A oft-repeated water vapor myth is that warm air can “hold” more water vapor than cool air because as the air warms its molecules move farther apart, making room for more molecules. This leads to the idea that as air cools its molecules move closer together, “squeezing” out water vapor.
If you look at what happens in nature, such as clouds beginning to form when the air rises and grows colder, the idea that condensation begins when the air grows too cold to “hold” the water vapor in it seems to make sense.
But, saying cold air can’t hold as much water vapor as warmer air is at best a metaphor for what happens. It’s a metaphor that can lead people astray as they try to understand weather.
To begin with, it doesn’t make any physical sense because the air around us is mostly empty space with molecules of nitrogen, oxygen, water vapor, and other gases zipping around at speeds in the neighborhood of 1,000 mph near the ground.
There’s plenty of room for water vapor molecules to join the mix. As we saw above, they would displace some of the nitrogen and oxygen molecules. We can think of these displaced molecules spreading out into nearby, drier air. As the air grows colder the average speeds of all of the gasses in the air slow down, but there is still plenty of room for water vapor.
The best way to see what happens when water evaporates into water vapor and when this vapor condenses back into water is to imagine a drinking glass full of water in a room where the air is still. We also need to imagine we can see water molecules.
If so, you’d see water molecules moving around at various speeds in the water in the glass glass and also in the air above the water as vapor. On the average, the molecules in the glass are moving slower than those in the air. The liquid molecules in the glass are, on the average, slow enough for intermolecular forces to hold them in the glass while they otherwise move freely.
The vapor molecules in the air are moving fast enough to overcome intermolecular forces.
In both cases the molecules are not all moving at the same speed, but at a wide range of speeds with most of these speeds relatively close to the average, but a few are moving much faster and others much slower.
Some of the molecules in the glass will be moving fast enough to escape the water and fly into the air; they evaporate into vapor. In a similar way, some vapor molecules in the air are going slowly enough to be pulled into the water by intermolecular forces when they hit the water; they condense into liquid.
If a breeze is not carrying away water molecules that evaporate into the air and the temperature doesn’t change the number of molecules evaporating and those condensing are roughly the same. The air isn’t becoming more humid and water is staying at the same level in the glass.
If you cooled the room the average speeds of the water molecules in both the water and the air would slow. You’d see more water molecules in the air moving slowly enough to stay in the water when they hit―they’ve condensed. At the same time fewer molecules in the water are moving fast enough to evaporate. The water level in the glass would increase.
If you warmed the room and the water the average speed of the molecules would increase and more would evaporate until a new equilibrium is reached between evaporation and condensation.
In the everyday atmosphere, if the air is cooled enough as it rises water molecules slow down enough to attach to tiny particles in the air known as condensation nuclei to begin forming fog or cloud drops. At ground level they form dew drops on grass and objects such as cars.
For more on why saying condensation begins when the air can no longer hold the water vapor in it is both wrong and can lead to wrong conclusions, see Alistair B. Fraser’s “Bad Clouds” page on his Bad Meteorology site on the Pennsylvania State University Web site.