I recently moved into a new house with a 2 acre lake behind it. We’ve had 15 consecutive days below freezing, and the last 12 of those have been below 20 degrees (Fahrenheit)! Six days ago I shoveled off a small spot to test out whether it would be good to ice skate on the surface, with the plan that yesterday I would shovel off a much bigger area. Yesterday morning, however, I woke up to a big area straight across starting to melt! The temperature was about 5 degrees F. This morning it’s even larger. The temperature did not exceed 20 degrees yesterday. Though there has been frigid cold it has been sunny. Is it possible that the area I shoveled caused this? The rest of the lake looks exactly the same (About 2” of snow all over it. No signs of melting anywhere else.) I’ve attached before and after pictures. Can’t figure out how in the world this could happen. Any ideas? 
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$\begingroup$ the air temprature is not the same as temprature of individual objects in the sun, an exposed dark surface may have exceeded the melting point. $\endgroup$– JohnCommented Jan 9, 2018 at 0:07
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$\begingroup$ Could your lake be formed by damming a stream (artificially or naturally)? Flowing water can cause this. There is also the possibility of underwater springs causing thin ice. There is a lake near here with warning signs because of this. Did you ask your new neighbors? $\endgroup$– Keith McClaryCommented Jan 9, 2018 at 21:41
3 Answers
My guess is that since ice (and the liquid water underneath it) is much better at absorbing thermal radiation than air, then even if the air temperature is below zero, the ice can heat up in the sun and melt a little bit. It will presumably then refreeze at night when there is no sun to keep it warm.
This was probably also happening on the surface of the snow as well before you cleared it but you didn't notice it because there was no colour change.
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$\begingroup$ I agree. When there is freezing temperatures and sun is shining snow and ice are constantly freezing and melting. As the temperatures have been below freezing the whole time the thickness of the ice will nevertheless increase continually. The image shows melting oddly linear in the middle of the lake as if something was melting it from below... $\endgroup$ Commented Jan 8, 2018 at 13:04
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1$\begingroup$ Its not melting in the middle so much the melt is pooling in the middle, which is not surprising, there is a known lifting effect of ice freezing near the edge of a lake or pond, this will result in an depression in the center. $\endgroup$– JohnCommented Jan 9, 2018 at 0:08
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$\begingroup$ There's a good chance the snow wasn't melting since it is more reflective. $\endgroup$– TomOCommented Jan 9, 2018 at 21:07
The pond water may be in contact with the water table underground. Several feet underground, temperatures don't vary much from the average annual temperature. If the average annual temperature is sufficiently above freezing, liquid water from the water table could be feeding the pond. Also, rivers can flow in winter even when air temperatures are somewhat below freezing for the same reason, because they are fed by groundwater at temperatures above freezing.
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$\begingroup$ Even without water flowing from the water table, the warm ground could heat the water by conduction. $\endgroup$ Commented Jan 9, 2018 at 22:58
There is ice below this melted section which shows the "heat" source is not from a water table or the ground beneath the water. It is obviously radiant heat from the sun. As can be seen on a super hot sidewalk, radiant heat can greatly exceed the temperature of the air. The location of the melting would predominantly be related to the areas that get the most sun but could also be related to thickness of ice beneath which could be related to many factors with the lake.
