What is the common reason for relative humidity decreasing in the afternoon?

Correct answer: Rising temperature increases the air’s capacity

Direct answer: A. Relative humidity compares the actual amount of water vapour in air with the maximum amount that air could hold at the same temperature, expressed as a percentage. As daytime heating raises air temperature, the air’s capacity to hold water vapour increases. If the actual amount of vapour does not increase by the same proportion, the ratio becomes smaller, so relative humidity falls. This is why relative humidity commonly decreases during the warmer afternoon even when the amount of water vapour has changed little. B is incorrect because oxygen removal is not the normal explanation. C is incorrect because water vapour does not always freeze in the afternoon; freezing requires very low temperatures. D is incorrect because air pressure does not become zero. Memory cue: warmer air can hold more vapour, so unchanged moisture gives lower relative humidity.