Just How Much Water Can Air Hold?

A FAQ in the dehumidification business is “How the heck much water can air hold?”

OK, a pound of 80°F. air in the atmosphere at saturation, 100% relative humidity, will contain .0022338 lbs. or 156 grains of moisture. You probably already knew there are 7,000 grains to a pound.

At 60% RH, how we rate our dehumidifiers, there would be 92 grains in each pound or air. That pound of air, in turn, occupies 13.3 cubic feet in the atmosphere, a cube about 2-1/2’ on each side. That’s what the weather was like at my house in Florida all through Christmas. Some people live in places like Madison, WI. A pound of their air would only contain 5 grains at 0°F.

Another way to think about this is a pound of water will occupy a minimum of 500 cubic feet at my house (did I mention I live in Florida?) and 15,000 cubic feet should a poor soul in Madison go outside. But back to my question, how much water can air hold? Answer: zero, zilch. Air doesn’t hold water! Rather, air and water vapor share the air space according to Dalton’s law of partial pressure: The air and water vapor each occupy the common (total) volume at the common temperature but at their respective pressures. The water vapor contributes up to one-half psi to atmospheric pressure in my town, the other 14.2 psi due to dry air.

Water vapor is air’s roommate that she doesn’t notice unless you bump into her in the bathroom or she wants to watch a different TV channel.