Winter Stratification - Thermal stratification

During extremes of winter the surface layer of the lake freezes or attains a temperature close to 0° Celsius. Under these conditions an inverse stratification develops. The water beneath the ice absorbs solar radiation passing through the ice and so remains relatively warm. When this warm water attains a temperature of 4°C it becomes dense and heavier. Consequently it sinks to the bottom where it mixes with bottom water of the lake which is warmed by heat conducted from the bottom mud. The result is higher temperature at the bottom, though the overall stability of water remains undisturbed.

That is, the less dense-surface water in the form of ice or at 0°C floats on the top of the warm, heavier water which is at an appropriate temperature of 4°C. Both of these layers remain stratified and do not mix during the winter season so that the lake is said to have undergone, winter stagnation or stratification.