 After the surface ice breaks up, the surface water is slowly warmed up by the sun, and in May–June, the upper or so becomes homothermic (same temperature throughout) at around because of water mixing.The sun continues to heat up the surface layer, and at the peak in August can reach up to about in the main sections and in shallow bays in the southern half of the lake.During this time, the pattern is inverted compared to the winter and spring, as the water temperature falls with increasing depth.