In which atmospheric layer do nearly all clouds form?

Clouds primarily form in the troposphere, which is the lowest layer of Earth's atmosphere. This layer extends from the surface of the Earth up to about 8 to 15 kilometers (5 to 9 miles) high, depending on geographical location and weather conditions. The troposphere contains most of the atmosphere's mass, including water vapor, which is essential for cloud formation. The cooling of air in this layer leads to condensation, as warm, moist air rises and expands. As it cools, it can no longer hold as much moisture, resulting in the formation of clouds.

In contrast, the stratosphere, which lies above the troposphere, is characterized by stable air and a temperature inversion, which prevents the vertical mixing of air masses and typically lacks the conditions necessary for cloud formation. The mesosphere, situated above the stratosphere, experiences decreasing temperatures and is also not conducive to cloud formation. Finally, the thermosphere is the uppermost layer of the atmosphere, where temperatures can become extremely high but also lacks the density and moisture required for clouds. Thus, the troposphere is the correct answer as it is the primary layer in which clouds exist.