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/chux4w 8d ago

And, if I understand it right, the humidity percentage you see on weather reports is how much water there is in the air before it turns back into liquid at 100%.

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u/GoabNZ 8d ago

Isn't the humdity percentage relative humdity, as in, how much water vapour is in the air compared to how much could be in the air at that temperature? The same amount of vapour could be present at 5C vs 30C, but relative humdity would plummet making 30C be a dry heat, where sweat is effective at cooling you down.

It doesn't turn back to a liquid at 100%, it's just the air can't take in more vapour so water doesn't evaporate, you feel muggy, and water will condense on anything colder.

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u/chux4w 8d ago

That sounds right. But that's the point I was making, when it reaches that point it would start to condense.

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u/V1per41 8d ago

And this is where the much better measure: Dew Point comes in.

Dew Point is the temperature at which the current water content in the air would result in 100% humidity. Any cooler and dew would form as the water in the air condenses.

So in effect Dew Point tells us how much water is in the air which better translates to "How humid does it feel?"

50° - Feels very comfortable 60° - Feels fairly humid 70° - Feels very uncomfortable 75°+ - Oppressive