why frozen lakes seem to unfreeze from the bottom up. I know the top freezes over first because of density but why does the I’ve seem to simply get thinner while it unfreezes rather than having a pool of water over the ice. Go easy if it sounds like a dumb question.
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It does unfreeze from top, but floats on water
Water flows from melting ice into the body of water, ice stays afloat, getting thinner
It’s certainly possible to have an inch or two of water sitting on top of the ice, but any more than that and it will usually find a way to pass through. The water will flow down through cracks and melt a drain hole, or will flow along the border between the lake and the land.
Try it yourself: get a cup that’s wider at the top than at the bottom, put some water at the bottom of the cup, freeze it, and then put more water on top. Unless the ice is very thoroughly stuck to the cup, it’ll bob upward as water flows around it and displaces it.
Along with the other responses, ice is very reflective and so is able to stay pretty cool when the only source of heat is the sun (as opposed to say, warm weather). However, the ground is not, and so it will heat up before the ice (you may have noticed that melting ice on ponds tends to melt around the edges first, since there is less of a barrier between the ice and the ground underneath). This warms the water, helping to melt the ice from below.
It’s based on the principle that heat rises – so yes it does melt from the top first and ice remains on the lower parts, but the remaining ice eventually floats up and melts into itself again to form the lake-face – this continues until there is nothing left but water.
Ice become porous from sublimation and heat melting and from everyday life you know that water will find a way no matter how thick and solids objects looks. Ice floating on water and water just goes down. Moreover lake ice have a lot imperfections (trapped bubbles, vegetation) and since warm water expands it breaks ice from below .
Dont forget that waves from open parts in big lakes goes under ice and add fractures. I once witnessed cracks on Saima at -30 with ice feet thick. Very loud thin cracks that becomes immidiately solid.
However In some cases a sudden heatwave after cold weather could create a pond on ice.
Lake ice doesn’t thaw just from the bottom. It thaws from the top, the inside, and the bottom.
Thawing on the top is driven mostly by warm air. If the weather suddenly turns warm and windy, there can be enough thawing on the surface to form a pool of water on top of the ice while the ice remains thick and impenetrable. I’ve seen that a few times.
Thawing inside the ice is driven mostly by sunlight that penetrates the ice. This weakens the ice and forms cracks and channels throughout. But ice floats on water, so if the melt water is able to go anywhere through the cracks, it’s down, not up.
The lake water beneath the ice is above the freezing point, of course, because it is not frozen. That water melts the ice from below if the ice isn’t cold enough to stay frozen. Water currents will accelerate thawing from below, which is why lake ice tends to thaw more quickly near an inlet or outlet.
How much of each kind of melting you get depends on the composition of the ice, the temperature of the ice, the temperature of the air, how windy it is, how sunny it is, the temperature of the water, and the currents in the water.
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