MadSci Network: Earth Sciences

Re: Which effects evaporation most-temperature,wind speed, or humidity?

Date: Mon Nov 13 19:36:41 2000
Posted By: John Christie, Faculty, School of Chemistry, La Trobe University, Bundoora, Victoria, Australia
Area of science: Earth Sciences
ID: 974050104.Es

OK Sarah, I'll try to answer, and I'll try to keep it simple.

Temperature, wind speed, and humidity can all affect evaporation a lot. They 
affect it in different ways. No one of these factors is always stronger than 
another. There will be circumstances where any one of the three could be most 
important. Probably, as a general rule, the order you have got them in is about 
right though.

If you have water in a half full closed bottle, some of that water will 
evaporate, and there will be water vapour in the air above the liquid level in 
the bottle. Liquid water will keep turning to vapour until it reaches the 
saturation vapour pressure, and then no more will. How much water vapour the air 
can hold depends very much on temperature.

At -20 degrees C = -4 degrees F, 0.10% of the air maximum can be water vapour
At 0 degrees C = 32 degrees F, 0.6% maximum can be water vapour
At 20 degrees C = 68 degrees F, 2.4% maximum can be water vapour
At 40 degrees C = 104 degrees F, 7% maximum can be water vapour

So hot air can hold a lot more water vapour than cold air can.

But a lake or an ocean or a puddle out in the open air is not like a closed 
bottle, and that is where the other two factors come in.

Humidity is simply a measure of how the amount of water vapour that an air parcel 
is actually holding compares with the maximum that it could hold. So, for 
example, if air at 68 degrees F contained 1.2% water vapour, and the most it 
could hold was 2.4%, we would say the humidity was just half, or 50% humidity.
Humidity is a funny measure, because on a really cold morning at -4 deg F, with 
100% humidity, the air is really very dry compared with a 104 deg F afternoon, 
with 20% humidity.
100% of 0.1% = 0.1%;  20% of 7% = 1.4%, or 14 times as much water vapour content!

When air gets close to 100% humidity it is no longer able to allow surface water 
to evaporate into it any more, so very high humidity is a strong factor 
preventing evaporation.

When air is very still, evaporation produces a layer of very humid air (100% 
humidity) just above the surface of a lake or puddle, and this can slow or stop 
further evaporation. The official "humidity" might be quite low but there is this 
small area of high humidity just over the lake or puddle. If there is a stiff 
breeze, that layer will keep getting swept away and replaced with drier air. That 
can help evaporation a lot. If the wind gets even stronger, and starts to make 
ripples and waves, that also speeds up evaporation a lot. When water evaporates, 
it chills, and if the water is still, a cold layer can form on the surface which 
slows further evaporation. A bit of wind helps circulation currents to form in 
the water as well, and keeps the warmer water coming to the top.

So that is how the three factors work. I hope you can work out the rest from 

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