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Ice Formation and Thickness on Ponds
Name: Timothy
Status: student
Age: N/A
Location: N/A
Country: N/A
Date: N/A
Question:
We have a relatively small farm pound,
(approximately 4 to 6 feet deep and covers a
surface area of about ½ an acre) on which myself
and my little brother skate and play hockey on
throughout the winter. We have been instructed
my our mom to take a cordless drill with us
every time we go out on the ice, with the drill
we make 4 to 6 holes randomly over the surface
to determine the thickness of the ice. If the
ice is not greater than 6 inches, we are not
allowed to play. However, I have noticed that
even after a week of lows hovering around the
-30°F and highs not getting above 0 mark (last
winter), the pond still does not freeze solid.
Why is that? Is it that the ice at a certain
point is so thick and strong that it does not
allow for the expansion of water to form more
ice, creating the case where we have sub-cooled water.
Replies:
Hi Tim,
Good question.
There is a great explanation in Paul Hewitt's
Conceptual Physics In the Temperature, Heat, and Expansion chapter.
A couple of things you have to remember, ice is
less dense than water, that is why it
floats. Water freezes at 0 degrees C and water is WEIRD!
You understand that water has a volume increase
when it changes to ice. There is another little
weird thing about water, it is most dense at 4
degrees C. So as water in your pond reaches 4
degrees C, it sinks to the bottom of the
pond. Any water that is less dense than this 4
degree water will then "float" to the surface
for cooling.In Mr. Hewitt's words:
"If the water below the surface is any
temperature other than 4 degrees C, any surface
water at 4 degrees C will be denser and sink
before it can be further cooled. So before any
ice can form, all the water in the pond must be
cooled to 4 degrees C. Only when this condition
is met can the surface water be cooled to 3,2,1
and 0 degrees without sinking. Then ice can form.
Thus the water at the surface is the first to
freeze. Continued cooling of the pond results
in the freezing of the water next to the ice, so
a pond freezes from the surface downward. In a
cold winter, ice will be thinker than in a milder winter.
Very deep bodies of water are not ice-covered
even in the coldest of winters. This is because
all of the water in a lake must be cooled to 4
degrees C before the lower temperatures can be
reached, and the winter is not long enough for
all the water to be cooled to 4 degrees C. If
only some of the water is 4 degrees C, it will
lie on the bottom. Because of water's high
specific heat and poor ability to conduct heat,
the bottom of deep lakes in cold regions is a
constant 4 degrees C. Fish should be glad that this is so."
I hope this sheds a little light on your pond
ice. Mom's got a good idea with the
drill. Keep playing hockey and keep thinking scientifically.
Martha Croll
Hi Tim,
Subcooling may play a role, but I would guess it is not the
main reason. I think it is just the warmth of the water and the ground
beneath it. Only in very cold areas (like Siberia) does the ground
freeze much deeper than a few inches. So beneath the pond, the ground
is still a balmy 50F or so. It takes a fair amount of energy to make
liquid water freeze even when it is already at freezing point. When the
ice above it freezes, it gives off heat to the water beneath it as
well. So while your pond is probably near or at freezing, there is
enough heat emanating up from the ground beneath it to keep it from
freezing completely.
Hope this helps,
Burr Zimmerman
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Update: June 2012
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