Why do humidity levels rise in the overnight hours, and then begin to drop after dawn?

Weather Blog

Dear Tom,
Why do humidity levels rise in the overnight hours, and then begin to drop after dawn?

Thanks, 
Jim Robinett 
Chicago

Dear Jim,

The ability of air to hold water vapor decreases as temperature decreases, halving for about every 20 degrees of temperature decline. Dew point is the temperature at which air is saturated with water vapor. If moisture is neither added nor removed from air, the dew point does not change. Relative humidity is the percent of saturation and reaches 100 percent when the air temperature drops to the dew point. Assuming a constant dew point, as temperatures fall overnight and approach the dew point, the humidity rises, usually peaking around dawn. After sunrise, the temperature begins to rise, and the relative humidity starts to drop, reaching its lowest values during the afternoon during the period of highest temperature.

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