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Q & A: effect of warmer air on temperature of water

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Most recent answer: 01/19/2016
What is the relationship between air temperature passing across a shallow pan say 10mm of potable tap water and the resultant water temperature assuming 10% relative humidity and an airspeed 15km per hr between 25 deg C and 45 degC for a starting tap temperature of 25 DegC. Will the warmer air speed up evaporation enough to make the water cooler and cooler in a given time despite the warmer air temperature?
- Bruce Sharman (age 56)
Red Cliffs, Victoria, Australia

Nice question. At first it might seem like these are two competing effects, and that it would be hard to guess which is bigger:

1) the warmer air warms the water
2) the warmer air speeds up evaporation, cooling the water. 

Let's think, however, about how the warmer air speeds up the evaporation. It's not by slowing down the rate at which water molecules return from the air to the liquid. In fact, the warmer, faster-moving ones tend to return more quickly. Instead, it's by greatly speeding up the rate at which water molecules can break away from the warmer liquid and go into the air. So that's the answer: unless the water is warmer, the evaporation won't speed up. So the bigger effect has to be the warming. The evaporation reduces that warming, but can't change the sign of the effect.

Mike W.


(published on 01/19/2016)

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