Of course humid air is heavier than dry air! Scientific facts that hold up my theory: When wet, my hair is heavier on my head than it is when dry. Fact. When dunked in milk, my generic chocolate sandwich cookie is heavier than it is when eaten straight from the package. Fact. When I cry, my eyes are heavier than when I don't cry. Not really fact, but it seems like it would be true-ish, because, you know, your eyes feel heavy when you're sad, right?
Here's the problem with all my facts (aside from the biggest problem, which is that I didn't bother to test them outside of my imagination): They all concern water, not water vapor. And it's water vapor that makes air humid, not actual drops of water. And once we understand how water vapor molecules work, we'll be able to prove the real truth, which is that humid air is actually lighter than dry air.
Let's start with Avogadro's law. This basic law of chemistry states that equal volumes of gases at the same temperature and pressure contain an equal number of molecules [source: Williams]. So it doesn't matter what the molecules weigh; it just matters that you have the same volume. And since the air in our atmosphere moves freely, molecules can float in and out.
But wait, you might think: If you're adding water molecules, why wouldn't that add weight? And the answer is that any water vapor that gets added replaces either nitrogen or oxygen in our free-moving air. Nitrogen and oxygen make up the majority of our atmosphere, and they're displaced -- or evaporated -- when water takes their place in the air. And water vapor molecules are lighter than both nitrogen and oxygen. In other words, humid air is going to have less heavy nitrogen and oxygen -- and lighter hydrogen and oxygen -- in its place. Remember that they have the same number of molecules, but the air with water vapor is simply less dense [source: Schrage].
So there you go. If temperature and pressure are the same, dry air will be heavier because it lacks the lighter water vapor molecules. And keep in mind that it's the interplay between dry and humid air that causes big storms. When dry, dense air moves under moist, light air, it lifts the moist air and makes the right conditions for thunderstorms. This "dry line" area -- where dry air pushes up humid air -- is seen right where you'd expect it: in the Tornado Alley of the southern-central United States [source: Schrage].