There are two kinds of humidity: relative humidity and absolute humidity.
Absolute humidity is the actual, physical amount of water in a certain amount of air.
What you see in weather forecasts is relative humidity and it’s a percentage of how “full” of water the air is. The maximum amount of water air can hold depends on its temperature. Warmer air can hold more water than colder air.
As temps drop, the actual amount of water in the air stays the same. But the maximum that the air can hold is decreasing. So relative humidity goes up.
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