If air rises in a mountain region but moisture is very low, why may cloud formation be limited?

Correct answer: Even on reaching the dew point, there will be little condensable water vapour

Direct answer: A. Rising air generally expands as surrounding pressure decreases and may cool. If it cools to its dew point, condensation can begin, but condensation still requires enough water vapour. When the starting moisture content is very low, only a small amount of vapour is available to become droplets. The air may therefore reach or approach saturation without producing a thick cloud or significant precipitation. A is correct because both cooling from uplift and adequate moisture are needed for substantial cloud development. B is wrong because rising air can cool through expansion, although the exact rate depends on conditions. C is wrong because mountains do not destroy all condensation nuclei. D is wrong because low moisture usually limits, rather than guarantees, heavy rain. Memory cue: uplift supplies cooling, but moisture supplies the material for clouds.