Evaporation rates

[Phillbo]

Does water evaporate faster when the relative humidity is lower ?

[chem geek]

Does water evaporate faster when the relative humidity is lower ?
Yes.

Of course, I might anticipate your next question which is how much faster does water evaporate as a function of the level of relative humidity. As a first approximation and with temperature, air density, and air flow (i.e. wind) held constant, the rate of evaporation is inversely proportional to the relative humidity (RH). So the rate is highest (say, 1.0) with 0% RH and is 10% lower (say, 0.9) at 10% RH, half the rate at 50% RH, 90% lower (say, 0.1) at 90% RH and is 0 (i.e. no evaporation) at 100% RH.

Richard

[Simmons99]

Yes.

Of course, I might anticipate your next question which is how much faster does water evaporate as a function of the level of relative humidity. As a first approximation and with temperature, air density, and air flow (i.e. wind) held constant, the rate of evaporation is inversely proportional to the relative humidity (RH). So the rate is highest (say, 1.0) with 0% RH and is 10% lower (say, 0.9) at 10% RH, half the rate at 50% RH, 90% lower (say, 0.1) at 90% RH and is 0 (i.e. no evaporation) at 100% RH.

Richard

Phillbo......so in other words - when it's pouring there is no evaporation - but when it's really dry...yes you have to top off the pool :)

LOL - I'm laughing again :D - Richard you might like my post on the anti-freeze thread.....:). I need to get more than 4hrs of sleep a night or I'm going to start getting banned from forums :p

[chem geek]

Though I answered the question of evaporation rates as a function of relative humidity, I now answer it with respect to temperature. The evaporation rate is proportional to the vapor pressure of water which is a function of temperature. At 20C (68F), the vapor pressure is 17.5 mm Hg; at 30C (86F) it's 31.8; at 40C (104F) it's 55.5

So as Simmons99 says, "when it's really dry you have to top off your pool" and more specifically when it's 104F outside then, all else equal, water evaporates over 3 times faster than at 68F. In hot desert areas with 104F temperature and RH of 10%, the water evaporation rate is over 5 times as high as the coastal area where I live at 86F and 70% RH (not right now, but back in July). And then there's differences due to wind...

There are products on the market that act as a liquid "blanket" to lower the rate of evaporation. Such products, typically alcohols such as cetyl alcohol, will cut the evaporation rate by more than half, but you have to constantly replenish this product (even though a little goes a long way since it's only a few molecules thick on the water) and it's putting more organic junk into your pool (more stuff for chlorine to oxidize, etc.). It is generally much better to use a pool cover to significantly reduce evaporation.

Richard