How does the top of a lake freeze

[zhengwei]

can somebody tell me how is the surface of a lake been frozen yet the water underneath the ice layer remains unfrozen? can someone give me detailed explanation of this. including the convection theory and density as well. i mean include whatever that is connected. thanks

 

[HallsofIvy]

Water has a very peculiar property- most materials contract as the cool; in particular, have a sudden sharp contraction as they change from liquid to solid- water, however, because it forms crystals, expands when it freezes.

Water ice is less dense than liquid water- it floats on top of the water.
Also ice is a good insulator. Once the ice has formed, it prevents much further heat loss from the water below it. "Convection" doesn't really have anything to do with it.

 

[zhaggy]

i think the property is referred to as 'anamolous expansion of water'
water tends to expand when cooled from 4degCelcius to 0degCelcius
hence water at 0celcius is less dense than water at 4+celcius..

but this brings another question to my mind..
ice when cooled below 0celcius must contract(?).. therefore when cooled enough.. should it not sink..?