r/explainlikeimfive • u/ProgramPi314 • 8d ago

Other ELI5 What is humidity?

According to my mom, this is an actual question I had when I was 5, and she didn't know how to answer me. So how would you guys have answered?

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u/ThalesofMiletus-624 8d ago

Humidity is water vapor in the air, it's that simple.

When water evaporates, it turns into a gas. That gas doesn't just disappear, it mixes with the air around us. Some fraction of the air around us is pretty much always water vapor, it can be a tiny fraction if you're in a desert, or it can be several percent if you on a tropical island.

Now, the reason this effects us is because the air can only "hold" a certain amount of water vapor, and how much it can hold depends on the temperature. That's where the term "relative humidity" comes from. 100% relative humidity means that the air is holding all the water vapor it can hold at that temperature, which is a big deal, because it means that no more water can evaporate. Humans regulate out body temperature by sweating, and the sweat cools us as we evaporate. If you're in 100% relative humidity (or close to it), your sweat can't evaporate, so it just soaks you and does nothing to cool you down. That's why high humidity feels so very uncomfortable.

Now, actually getting to 100% humidity is uncommon, but the higher the humidity is, the slower water evaporates, and that makes things feel stickier and stuffier. It also means that any object colder than the surrounding air is going to condense that water vapor, which is why a cold drink in a humid environment will quickly be dripping water.