Winds and Drying
I am wondering how wind affects evaporation? I keep going to these web
sites that tell me that you dry faster if its windy out and that does not
answer my question. Any information you can give me is great. I will be
Believe it or not, at ordinary temperatures all molecules are in a state of
incessant motion -- vibrating, jiggling and slipping about. The warmer it is
(the more thermal energy the molecules have), the more they move.
Even though water molecules have a tendency to hold on to each other,
molecular motion encouraged by warmth (heat) makes it increasingly difficult
for them maintain their bond to each other. Whenever a water molecule receives
enough energy to break loose, it can either keep on moving away from the wet
surface or be bumped back into it by collision with nearby molecules in the
If the nearby air is moving, water molecules that do manage to jump free
wet surface are swept away rather than re-deposited. Thus, more wind means
faster drying. Of course, this process works even better when the moving
airstream is of low humidity.
What you have read is correct, you dry faster when the wind is
blowing. However, the unsaid assumption is that the air is dry. If the air
is dry but the wind is not blowing, then the water which evaporates from you
will saturate the air surrounding you with water, preventing additional
water from evaporating. If the air is dry and the wind is blowing, then the
water which is evaporated will be carried away, bringing more dry air in
contact with you, allowing further evaporation. (The process of evaporation
requires a large amount of energy. This energy is needed to give individual
water molecules enough energy to escape the liquid and become gaseous. This
is why you get so cold when you stand out in the wind when you're wet. The
energy needed to liberate water molecules from the liquid is taken from your
If the air is wet (humid is the correct term), then the air will
already have a lot of water in it. In this case, evaporation will be
prevented, regardless of whether the wind is blowing or not. If you have
spent time in Florida or anywhere else where its hot and near the water, you
know that once you get clothes wet, they stay wet.
At a given fixed temperature, a liquid such as water has a certain
pressure of its vapor that is in equilibrium with that liquid. For example,
water at 25 C. has a vapor pressure of about 25 mm of mercury. If the
temperature is held fixed at 25 C. the liquid water "wants" to keep enough
vapor in contact with it at 25 mm of mercury. If a draft or wind carries
some of the water vapor away, the liquid water "tries" to re-establish the
25 mm of mercury by evaporating more liquid. It does so by increasing the
rate of evaporation.
I apologize for using terms like "wants" and "tries" which of course implies
the water is "thinking", which it isn't, but using these terms avoids having
to use more technical terminology that would only obscure the point of your
Quantitative analysis of this is complicated but the basics physics is simple.
Evaporation occurs when the liquid molecules close to the free surface of the
liquid (i.e., at the liquid-air interface) gain enough energy to bounce around
and ultimately separate themselves from the liquid and fly off. Any process
that increases this agitation tends to enhance evaporation.
Wind does two things. First, it helps lift off the molecules that are about to
lift off, and this allows new liquid molecules to get in line for lift off.
Second, it moves those liquid molecules that have already evaporated away from
the surface providing a virgin region in which newly evaporating molecules do
not have to compete with those that have already evaporated. Moving them away
also reduces the chances of the vapor molecules go back and stick to the
liquid surface again.
Technically speaking, we know that there is a vapor-rich layer right above the
liquid-air interface. Obviously, there is more vapor close to the liquid
surface that further away from it. What wind does is to agitate, thin, and
even break apart this layer; this all tend to enhance evaporation.
I hope this explanation is helpful.
Ali Khounsary, Ph.D.
Advanced Photon Source
Argonne National Laboratory
You know that humidity affects evaporation, right? If the air is already
saturated with water vapor, the rate at which additional water evaporates
into the air is going to be matched by the rate at which water vapor
condenses back into liquid.
Well, if the air is not saturated with water vapor, and there is no
wind, then the layer of air very near the water surface is going to get
saturated with water vapor after a while. If there's wind, that
saturated air layer is going to be swept away and replaced by air which
is not saturated with vapor.
When an object is immersed in a moving fluid, the rate
of heat transfer is governed by, amongst other things,
the velocity of the fluid (in this case, air).
Evaporation occurs when the water molecules at the
surface gain enough heat (in this case, from the air
above it), to change phase (evaporate to a gas).
Increasing the wind speed would increase the heat
transfer rate, which would allow the water molecules
to absorb heat faster, which would allow them to
change to steam quicker, which, by definition, is an
increase in the evaporation rate.
Hope this helps
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Update: June 2012