What criterion is used to define the major atmospheric layers?

• A. How temperature changes with altitude
• B. The concentration of water vapor
• C. The amount of oxygen
• D. The presence of life

Answer: (A)

Temperature variation with altitude defines the major atmospheric layers. Each layer is identified by how temperature changes as you rise. In the lowest layer, temperature decreases with height and ends at the tropopause. Above it, the stratosphere warms with altitude due to ozone absorbing UV radiation, bounded by the stratopause. Above that, the mesosphere cools with height until the mesopause, and in the thermosphere temperature increases again with height. Because the defining feature is the pattern of temperature change with altitude, not moisture, oxygen content, or life, these layers are distinguished by their characteristic temperature gradients rather than composition or biology.