Can warmer air 'hold' more water vapour?

E. Linacre and B. Geerts


It is only a rough shorthand to say that cold air can hold less than warm air, as elegantly pointed out by Alistair Fraser at his website and by Craig Bohren in his popular book Clouds in a glass of beer(1). Removal of the air (i.e. of the gases other than water vapour) makes no difference to the process of condensation, so the air temperature is irrelevant. What matters is the temperature of the tiny, transient droplets of moisture within the vapour, even within apparently dry air, and the consequent energy of these droplets’ molecules. (Temperature is simply a measure of that energy.) If the water molecules are so energetic that their rate of escape from a droplet exceeds the rate of capture from the atmosphere (which depends on the water-vapour pressure, ie the concentration of water molecules in the space), the droplet will be depleted and vanish before it has formed. In this case, the vapour pressure in the air is less than the equilibrium (or saturation) vapour pressure. However, if the atmospheric concentration of vapour molecules is sufficient for a rate of capture equal to the rate of escape, we say the atmosphere is ‘saturated’, and the droplet persists. Beyond that, the droplets actually grow if the atmospheric water-vapour pressure is even slightly greater, because this leads to a rate of capture more than the rate of escape. The growth of the droplets produces clouds or dewfall.

            In brief, the rate of escape of water vapour molecules from an embryonic droplet is less in a cool environment, and so a lower concentration of atmospheric water molecules provides a matching rate of capture. This means that a lower water-vapour pressure provides saturation.


(1) Bohren, C.F. 1987. Clouds in a Glass of Beer. (Wiley) 195pp.