Avogadro’s Law explains why humid air is deceptively heavy

From the earliest days of sports announcing, on muggy nights baseball announcers have remarked about how baseballs do not travel as far due to the heavy, humid air.

On these humid Kona days, the air does feels heavy, like walking underwater, or hard to breathe with heavy lungs.

As intuitive as that may sound, the muggy air fools our senses. The fact is that humid air is lighter than dry air.

The reason is simple. It derives from a fundamental law of physics: Avogadro’s Law, which states that equal volumes of gas contain the same number of molecules at the same temperature and pressure.

There is an easy analogy for the way in which Avogadro’s Law explains the lightness of humid air. Picture two identical jars of equal shape, each with 10 lead weights inside. They weigh the same and contain the same number of objects.

Now remove two of the lead weights from one jar and replace them with two plastic beads. The jar with the beads weighs less but it still contains 10 items.

The situation is the same for air, but involves molecules instead of weights and beads.

Water vapor is the third most abundant gas in the atmosphere, except when it is not. Water is not usually on the list as a component of atmospheric gas along with nitrogen and oxygen because it varies from zero to 4 percent by weight.

Water vapor composes up to 4 percent of the atmosphere in a sultry equatorial rain forest and ocean. Otherwise, it is typically in the 2 percent to 3 percent range.

Water vapor is lighter than air. Nitrogen and oxygen, which compose a little more than 99 percent of air, are diatomic gases. That means they exist as molecules composed of two atoms.

Each nitrogen atom weighs 14 units, so a nitrogen molecule of two atoms weighs 28 units. Each oxygen atom weighs 16 units, so an oxygen molecule of two atoms weighs 32 units.

Water molecules consist of one oxygen atom of 16 units and two hydrogen atoms of one unit each, for a total weight of 18 units. Each water molecule weighs only 64 percent as much as a nitrogen molecule and 56 percent as much as an oxygen molecule.

Imagine for illustrative purposes a bubble of dry air at one atmosphere of pressure and 75 degrees. To further simplify, let us say this sphere contains exactly 1,000 molecules. This represents a minuscule volume because molecules are very, very small.

Adding water molecules to the bubble would force some of the nitrogen and some of the oxygen out of the volume because it must still contain 1,000 molecules. With the lighter water molecules displacing the heavier nitrogen and oxygen molecules, the volume overall must weigh less than when it was filled with only nitrogen and oxygen.

The humid air seems heavier because each breath draws in fewer oxygen molecules, as if at a higher elevation.

As for the baseball, it is not the air that is heavier. It is the baseball: It has absorbed some water from the atmosphere. It is also less rigid because the damp leather is softer and less resilient.