Perceptions can be misleading. An example of this is the fact that humid air is light compared to air with low humidity. The expression, “It’s so humid, you can cut the air with a knife,” illustrates the feeling that humid air is oppressive and, therefore, heavy. Actually, air that is more humid is lighter than air with low humidity. The explanation is fairly simple.
Air is about 78% nitrogen and about 20% oxygen. Water vapor accounts for about 1% on average, but the amounts vary from place to place as humidity goes up and down. It turns out that each molecule of water vapor is lighter (18 grams/mole) than each molecule of either nitrogen (28 grams/mole) or oxygen (32 grams/mole). So the more water vapor (humidity) there is in the air around you, the lighter the air is. If you feel sluggish in sticky weather, it isn’t because of the weight of the air.
