I am currently thinking through evaporation over lakes, specifically the Laurentian Great Lakes (a complex subject, I know). Particularly, I am trying to wrap my head around why evaporation peaks in the fall and winter. Based on what I have read, this fact is due to the vapor pressure gradient that exists between relatively warm water and dry air (dry in an absolute sense because the air is cold) and the high winds which continually replace that dry air over the water.
I have also read that this evaporation from the Lakes has a cooling effect on the Lakes themselves, causing a temperature decrease in the Lakes. I have seen it insinuated that over-lake evaporation is synonymous with a latent heat-flux. Based on what I have read, latent heat is an exchange of energy to a substance without a change of temperature of the substance. For water transitioning from a liquid to a gas, the required amount of energy to cause this phase change is called the enthalpy of vaporization or the latent heat of vaporization.
When the air is colder than the water, where is the energy coming from to supply the latent heat of vaporization that causes evaporation (assuming its a cloudy day)? If the vapor pressure gradient is the main driver of the rate of the evaporation, how can the evaporation still be said to be a latent heat flux, i.e., how is the transfer of particles based on a pressure differential called a transfer of heat, albeit a latent transfer of heat? Does a latent transfer of heat mean that the water particles which evaporate do not change temperature, though the liquid water which they leave behind decreases in temperature?
Sorry, that's more like three questions! Hopefully they are not stupid ones 🙂 it seems this is a complex topic, and my initial intuition that evaporation is always higher when the air is warmer than the water (thus transferring heat energy to the water, increasing the energy of the molecules, leading to increased evaporation rate since the water molecules are now higher energy) is simplistic and even wrong, as it depends on so many more factors than that. Our world is not a simple one, physically at least!
Best Answer
The reason that this seems complicated is that there are two things happening at the same time. Both heat transfer and mass transfer are occurring.
The heat to provide the energy of vaporization is coming mostly from the lake water. There is a temperature gradient established within the water (with depth) as a result of the evaporation. The temperature at depth is higher than at the surface, and this causes heat to flow from depth to the surface. The rate of upward heat flow approximately matches the rate of evaporation times the heat of vaporization.
But, in order for vaporization to occur, the equilibrium vapor pressure of the water at the surface temperature must exceed the partial pressure of water vapor in the air above. This difference in vapor pressures provides a driving force for water vapor to diffuse into the overlying air and be swept away by the air currents.
So, in the situation you describe, both heat transfer and mass transfer are present to determine the overall rate of evaporation and surface cooling.