[HPsquared]

Another interesting temperature dependence of water is the negative thermal expansion between 0°C and +4°C. The maximum density is at +4°C.

Therefore, between 0 and 4°C the usual law of natural convection (heat rises) is inverted and you get "heat falls" instead.

What this means is that in winter, the ice on top of water can be frozen but the water can be warmer further down and resist further heat transfer. Natural convection will not act to cool the water from above, and a stable stratified temperature gradient can form. This allows bodies of water to remain liquid in winter for longer than you'd expect in a "normal" liquid.