Quote:
Originally Posted by This'nThat
No, water freezes at 32 degrees. However, the lake can't freeze until all water underneath is at 39 degrees (4 degrees C). Water is most dense at 39 degrees, so water at that temperature sinks, and warmer lake water rises to the surface -- which also must be cooled to 39 degrees -- which sinks, is replaced by warmer water, and so on until everything is at 39 degrees. When that temperature is reached, there is no more warm water to rise to the surface. Therefore, the surface water can now begin to freeze and ice begins to form and stay on the lake.
This "water density at 39 degrees" also explains why shallow lakes freeze quicker than deep lakes like Winni. Deep lakes have much more water that needs to be cooled to 39 degrees. It also explains (in part) why Alton Bay can freeze before the rest of the lake, or even other parts of Alton Bay (as mentioned by another poster here) -- because it is more shallow and gets to 39 degrees a lot quicker.
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Nice job TNT. I was gonna take a crack at explaining it after I posted it, but I knew someone else with a better way of explaining it would come along. I remember former Channel 4 meteorologist Bruce Schwoegler talking about the 39 degree principle on TV when I was a kid. He always made references to Winnipesaukee during "ice in/out" season.
BT