Pond Freezing Process
Name: Patti P.
We have a fish pond with a viewing window. I can see the layer of ice that covers the
entire pond, a layer of air followed by the unfrozen pond water. As the cold days go by, the layer
of ice thickens. Herein lies the questions that none of us seem to be able to answer:
1. Is there always an air layer under frozen pond or lake ice?
2. How does the ice get thicker with this air layer between the water and the ice? Does the water
vaporize into the air layer and then freeze on the bottom of the ice layer making it thicker?
Hopefully you can solve this mystery for us. I also explained this to my students and have them
trying to figure it out. We have been having fun coming up with theories but I'd like to be able
to give them an answer. Can you help?
What a great opportunity having an observation window in your pond. Perhaps I can help shed some light
on your question.
First, water is the most dense at about 38F. (There is an exact number, but it escapes me at the
moment.) So at this temperature, it sinks to the bottom. As it cools further, it becomes less dense
and rises to the top.
At the top you have a layer of gas (likely created by the plants living in the pond) Air is a rather
good insulator, but because it is a gas it convects. In other words the cooler gas (cooled by the ice
on top) sinks to the top of the water, but the warmer gas (relatively speaking) raises from the water
to the bottom of the ice (taking the place of the falling gas). This convection current moves water
molecules (which evaporate from the top of the water even at cool temperatures) up to the bottom of
the ice layer.
The water molecules are already lacking in energy and approaching the freezing point. When they
contact the bottom of the ice, they sublime - go straight from a gaseous state to a solid.
If my theory is correct, it is likely that the bottom of the ice looks rather ragged. Sublimed
water (like frost) generally forms larger crystals that appear obtuse and jagged. If you would
break a piece of ice from the pond and turn it over, it would not have a smooth bottom.
Good question and a great opportunity for an investigation.
I also have a pond with a viewing window. While I can easily see many air bubbles trapped under and
in the ice, I have never seen a circumstance such as you describe -- a layer of air trapped between
the liquid water below and the ice layer above. Based on my observation of my pond and many years of
examining frozen lakes and ponds, I think what you describe is rather rare and I am unable to formulate
a reason for its occurrence.
If a layer of (compressible) air were present under all ice layers, the ice would have to be much
thicker to support a load. Water in contact with the bottom of the ice layer supports the ice because
water is incompressible. Any imposed load is transferred through the ice to the underlying water.
You can demonstrate this for yourself by poking a hole in the ice and then repeatedly pressing on the
unbroken ice at some distance from the hole. You'll note that each compression stroke results in water
gushing up through the hole. If there were a layer of air under the ice, little or no water would flow
through the hole because the air would absorb the flexure of the ice and be less able to transfer the
force you apply to the underlying water.
There is not always or even typically a layer of air between the frozen
and unfrozen water in a lake or pond. As you probably know, water is most
dense at about 4C. When it reaches 4 it sinks. At 4C there will be very
little vapor pressure. The air in your case can be from numerous sources.
Turbulence, underground springs are two that come to mind. I have also
seen some lakes freeze and then because of drainage without equal
replacement have a substantial space between the ice and water...very
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Update: June 2012