Density stratification describes the layers of water in a body of water with a layer of cold dense water at the bottom and, at temperatures above freezing, a warmer layer floating on top. When water begins to freeze, as water on the surface cools to 4 °C, 39 °F, it sinks. Eventually, the body of water will fill up with water at 4 °C and surface water will cool below 4 °C and freeze. Thus, in freezing conditions, water at depth will be warmer than the surface water in contact with the covering layer of ice.