Why does the vertical distribution of water vapour generally decrease with height?

Correct answer: Because main sources are the surface and oceans, and cold upper air has lower capacity

Direct answer: A is correct. Most atmospheric water vapour enters the atmosphere through evaporation from oceans, lakes, soil, vegetation, and other surfaces near Earth. As air rises, it generally becomes cooler. Cooler air has a lower saturation capacity, so it cannot hold as much water vapour before condensation begins. Rising air may therefore lose moisture through cloud formation and precipitation, while the main moisture sources remain below. These two connected facts—surface-based sources and decreasing holding capacity in cold upper air—explain why water vapour usually decreases with height. A is correct because it includes both factors. B is wrong: gravity does not destroy water vapour, and it does not simply increase in the way stated. C is wrong because most atmospheric vapour comes from Earth’s surface, not space. D is wrong because temperature generally decreases through much of the lower atmosphere, though it is not identical at every height. Memory cue: moisture comes from below, and colder air aloft holds less.