In lakes, different layers of water blend in the spring and in the autumn. The blending is called spring turnover or autumn turnover.

Water is heaviest when its temperature is +4°C. When the air cools down in the autumn, the water on the surface cools first and starts sinking down, until the whole mass of water from surface to bottom is about +4 degrees. After that, the surface water cools some more and finally freezes. In the spring after the ice has melted, the cold water on the surface soon reaches +4 degrees, and the whole mass of water has again the same temperature for a while, until the water on the surface warms up some more.