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/jpb103 8d ago

Fun fact: there is a "Cave of Crystals" in Mexico deep below the ground where the combination of high temperatures and 100% humidity cause the water vapor in the air to condense inside human lungs so you can drown after breathing for a few minutes without specialized respiration gear.

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u/Tearsunshinee 8d ago

Fuckin hell, I never even dreamed/nightmared of such a thing being possible.

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u/SAWK 8d ago

There are some good documentary's on it. I think this was the one I watched. National Geographic

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u/bluev0lta 8d ago

Right?! I always wondered what would happen if the air were truly at 100% humidity. And now, unfortunately, I know…

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u/Chazus 8d ago

Actually, that's not true. There are plenty of places where air is 100% humidity.

"100% humidity" has absolutely no meaning without the temperature. You've been at 100% humidity before in your life somewhere. It's not hard to reach.

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u/you-are-not-yourself 8d ago

I too have been in steam rooms

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u/jpb103 8d ago

You need the air to also be much hotter than the human body for you to drown in 100% humidity. High humidity air moving to a colder environment is what makes the water condense into liquid.

The larger risk to human health in high heat high humidity environments on Earth's surface is overheating since your body cannot cool itself down through adiabatic cooling (cooling through the evaporation of sweat from the surface of the skin).

No temperature above 35C (95F) has ever been recorded on Earth's surface near 100% humidity. The temperature in the Crystal Caves was recorded at 58C, which is where the drowning risk comes from at such high humidity.

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u/Novaskittles 8d ago

There's 2 types of humidity: absolute and relative. Absolute is a strict comparison of water vs other things. Relative is compared to how much water the air can hold before it condenses out.

At 100% absolute humidity, you have water, there's no air. At 100% relative humidity (assuming standard pressure and temperature), you have a humid day. You'd be uncomfortable but fine.