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Name: Donald P. G.
Status: educator
Age: 60s
Location: N/A
Country: N/A
Date: 2001


Question:
Given two lakes having equal water temperatures, equal ambient temperature and relative humidity, and equal daytime radiational heating and nighttime radiational cooling with one lake at sea level and the other at an elevation of say 5000 feet above sea level will the lakes experience equal evaporation rates or will the 5000 foot elevation lake experience a higher evaporation rate and if so what are the governing equations?

This question presumes that the lake temperature is such that evaporation rather than condensation will occur.

Rephrased the question is: With all other factors being equal does a significant change in barometric pressure influence the evaporation of water from a water surface?


Replies:
Donald,

The condition that you list, relative humidity the same, implies that there is approximately the same density of water vapor in the air at both locations, even though the atmospheric pressures are different. Since vapor pressure is proportional to atmospheric pressure (times .001) to a greater degree than saturation vapor pressure is (.0000035), your scenario is unlikely.

Other conditions needed for this scenario are: the same wind speed, lake dimensions, local geography (affecting turbulence and thus evaporation rate), and water composition.

Assuming that the scenario exists, there would be slightly less water vapor in the air at the higher elevation. Therefore, water can evaporate very slightly faster from the lake at the higher elevation.

Evaporation under these tightly controlled conditions would be completely proportional to the difference between the saturation vapor pressure at the lake surface and the vapor pressure of the air. If this difference increased 10% (drier air in the absolute sense) then evaporation would increase 10%. If the air had more moisture, evaporation would decrease.

Because of the complicated dependence upon turbulence, temperature, radiation, water composition, and atmospheric pressure, I would prefer not to try to present the governing equations for evaporation. These equations are many and complicated and are not normally handled except in atmospheric computer models. There are many good technical textbooks that cover this subject, including ones by a leading expert that I know, Wilfried Brutsaert. If you search for his name on Amazon.com or a scientific textbook site on the internet, you will probably find his books listed.

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory


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