The water above continues to cool until it freezes, then the ice provides an insulating layer that keeps the denser water beneath from cooling as quickly.
show that water has a maximum density at 4°C. In winter, ponds and lakes are cooled from above.
Convection then occurs throughout until a dense layer of water at 4°C forms on the bottom.
The comparatively warm layer on the bottom enables aquatic life to survive.
Were water not to have its maximum density at a temperature above its freezing point, then ponds and lakes would freeze throughout and, because heat conduction between air and ice is such a slow process, possibly fail to thaw even during the summer months.

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