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Q & A: Does wind speed affect the evaporation rate of water?

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Most recent answer: 10/22/2007
Q:
Does wind speed affect the evaporation rate of water? If so, how?
- Richard (age 11)
Advent Episcopal, Alabama
A:
Hi Richard,
The speed of air that is flowing across water will affect how quickly the water evaporates. The three main parameters that control the evaporation rate of a body of water are: the surface area, the temperature, and the partial pressure of water in the air.

First consider the surface area. As a body of water gets more spread out, then more of the water particles are exposed to the air and will therefore be given more of a chance to evaporate.

The temperature of the water says how energetic the water molecules are. The more energetic the molecules are, the more likely they are to break free from the liquid and get into the air.

Finally, the partial pressure of water in air is a measure of how much water is already in the air. This is important because just as the water evaporates a water molecule into the gas, the water molecules can go back from the gas into the body of water. When these two processes proceed at equal rates the net evaporation rate of water from the body of water stops.

When the rates are equal, so there's no net evaporation, we say the "relative humidity" is 100%.  Relative humidity, determined by the temperature and the water vapor partial pressure, says how much water is in the air as a fraction of the total amount the air can hold when "saturated".

To answer your question of how the wind speed affects evaporation we just have to realize that when the wind blows it will sweep away the air-borne water particles from the air above the body of water. This will reduce the humidity of the air close to the evaporating water reducing the rate of water molecules going back into the liquid. Of course, if the blowing air is already saturated with water (100% humidity) it won't matter how fast the air is blowing, net evaporation won't happen.
Thanks for the question,
James (small mods by mw)

(published on 10/22/2007)

Follow-Up #1: water in air

Q:
How does air "hold" water?
- Steve (age 15)
Michigan
A:
Nice question. The air doesn't really 'hold' the water, that's just a loose expression. If you took all the air away, there would still be water vapor (gas) above the liquid water. The density of that vapor depends a lot on the water temperature, when the vapor and liquid have reached equilibrium, with equal numbers of water molecules going from each to the other.  That vapor density doesn't depend much at all on whether there's any air (nitrogen and oxygen) around, so that's why it really isn't right to say that the air 'holds' the water.

Mike W.

(published on 10/22/2007)

Follow-up on this answer.