Abstract: It is common knowledge that hot air rises, so we might expect that hot water should rise above cold water. However, in very cold freshwater, the surface water is typically colder than the water below. This counter-intuitive temperature structure results from the density of freshwater depending nonlinearly with temperature.

Many lakes transition between the intuitive hot-over-cold temperature stratification in the summer, and the reverse cold-over-warm stratification in the winter. The division between these two regimes occurs at the temperature of maximum density (TMD; for fresh water, TMD≈4ºC). By understanding how the transport of heat is changed during this transition period near TMD, we will gain a better understanding of when lakes will freeze, and how that's going to change in an evolving climate.