Why is water vapour called a variable gas of the atmosphere?

Correct answer: Its amount changes with place and time

Answer: A, its amount changes with place and time. Water vapour is the gaseous form of water, and its concentration is not uniform throughout the atmosphere. Near oceans, lakes, rivers, wet soil or dense vegetation, evaporation can add more vapour to the air. Temperature also matters because warm air can generally hold more water vapour than cold air. Cooling, condensation, rainfall, wind and distance from water sources can reduce the vapour content. Therefore the amount differs between a humid coast and a dry desert, and it can also change between day and night or between seasons. A is correct because it expresses this spatial and temporal variation. B is wrong because the amount is not constant. C is wrong because water vapour occurs over many regions, not only mountains. D is wrong because temperature, evaporation and condensation strongly affect it. The phrase variable gas refers to changing quantity, not to a gas that changes its chemical identity. Memory cue: water vapour varies wherever water supply and temperature vary.