When air is cooled to a point at which the air's capacity to hold more water vapor is zero, saturation is reached.

Like all clouds, it forms when the air reaches its dew point, which is the temperature to which an air mass must be cooled for the water vapor in it to condense into liquid droplets.

And we found that the gas-liquid coexistence point, the vapor pressure of the water, was less than if it were pure.