# Vaporization in function of relative humidity

I am wondering how the relative humidity affects the vaporization?
I saw on an other post that it doesn't affect it, but in my personal experience, it takes much more time to dry clothes when the relative humidity is high, so it's hard for me to believe that it does not have a big effect. What is the effect of relative humidity on evaporation speed?

Yes, relative humidity definitely affects drying rate. At 100% there won't be any drying. The other post is not discussing drying rate, but instead the energy needed for water to enter the gas phase.

See the following reference for information about humidity dependence of drying rate:

The simple answer is that "rate of evaporation" is the net flow of molecules from the liquid phase to the vapor phase. This is the difference between "molecules leaving the liquid" and "molecules entering the liquid". The definition of "relative humidity 100%" is basically that point where these two quantities are the same. So yes - the net rate of evaporation will slow down as relative humidity increases.

This is the reason that relative humidity plays a part in "comfort index" which is a combination of temperature and humidity. Warm dry air feels more comfortable than slightly cooler, but "wetter" air - because in the former case the body can more easily regulate its temperature by evaporation than in the latter case.

First of all, we need to use the good way to talk about humidity, which is in terms of vapor pressure. If water (or any given non exotic liquid a) is present in a system with global pressure p and temperature t, then it will reach equilibrium when the partial pressure of vapor of a in the gaz phase has reached the vapor pressure $p^*_a(t,p,[X_a])$(Where $X_a$ Would be the molar fraction of substance of interest a in the solution if there are multiple substances in the liquid). That being said, usual hygrometry level are given in percentage of this vapor pressure so that 100% should mean nett condensation starting, for any given conditions.

Of course this is just equilibrium thermodynamics, but the process itself of evaporation fits the usual transfer theory very well, so that the speed of evaporation linearly depends on the "distance" from equilibrium and size of interface: $$J[mols^{-1}]=\mathrm{constant}\cdot(p^*_{H_2O}(t,p_{atm})-p_{H_2O})\cdot\Omega$$

So in conclusion, yes of course, ambiant humidity will heavily impact evaporation speed (ever tried drying a towel in a sauna?)

In open air, water evaporates without affecting much air's vapor-pressure so the decreasing water's surface-temperature will eventually reach temperature of the air's vapor-pressure. How much water evaporate is determined by initial water temperature and the ratio of latent heat to specific heat. The evaporation rate depends on air's and water's flow speed and turbulence above and below the surface. In close limited room, water evaporates so air's vapor-pressure will eventually reach that of water's surface-temperature. how much water evaporate is determined by initial vapor-pressure and the room's volume