It is useful to know how the temperature of maximum density changes with depth (e.g., from 3.94 °C at the surface to 3.39 °C at 500 metres depth [38.10 °F at 1,500 feet]). Because the temperature of maximum density of most lake waters is close to 4 °C (39 °F), and ice forms at temperatures close to 0 °C in response to surface cooling, vertical mixing takes place. When density increases with depth, the lake is said to be stable. Unstable conditions exist when density decreases with depth. Cooling at the surface to temperatures below 4 °C establishes stability based on a negative thermocline (a positive thermocline is a vertical decrease in temperature with depth), because density will increase with depth. Ice then forms at the surface, enabling liquid water to exist beneath the ice in lakes, unless they are shallow enough to freeze to the bottom.

During the warming season, after ice has melted, heating increases the density of the surface waters, causing them to sink until stability is achieved. When surface heating proceeds above the temperature of maximum density, this process ceases, and the vertical thermal structure maintains and strengthens its stable condition, based on a positive thermocline. Turnovers tend to be seasonal.

Mixing due to cooling or warming processes that increase the density of surface waters sufficiently to cause them to sink results in what is termed circulation, or overturn, of lake waters. Lakes that cool to below 4 °C in winter experience two turnover periods, as just described, and are called dimictic lakes. Most lakes in temperate regions fall into this category. Lakes that do not cool to below 4 °C undergo overturn only once per year and are called warm monomictic. Lakes that do not warm to above 4 °C also experience only one overturn period per year and are called cold monomictic. There are many examples of the former, including lakes in the tropical regions and generally as far north as about 40° latitude. The cold monomictic type, however, is less common but can be found at high latitudes and at high altitudes (in the Alps, for example).

All the types described that circulate at least once throughout are called holomictic. It is possible, however, for lakes to be stable despite the thermal processes that normally induce overturn, owing to the existence of a positive salinity gradient with depth (chemocline). This type is called meromictic, and, in those cases where stability is permanent in at least part of the lake, the deep waters do not experience overturn and consequently are deoxygenated. Three principal origins of meromixis have been recognized. Ectogenic meromixis results from either the intrusion of seawater into a lake, as in the case of flooding from an unusually high sea level (e.g., Hemmelsdorfersee, in Germany), or the introduction of fresh water through land drainage and precipitation to a saline lake (e.g., Soda Lake, Nevada). Crenogenic meromixis is due to the introduction of saline water by springs, and biogenic meromixis is due to the uptake of salts from the lake sediments. North American examples include Lake Mary, Wisconsin, and Sodon Lake, Michigan.

A strong vertical salinity gradient that exists in the upper portion of a lake will affect the thermal structure by inhibiting the downward mixing of heat. In holomictic lakes, however, the downward mixing of heat due primarily to wind action usually compresses or concentrates the thermocline until it essentially separates an upper layer (epilimnion) from a lower layer (hypolimnion), each possessing weak or nonexistent vertical thermal gradients. The thermocline normally begins to grow at the beginning of the warming season. As summer passes and autumn commences, it intensifies and deepens. The onset of the cooling sees the beginning of the decay of the thermocline from above, although it usually continues to deepen until it is completely destroyed. The process just described is commonly found in lakes in temperate regions and is a seasonal phenomenon. During any period of strong warming, one or more shallower thermoclines may be observed to develop and move downward to the seasonal thermocline.

The heat budget of lakes

The heat budget of a lake includes several major factors: net incoming solar radiation, net exchange of long-wave radiation emitted by the lake surface and the atmosphere, transfer of sensible heat at the surface interface, and latent-heat processes. Those processes that are usually of much smaller importance include net inflow and outflow of heat advected by streamflow, precipitation, and groundwater flow, conduction from terrestrial heat flow, and dissipation of kinetic energy. In some cases, however, river inflow may be of more importance, such as where flow is from a nearby glacier or where the volume inflow is a significant fraction of the lake volume. Within a large lake the heat budget considerations for a particular location must also take into account the local advection of heat within the lake by currents.

Incoming solar energy varies seasonally and with the latitude and is greatly influenced by cloud cover. The fraction that is reflected away from the lake surface depends upon the solar angle, the turbidity of the atmosphere, and the wave state, or surface roughness. In middle latitudes this ranges from about 6 percent in summer months to about 14 percent in winter.

The amount of radiation emitted by the lake surface is proportional to the fourth power of the surface temperature, whereas the radiation emitted by the clouds and atmosphere overlying the lake depends primarily upon the amount and height of the clouds and the temperature and moisture content of the atmosphere near the lake surface.

The fluxes of sensible heat and moisture at the lake surface are of great importance yet are still poorly understood. They depend upon the vertical gradients of temperature and vapour pressure above the water, respectively, and upon the factors that influence the transfer processes, such as wind and atmospheric stability. The transfer of sensible heat may be either into or out of the lake surface, usually on a seasonal basis but also sometimes on a diurnal basis. It is also possible but less likely for condensation to occur on a lake surface.

Heat flow through the bottom of lakes is normally of small significance, but exceptions exist. In a very deep lake where low rates of heating are important, such as Lake Baikal, Russia, the results may be detectable. In some ice-covered lakes where other sources of heating are small, heat flux through sediments also has been shown to be significant.

The dissipation of wind energy that has been transferred to water movements is quite insignificant, as is the effect of heat transfer due to chemical and photosynthetic processes.

In latitudes and altitudes where ice is a factor, the latent heats of fusion and of evaporation of ice must also be considered within any heat budget considerations. Heat balance studies have been performed for lakes that are always ice-covered. Solar radiation is often an important factor where ice thickness and consistency permits penetration. The heat balance of the ice is often difficult to assess, as long-wave radiation and evaporation factors are not easily measured and are very important. The exchange of sensible heat may not be large during summer months in these cases but is likely to be significant in the colder months. Several lakes that are ice-covered have been shown to be meromictic; two examples are Lake Tuborg, Ellesmere Island, and Lake Bonney, Antarctica.

Heat balance measurements or estimates have been made for many lakes throughout the world. Results show that the difference between the highest and lowest heat content for each lake varies from around 5,000 calories per square centimetre for high and low latitudes to around 45,000 calories for some midlatitude lakes.

The relative importance of each of the major terms of the heat budget is shown by data for two North American lakes: Lake Ontario, a large, deep, middle-latitude lake; and Lake Hefner, a relatively small, shallow lake in Oklahoma. The energy unit frequently employed is the langley (one gram calorie per square centimetre), and the figures given are approximate monthly means of langleys per day. Net solar radiation input to Lake Ontario varies from 80 to 600 (Lake Hefner varies from 200 to 600), midwinter to midsummer. Net losses due to long-wave radiation from Lake Ontario are nearly 100 throughout the year (Lake Hefner varies from 100 to 200). Evaporation losses for Lake Ontario vary from 250 in midwinter to slightly negative values in early summer (Lake Hefner varies from 450 in late summer to 150 in spring). Conduction of heat from the surface of Lake Ontario varies from 250 in winter to about minus 100 in summer (Lake Hefner varies considerably from 80 to -80 for the same time interval).

Heat transfer

Heat added to a lake at the surface is usually mixed mechanically downward as a result of wind action. This process keeps the upper portion of a lake relatively uniform thermally. Consequently, a thermal gradient (thermocline) becomes established between the upper mixed layer (epilimnion) and the deep portion of the lake (hypolimnion). In shallow lakes or shallow portions of large lakes, the thermocline will eventually intercept the lake bottom so that no hypolimnion exists. Normally, as the heating season progresses, the thermocline intensifies and deepens. Secondary thermoclines may develop in the epilimnion, and these will migrate downward to the main seasonal thermocline. On very warm still days, a thin surface layer may store heat before a mixing episode transfers heat downward. When the cooling season commences, the mixing that tends to destroy the thermocline is enhanced by vertical convection. If the cooling continues until the entire thermocline is eliminated, the lake becomes essentially isothermal and no longer exhibits the characteristics of a two-layered system.

When a lake is stratified, the most important process for downward transfer of heat to the hypolimnion is through eddy conduction. The coefficient of eddy conductivity is determined empirically and varies substantially from lake to lake. Mixing processes are generally more active in coastal areas, so that isotherms can be expected to slope downward toward shore. In large, relatively unprotected lakes, wind stress at the surface causes convergence or divergence or both of shallow waters along coastlines. Isotherms will slant upward toward the shore, and hypolimnion water may even become exposed at the surface. These occurrences are of great importance with regard to the distribution of heat within stratified lakes.

Heat introduced to lakes in large quantities, as a waste product of cooling processes in power-generating plants and other industrial concerns, is presently viewed with some concern as a pollutant, especially in small lakes. If the heat is injected at the surface it will spread initially according to the momentum of the influent and the speed and direction of ambient surface currents. When the initial momentum is sufficiently dissipated, the heat will spread mainly as a consequence of turbulent mixing processes. Throughout these events, substantial losses of heat to the atmosphere may occur, so that the full effects of the thermal input are not borne solely by the lake. Temperature values at the surface, adjacent to the influent-heat source, may be raised to a very high level—as much as several centigrade degrees. Under certain conditions fish-activity tolerances may be exceeded, and undesirable algae and plankton production may be stimulated.

If waste heat is not released at the surface but is diffused over a large depth range or injected at depth, the large local-surface-temperature problem is avoided. Losses to the atmosphere in this case, however, are also greatly reduced, and the net heat input to the lake as a whole is much greater. Over a long period, this may prove to be more detrimental to the general ecology than near-surface injection.