How can increasing water vapour in air affect cloud formation height?

Correct answer: The cloud base may become lower because less cooling is needed for saturation.

The correct answer is A. As an air parcel rises, it usually cools because it expands under lower surrounding pressure. Cloud begins to form when the parcel cools to its dew point and reaches saturation. If the parcel contains more water vapour, its dew point is higher and the difference between its temperature and dew point is smaller. Therefore, it needs less additional cooling and may reach saturation at a lower altitude, producing a lower cloud base. A is correct, although the actual cloud-base height also depends on the temperature profile, lifting strength, mixing and other atmospheric conditions. B is wrong because increasing moisture does not automatically move a cloud into space. C is wrong because water vapour is essential for cloud formation, so more moisture can support it when lifting and cooling occur. D is wrong because saturation is approached as air becomes relatively more moist, not more dry. Memory cue: smaller temperature–dew-point spread means a shorter rise to cloud base.