Why can relative humidity become lower in the afternoon even when actual vapour remains almost the same?

Correct answer: Afternoon heating raises saturation vapour pressure

Direct answer: A is correct. Relative humidity compares the actual amount or vapour pressure of water vapour with the saturation value at the same temperature. During the day, afternoon heating raises air temperature. Warm air has a higher saturation vapour pressure, meaning its capacity for water vapour is greater. If the actual vapour content changes little while the saturation value increases, the ratio becomes smaller, so relative humidity falls. A is correct because it identifies the change in the denominator of the comparison. B is wrong because vapour does not automatically become zero. C is wrong because air continues to contain molecules. D is wrong because condensation nuclei do not simply disappear every afternoon, and their absence is not the main reason for this daily RH change. Memory cue: warmer air can have the same moisture but a lower percentage of saturation.