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Name: Nikki
Status: student
Age: 14
Location: N/A
Country: N/A
Date: 1999-2001


Question:
Hello,

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 very greatful.


Replies:
Nikki,

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, increased 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 air.

If the nearby air is moving, water molecules that do manage to jump free of the 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.

Regards,
ProfHoff


Hello Nikki.

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 body heat.)

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.

Jim Rubin


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 question.

Vince Calder


Hello,

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.

AK

--
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.

Tim Mooney


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

-Wil Lam



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