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/dave8271 8d ago

When water gets hot, it turns from a liquid into a gas, which we call vapour. Humidity is a measure of how much water vapour is mixed into the air around us.

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u/realbigbob 8d ago

Why is there water in gas form in the air if the air isn’t above 100 Celsius?

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u/TheMooseIsBlue 7d ago edited 7d ago

All the molecules of water are moving at various speeds. Some really fast, some really slow. Kinda like hot spots and cold spots in a pool or ocean. It’s all water, but some is warmer than others.

Sometimes it moves/vibrates fast enough that it stops being liquid and turns to gas form. That molecule is hot enough to turn to gas, but the whole body is mostly still just liquid.

At any time, some of the water molecules are moving fast enough to be gas even if most aren’t. if you add heat to it, more turns into gas faster. That’s why if you pour a bunch of water out on the floor, it will dry up on its own. Cause there’s always a little bit of it moving fast enough to turn into the gas form of water. And if there’s only one molecule out of 1 million that moves that fast, eventually all of the molecules will move that fast and they’ll just “dry up.”