The troposphere is the lowest atmospheric layer. All human activity takes place here. Everyday weather phenomena, such as clouds and storms, mainly happen in the troposphere. Here temperature decreases with increasing elevation.

The troposphere is thickest in the equatorial and tropical regions, where it stretches from sea level to about 16 km. It thins toward the poles, where it is only about 6 km thick.

The troposphere contains significant amounts of water vapor. When the water vapor content is high, vapor can condense into water droplets, forming low clouds and fog, or the vapor can be deposited as ice crystals, forming high clouds. Rain, snow, hail, or sleet— collectively termed precipitation—are produced when these condensation or deposition processes happen rapidly. Places where water vapor content is high throughout the year have moist climates. In desert regions water vapor is low, so there is little precipitation. When water vapor absorbs and reradiates heat emitted by the Earth’s surface, it helps to create the greenhouse effect—a natural phenomenon that is responsible for warming the Earth to temperatures that allow life to exist.

The troposphere contains countless tiny particles that are so small and light that the slightest movements of the air keep them aloft. These are called aerosols. They are swept into the air from dry desert plains, lakebeds, and beaches, and they are released by exploding volcanoes. Oceans are also a source of aerosols. Strong winds blowing over the ocean lift droplets of spray into the air. These droplets of spray lose most of their moisture by evaporation, leaving tiny particles of watery salt that are carried high into the air. Forest fires and brushfires also generate particles of soot as smoke. And as meteors vaporize as they hit the atmosphere, they leave behind dust particles in the upper layers of air. Closer to the ground, industrial processes that incompletely burn coal or fuel oil release aerosols into the air as well. Aerosols are important because water vapor can condense on them to form tiny droplets. When these droplets grow large and occur in high concentration, they are visible as clouds or fog. Aerosol particles scatter sunlight, brightening the whole sky while slightly reducing the intensity of the solar beam.

The troposphere gives way to the stratosphere at the tropopause. Here, temperatures stop decreasing with altitude and start to increase. The altitude of the tropopause varies somewhat with season, so the troposphere is not uniformly thick at any location.