With the onset of cold weather on the surface of lakes formed a thin crust of ice, which is a consequence of lowering the water temperature to negative values. But in winter, when the temperature drops below 30 degrees of a frost, on the surface of lakes formed an impressive layer of ice, but large lakes do not freeze ever. Why is this happening?
It turns out that when the water temperature begins to drop, in confined waters very interesting things happen. Freshwater, due to its unique molecular structure, has a maximum density at a temperature of +4 º C. And when the water temperature continues to drop, the lake there is a separation of the layers, formed by the seasonal thermocline.
Water with a temperature of 1-2°C is always lighter than the layer of water with temperature +4°C, which is located on the bottom. Due to the weak circulation of water masses (and we remember that it’s not a river, namely deep lake), active mixing and equalization of temperature occur. For this reason, water with a temperature of about +4 degrees is always at the bottom of the pond. Gradually increasing layer of ice and more cold water in the upper part of the reservoir do not allow to freeze the lake to the bottom. Fish and other aquatic inhabitants continue to live in the lake without fear to turn into a piece of ice.
Of course for small lakes, this is not the case, and with the arrival of freezing temperatures, they can freeze to the bottom. Prudent fish, generally leave such dangerous for the winter place and go to the nearby rivers or deeper lakes.