The map represents the mean value (in millimetres) of the annual loss of water through the evaporation process from the surfaces of open water bodies, such as ponds and shallow lakes and reservoirs based on the 10-year period 1957 to 1966. The greatest mean annual lake evaporation (more than 900 millimetres) occurs in southwest Saskatchewan and southeast Alberta. The smaller means (less than 100 millimetres) appear in the Arctic Islands. The mean annual lake evaporation across Canada generally decreases from south to north. The map also shows the location of the stations, which are part of the "Class A pan evaporation network" used for the analysis and additional stations operating in 1974.The rate at which water evaporates from a lake depends primarily on two factors: first, the rate at which energy is supplied to the evaporating surface to effect the change of state of water to water vapour (requires 2.47 joules per kilogram) and secondly, the rate of diffusion of water vapour away from the surface. The main energy supply for evaporation is generally through the heating of the upper part of the lake by the sun, although in some cases the net energy advected into the water body, by streams for example, may also be important. For a specific lake surface temperature, the rate of diffusion of water vapour is determined in a complex manner by atmospheric temperature, humidity, and wind speed. For small, shallow water bodies evaporation is greater for sunny days during the summer when the water temperature is high, the humidity is low, and winds are brisk. For deeper lakes, heat storage becomes an important consideration and evaporation is not as closely associated with the daily energy input by the sun's radiation. For example, large amounts of water evaporate from deep lakes during the autumn when their surface temperatures are much higher than air temperatures, while the smaller lakes, because of lack of energy storage, evaporate very little. The converse takes place during late spring and early summer when the large deep lakes evaporate very little because of their relatively low surface temperatures.