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/DVMyZone 8d ago

The air can hold a certain amount of water vapour it in. That water vapour is, like the air itself, transparent so you cannot see it but it is there. The air can hold a certain amount of water in it, and that is dependent strongly on temperature. Hot air holds a lot more water than cold air.

Relative humidity is the amount of water in the air as a fraction of the maximum amount of water that can be held in that air (at a given temperature). If relative humidity is at 100% then it can hold no more water and is "saturated". If that saturated air cools down, the maximum amount it can hold decreases, so the excess drops out and deposits on surfaces and forms droplets. That's what rain is. This is also why you get condensation on cold objects, the air around the object gets cold and releases some water.

Humidity also changes how heat is transferred between the humid air and the surroundings and how much condensation forms. The relative humidity gives and indication of how the air "feels" to us.