Buzz
On hot and humid days, the air can feel heavier, but in reality, it's not. The perception is more about the discomfort caused by the moisture in the air.
Carlos Sanchez/ThinkStockImportant Points to Remember
- In reality, humid air is lighter than dry air since water vapor molecules replace the denser nitrogen and oxygen molecules in the atmosphere.
- This aligns with Avogadro's law, which states that gases of equal volume at the same temperature and pressure contain an equal number of molecules.
- The interaction between humid and dry air plays a significant role in weather events, including thunderstorms.
Obviously, humid air is heavier than dry air! Here are some scientific facts that support my theory: When my hair gets wet, it feels heavier than when it's dry. Fact. When I dip my generic chocolate sandwich cookie in milk, it becomes heavier than when I eat it directly from the package. Fact. When I cry, my eyes feel heavier than when I'm not crying. Not entirely factual, but it does seem like it could be true, as your eyes tend to feel heavy when you're emotional, right?
Here's the issue with all my facts (aside from the major one, which is that I didn't bother to test them beyond my imagination): They all relate to water, not water vapor. It's the water vapor that contributes to humidity, not the liquid droplets of water. Once we comprehend how water vapor molecules behave, we can finally demonstrate the real truth: humid air is actually lighter than dry air.
Let’s begin with Avogadro's law. This fundamental principle of chemistry tells us that equal volumes of gases at the same temperature and pressure will contain the same number of molecules [source: Williams]. The weight of the molecules doesn't matter as much as the volume. Since air in our atmosphere is free-flowing, molecules move in and out without restriction.
But hold on, you might wonder: If you're adding water molecules, wouldn't that increase the weight? The answer is that any water vapor introduced replaces nitrogen or oxygen in the air. Nitrogen and oxygen are the main components of our atmosphere, and when water takes their place, they are displaced or evaporated. Water vapor molecules are lighter than both nitrogen and oxygen. So, air with water vapor contains fewer heavier nitrogen and oxygen molecules, and more of the lighter hydrogen and oxygen. While the molecule count stays the same, the air with water vapor becomes less dense [source: Schrage].
So, here's the bottom line: When temperature and pressure are equal, dry air will be heavier because it lacks the lighter water vapor molecules. It's also important to remember that the interaction between dry and humid air leads to intense storms. When dense, dry air moves beneath moist, lighter air, it pushes the moist air upward, creating ideal conditions for thunderstorms. This 'dry line' zone—where dry air forces up humid air—can be found right where you'd expect: in the Tornado Alley region of the southern-central United States [source: Schrage].
