How can absolute humidity stay constant when cooling air isobarically?

[dobry_den]

Hi! My Physics introductory book states the following: "A dew point is a state defined by the temperature to which the air has to be cooled isobarically (with constant absolute humidity) for the vapour to become saturated."

Humidity is defined as m/V, where m is weight of the vapour, V is volume of the air.

My problem is that when we isobarically cool gas, its volume decreases, so the absolute humidity has to rise (the weight of vapour should be constant?). So how can we isobarically cool air and at the same time preserve its absolute humidity?

 

[member 553083]

The key to understanding this is the fact that when air is cooled, the amount of water vapor it can hold decreases. This is due to the decrease in pressure and the decrease in temperature. As a result, when air is cooled isobarically (with constant absolute humidity), the amount of water vapor in the air increases as the temperature drops. Eventually, the air will reach its dew point, which is the temperature at which the air has reached its saturation point. At this temperature, the air cannot hold any more water vapor, so it condenses into liquid droplets and forms dew.