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Water Evaporation Threshold
Name: Haythm I.
Status: educator
Age: 20s
Location: N/A
Country: N/A
Date: 11/26/2004
Question:
At which temperature the evaporation of water begins?
Replies:
Dear Haythm,
Evaporation is a surface effect. The temperature of a body of water
represents an AVERAGE value of the kinetic energy of the water molecules in
the water. Some of those molecules have a "higher than average" kinetic
energy and break free of the surface to become water vapor. As this happens
over time, the remaining body of water becomes slightly cooler (as water
molecules with higher kinetic energy have "left"). That is why sweat
evaporating from your body provides a mechanism to cool you off.
This occurs at any temperature of water between the melting point and the
boiling point -- with more evaporation occurring with a body of water that
has a higher average temperature (that is, closer to the boiling point). It
can even happen below the freezing point (via sublimation -- when water goes
directly from a solid (ice) to water vapor) and this explains why the ice
cubes in your freezer sometimes seem to "evaporate".
All of this is assuming that you are at standard pressure of 1 atmosphere.
As the atmospheric pressure changes, water will boil and freeze at different
temperatures.
Here is another answer in the Newton Archives that also provides some useful
information:
http://www.newton.dep.anl.gov/askasci/phy00/phy00130.htm
Todd Clark, Office of Science
U.S. Department of Energy
Haythm -
Evaporation does not have any sharp beginning point.
It just gets slower and slower as the temperature goes down.
Even ice evaporates.
For each 10 degrees C colder, it gets a factor of two or so slower.
Actually, the colder it gets, the more it slows down for each 10 degrees C.
For slow evaporation which does not make the water much cooler than the
surrounding environment,
the evaporation rate is proportional to the vapor pressure of water at
that temperature.
It is also proportional to the exchange rate of the air immediately over
the water.
The water vapor must escape to elsewhere by either air movement or diffusion,
or else it will build up in concentration until it is in equilibrium with
the liquid or solid,
and then evaporation has been stopped.
Evaporation rate is very situation-dependent in that sense.
So we are talking about approximations when we compare rates at different
temperatures.
So find a table of the vapor pressure of water from 0 degrees C down to
perhaps -100 C.
Those numbers will be proportional to the evaporation rate of ice at those
temperatures.
From my CRC reference book:
Temperature Pressure
(degrees C) (mmHg)
-90 C 0.00007 mmHg
-80 C 0.0004 (factor 5 per 10C)
-70 C 0.0019
-60 C 0.008 (factor 4 per 10C)
-50 C 0.030
-40 C 0.097 (factor of 3 per 10C)
-30 C 0.29
-20 C 0.78
-10 C 1.9
0 C 4.6 (freezing)
10 C 9.2 (factor of 2 per 10C)
20 C 17.5
30 C 32 (factor less than 2 per 10C)
...
100 C 760 mmHg (boiling)
Seeing the 4's in the table above, I can say that:
At -80 C ice evaporates about 10,000 times slower than it does at 0 C.
And in a freezer at -10C, the ice should evaporate
about 10 times slower than water at room temperature.
Jim Swenson
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Update: February 2012
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