Lakes freeze over from top to bottom. But when it gets even colder, something strange happens to the water. The density of liquid water depends on its temperature, and water is densest at about 40 degrees. The ice can form at the edge of the lake, and then a warm sunny day can come and melt it again.
Most lakes and ponds do not freeze completely because the ice (and eventually snow) on the surface insulates the water below. If water were most dense as a solid, lakes would freeze from the bottom up and eventually become ice altogether.
Why doesn’t the water freeze at the bottom of the lakes in winter?
The amazing fact that water freezes at the surface of a lake, but not below, allows plants and fish to survive the winter in northern lakes and ponds. It is this property of water that allows ice to wedge cracks in pavements and rocks and make cans and bottles of soft drinks explode in the freezer. Cold-water fish like trout and salmon remain more active in the water column, eating other organisms. When the ice forms, it floats and stays on the surface of the lake until a solid layer of ice has formed.
Can water sink below freezing point?
They only stick together to form ice if they can be pressed very tightly together in a certain orientation. Dust can have different electrical charges and this could change the temperature at which clouds form, which in turn could affect global temperatures. While dust or impurities usually form a nucleus, very pure water only crystallises when the structure of liquid water molecules approaches that of solid ice. To keep water liquid, you have to make the water as pure as possible and make the system as unfavourable as possible for nucleation.
On a positively charged surface, the water freezes from the bottom up, on a negative surface from the top down.