In Lake Baikal, the water temperature varies significantly depending on location, depth, and time of the year. During the winter and spring, the surface freezes for about 4–5 months; from early January to early May–June (latest in the north), the lake surface is covered in ice. On average, the ice reaches a thickness of , but in some places with hummocks, it can be more than . During this period, the temperature slowly increases with depth in the lake, being coldest near the ice-covered surface at around freezing, and reaching about at a depth of . 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. As the autumn begins, the surface temperature falls again and a second homothermic period at around of the upper circa occurs in October–November. In the deepest parts of the lake, from about , the temperature is stable at with only minor annual variations.