Relative humidity may fall in the afternoon while vapour amount remains nearly the same. What is the reason?

Correct answer: Afternoon temperature increases saturation capacity

Direct answer: A is correct. Relative humidity is the amount of water vapour present compared with the maximum amount the air can hold at that temperature. In the afternoon, the ground is usually heated by the Sun, so air temperature rises. Warmer air has a greater capacity to hold water vapour before becoming saturated. If the actual vapour amount stays nearly unchanged, it becomes a smaller fraction of this increased capacity; therefore relative humidity falls. For example, the same quantity of vapour may make cool morning air nearly saturated but make warmer afternoon air less saturated. A is correct because it states the increase in saturation capacity. B is wrong: vapour does not disappear merely because it is afternoon. C is wrong: sunlight does not turn air into liquid water. D is wrong: air pressure is not always zero and is not the main reason here. Memory cue: same vapour plus higher temperature usually means lower relative humidity.