Temperature and Evaporation
Name: Chuck M.
Date: Tuesday, April 30, 2002
I always thought that higher temperatures cause more evaporation.
Is it possible then that more evaporation takes place on the Great Lakes in fall and
early winter when the difference between the water and air temperatures are maximum
than in the heat of summer when the air temperature is in the 70s and 80s?
I have heard it said that this is true, but it seems to fly in the face of physics
to me, even though winds tend to be higher during the fall and winter seasons.
Evaporation rate does depend upon temperature (the absolute temperature in
kelvins) and typically this will range from 260 to 313 kelvins -- the
log(vapor pressure) is proportional to 1/T. so while temperature is a factor
these functional dependences tend to "squash" the temperature effect. Even
more dominant are: 1. Relative humidity, which is lowest in the late fall
and winter, and 2. wind speed, which is higher around Lake Michigan in the
winter FOR SURE!
Sunlight penetrates the water to heat it up very slightly
and air temperatures can warm the water also, but these
play smaller roles in evaporation of water than the amount
of water vapor in the air above the water.
Evaporation from a water body with no internal heating
(like a hot spring) is controlled by the water vapor content
of the air above the water and by the amount of turbulence
in the air that can take evaporated water vapor away from
the water surface.
The less water vapor in the air, the more water can be
evaporated from the lake, no matter what the temperature.
The more turbulence (usually increases with wind speed),
the faster the evaporated water vapor is removed from the
near the water surface, thereby increasing the rate of
David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
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