Which layer of Earth's atmosphere contains the majority of the atmosphere's water vapor?

• A. Stratosphere
• B. Troposphere
• C. Mesosphere
• D. Thermosphere

Answer: (B)

Water vapor is concentrated near Earth’s surface because evaporation happens there and weather processes keep humidity high in the lowest part of the atmosphere. The layer closest to the ground—the one where air is warm and dense and clouds form—is where most of the atmosphere’s water vapor lives. As you rise, the air becomes thinner and cooler, so it can hold less moisture and the water vapor is less prevalent. That’s why the higher layers—stratosphere, mesosphere, and thermosphere—are much drier. The stratosphere in particular tends to have far less water vapor because its stability and temperature structure limit upward mixing of moisture.