Every night when the sun sets, the air temperature decreases. How does the relative humidity change (increase, decrease, or stay the same) when the air temperature decreases?

The relative humidity rises when the water vapor level appears to be the same but the temperature drops. The relative humidity reduces if the water vapor level remains the constant and the temperature increases. This is because of the reason that the colder air does not need as much humidity as warmer air to get saturated.