It doesn’t necessarily. As the temperature drops at the end of the day, absent some mechanism to remove moisture from
the air (e.g. precipitation, weather patterns, etc.), the absolute humidity and dew point can stay constant. The relative humidity will go up because that’s a measure of how much moisture air can hold and warmer air can hold more moisture. So it goes from low relative humidity to high relative humidity as the air temp drops at the end of the day, but the amount of moisture in the air stays about constant.
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