Together with spring, fall is the time of the year that sees a spectacular natural phenomenon unfolding here in Kuopio, Finland. The city has a total area of 2,317.24 square kilometre and water area of 719.85 square kilometre. Thus, about a third of the total area is water. That is a fair amount of water. In comparison, water area in Washington, D.C. is just about 10% of the total area.
The specific heat of water is high, which means that adding heat to water changes temperature relatively little, which is important to organisms living in water bodies at tropical latitudes. With the end of winter, due to increased solar radiation also beyond 50 degrees north latitudes the water temperature of lakes can increase dramatically, in Kuopio from just slightly above zero to around 25 degrees Celsius. Though, the mass of the water of many lakes here in Kuopio is not huge, the amount of heat necessary for such a temperature increase is of course substantial. Water has also high latent heat of freezing. This means that water releases a lot of heat while freezing.
Hence, a third of the surface of the city of Kuopio absorbs a lot of heat between spring and early fall and releases a lot of heat before it freezes, cooling only slowly. Instead, during fall air cools relatively fast here at 60 degrees north latitudes. Due to the large water masses, however, the air does not cool as fast as it would without the beautiful lakes. As a consequence, the period during fall that air temperature in Kuopio hovers around freezing point is relatively long, I would argue well beyond a month. There is a relative steep drop in air temperature between end of August (+15) and the end of September (+5) and then it might take two months until it drops from just below zero to well below -10, or -20, degrees Celsius. That is when a layer of ice covers the lakes. In spring something similar occurs, while the ice cover melts, due to water’s high latent heat of melting. Once solar radiation increases end of February, air temperature rises relatively fast from a nice -20, or -30, to just below 0 degrees Celsius. Then, the period while air temperature hovers around freezing point is relatively long, again, as a lot of heat goes into melting ice. Only when snow melted and lakes are free of ice cover air temperature rises again.
This phenomenon in fall is also a main factor and reason why permanent snow cover in Kuopio starts rather late, possibly only mid-end of December. Naturally, I’m simplifying things here because in reality a range of factors affect the time of freezing or permanent snow cover, such as solar radiation in summer (intensity and duration) and precipication. However, if you check out this figure you can spot the periods I discussed here (notice the almost flat, gray-colored, 5-year average of November and, similarly, the practically flat average of the black-colored plot [for 2011] of March).