# Relative Humidity, vapor pressure, partial pressure

The partial pressure of water in the mixture is the equilibrium vapour pressure of water at the temperature specified.

Relative humidity (RH) is the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at a given temperature.

So at all temperatures Relative Humidity is 1 . But this isn't so. Where am I wrong?

• Start with re-examining your definition of partial pressure. When talking about RH, the partial pressure we use in the calculation is the pressure of water vapor in the air-water mixture. It is not the vapor pressure of water at that temperature, where vapor pressure is the amount of gas phase water that is in equilibrium with liquid phase water; I think this is where your mistake is. – J. Ari Jul 21 '17 at 12:33
• Thanks. Does this mean that if we are working in a closed vessel the partial pressure of water is same as its vapour pressure? – user161158 Jul 21 '17 at 12:43
• @J.Ari And thus partial pressure of water only changes with temperature in a closed vessel? – user161158 Jul 21 '17 at 12:51
• As long as there is nothing that is affecting the expected vapor pressure from developing (salt effects, water reactive atmosphere), then your statement in the comments is correct for the case of an enclosed vessel. – J. Ari Jul 21 '17 at 14:26