The normal lapse rate of temperature of Earth's atmosphere drops to 0°

The normal lapse rate of temperature of Earth’s atmosphere drops to 0°C at the

upper part of ionosphere.

upper boundary of the tropopause.

lower part of mesosphere.

upper boundary of stratopause.

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This question was previously asked in

UPSC NDA-2 – 2019

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The temperature of the Earth’s atmosphere generally decreases with altitude in the troposphere (normal lapse rate), increases in the stratosphere, decreases again in the mesosphere, and increases in the thermosphere. The temperature reaches a relative maximum around 0°C at the upper boundary of the stratopause.

The stratopause marks the boundary between the stratosphere and the mesosphere. In the stratosphere, temperature increases due to absorption of UV radiation by ozone. This increase stops at the stratopause, where the temperature is typically around 0°C before decreasing sharply in the mesosphere. The “normal lapse rate” (temperature decrease with height) applies to the troposphere and mesosphere. The question phrasing is slightly ambiguous (“rate… drops to 0°C at the”), but in the context of atmospheric layers and boundaries, it most likely refers to the temperature value reaching around 0°C at a boundary where the lapse rate changes direction.

The approximate temperatures at key boundaries are: Tropopause (top of troposphere): -50°C to -60°C (highly variable). Stratopause (top of stratosphere): around 0°C. Mesopause (top of mesosphere): -90°C to -100°C (coldest point). Temperature increases rapidly in the thermosphere.

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