Why is most atmospheric water vapour found in the lower troposphere?

Correct answer: Because moisture sources are near the surface and oceans

Direct answer: A is correct. Most atmospheric water vapour enters the atmosphere through evaporation from oceans, seas, lakes, moist soil, and transpiration by plants. These sources are concentrated at or near Earth’s surface, so the lower troposphere receives the greatest supply. As air rises, it generally becomes colder and its capacity to hold water vapour decreases. Some vapour therefore condenses into clouds or is removed as precipitation, leaving much less vapour at greater heights. A is correct because it identifies the main location of moisture sources; the cooling of rising air also supports the vertical decrease. B is wrong because the upper atmosphere is not always warmer and warmer air alone would not explain the source. C is wrong because atmospheric moisture mainly comes from Earth’s water cycle, not from space. D is wrong because evaporation is active near the surface. Memory cue: moisture starts low and rising air usually loses it through cooling and precipitation.