Applied to lakes in which only one seasonal period of free circulation occurs. In cold monomictic lakes, typical of polar latitudes, the seasonal overturn occurs briefly in summer; in other seasons, surface-water temperatures are below 4 °C, which induces density stratification. In warm monomictic lakes, typical of warm temperate or subtropical regions, the seasonal overturn occurs in winter; at other times thermal stratification, with the formation of a distinct epilimnion, prevents free circulation through the depth of the lake.