Derived insolation and water budget evaporation data for Lake Superior and Lake Ontario are analyzed in terms of both the regional and the water surface energy balances. The results indicate that the seasonal pattern of evaporation is governed by heat storage changes, and that these changes are closely associated with atmospheric energy export from the lake. This finding provides a physical basis for the simple empirical relationships between monthly evaporation and island-to-mainland temperature differentials that are developed from the water budget evaporation data. Substantial atmospheric energy export, a concomitant of heat storage changes, reduces the energy available for evaporation. Therefore, evaporation from a large deep lake is less than that from a large shallow lake under comparable climatic conditions. The analysis also provides speculative reasoning and evidence to indicate that evaporation from a large deep lake is closely related to the radiant heat transfer to the sky.