Final answer:
The tropopause marks the upper boundary of the troposphere and is the point where the stratosphere begins, characterized by an increase in temperature due to UV radiation absorption by ozone.
Step-by-step explanation:
The troposphere is the lowest layer of Earth's atmosphere, where the temperature decreases rapidly with increasing elevation, reaching values near 50 °C below freezing at its upper boundary, which is known as the tropopause. The stratosphere begins above the tropopause and extends up to about 50 kilometers above the Earth's surface.
This layer of the atmosphere is characterized by a gradual increase in temperature with altitude, primarily due to the absorption of ultraviolet (UV) radiation by ozone, which is present in higher concentrations in the stratosphere.
Instruments like the Stratospheric Observatory for Infrared Astronomy (SOFIA) can operate effectively in this part of the atmosphere because they are above most of Earth's atmospheric water vapor, which interferes with infrared observations.