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?