Weather Talk: Hot, humid air is light, not heavy
Perceptions can be misleading. An example of this is the fact that humid air is light compared to air with low humidity.
One common 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 percent nitrogen and about 20 percent oxygen. Water vapor accounts for about 1 percent 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 than each molecule of either nitrogen or oxygen. 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.