– How can humidity in the air be measured as a percentage? How much humidity is 100%?
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Hot air can hold quite a bit of moisture.
Cold air cannot hold very much at all.
Imagine it like a cup that changes size based on temperature.
It’s hot outside, and the cup is half full.
We’d say the hot air has 50% as much of the air in it as it could ever hold.
Now the temperature drops, and the cup starts to shrink.
Same amount of water, fills a larger percentage of a smaller cup. About 3/4 of it. Now the air has 75% of all the water it could ever hold.
Now the temp drops again and the cup shrinks further.
Now we’re at 100%. The smaller cup is completely full with the same amount of water.
And now, the temp drops one last time.
The cup shrinks. It’s now too small. The water spills over and drips everywhere.
There’s more water in the air than the air could actually hold. So it percipitates out.
If the temperature is 32 degrees f or lower, the water will freeze and form into snow, hail, sleet…
If the temperature ks ABOVE 32 degrees it will stay water and fall out of the air as rain.
If the temperature is 212 degrees f or higher, perhaps wet ocean air around a volcano, cooling down as it moves away, it will turn to steam.
The amount of water in the air, expressed as a percentage of the maximum amount of water that air could possible hold at that temperature and pressure…
So… the amount of water relative to the maximum.
Hence: “relative humidity”
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