# Vapour pressure vs saturation pressure; too much confusion

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1. Nov 4, 2016

### Ravi Singh choudhary

It was only the psychrometry came; I read about vapour pressure is equal to the saturation pressure at 100% relative humidity. While before even in the textbooks both terms are used frequently as same. I fully understood what saturation pressure is; learned during phase change phenomenon of water. Again I learned about vapour pressure defined in closed container and through physics forums I understood about non equilibrium phenomenon in open container. Somehow I am not able to relate humidity, vapour pressure and saturation pressure but I think I know them independently well. Please help me.

2. Nov 4, 2016

### BvU

(Relative) humidity is usually expressed in percent and can only occur if there are other gases present in non-equilibrium cases. (i.e. equilibrium $\equiv$ 100 % humidity).

3. Nov 4, 2016

### Staff: Mentor

Relative humidity does not only apply to non-equilibrium situations; for example, it can also apply if there is no liquid present. The relative humidity is defined as the partial pressure divided by the equilibrium vapor pressure at the temperature of the gas mixture, times 100%.

4. Nov 4, 2016

### BvU

Thanks Chet, I didn't think of that! Too much liquid around in the daily rain