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Name: Timothy
Status: student
Age: N/A
Location: N/A
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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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