemist. I guess it was more the biologist than the chemist in
me that made me understand freezing water. It is important to a chemist
that pure elements and compounds have specific freezing points, but to a
biologist it could be a matter of life and death for an organism and the
unique properties of water may well have been a significant condition
for the creation of life.
One
of these unique properties so well evidenced on Flat Lake was that water
has a maximum density at 4 degrees centigrade (39.2 degrees Fahrenheit).
As the water on the surface of the lake cools and its molecules reach 4
degrees C, the dense water sinks to the bottom of the lake, causing any
warmer water to take its place on the surface. As the layer of dense
water on the lake bottom deepens, the interface, or thermocline, between
it and the warmer layer of water above, rises. When the thermocline
reaches the surface, the lake has turned over and can cool further, then
freeze.
Thus,
gradually, the whole lake reaches 4 degrees C before the surface can get
any cooler. It is only after the entire water column has turned over and
the thermocline has reached the surface in this manner that the surface
water can cool below 4 degrees C to 0 degrees C and freeze. I use the
term water column here because the shallower areas of the lake freeze
first since the water column there has less water to turn over. It is
usually not the case that an entire lake turns over and freezes at once,
but if it has been windy, keeping the surface from freezing, the water
temperature can fall below 32oF, or supercool. Then everything can
freeze at once when the wind calms, a process Carolyn refers to as
slamming shut.
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