The lowest layer of Earth’s atmosphere, containing most weather and water vapor, is the troposphere.
The troposphere begins at Earth’s surface and extends upward to the tropopause. Its thickness varies with latitude and season, reaching higher altitudes in the tropics than near the poles. It contains about 80% of the atmosphere’s total mass and roughly 99% of its water vapor and aerosols.
Weather develops there because the troposphere contains the ingredients and motion needed for clouds, rain, snow, storms, and winds. Air generally becomes colder with increasing altitude, encouraging convection: warm air rises, expands, and cools, while denser cool air sinks. Water vapor condenses as air cools, releasing latent heat that helps power thunderstorms and tropical cyclones.
The stratosphere lies immediately above the troposphere and contains most of the ozone layer, but comparatively little water vapor. The mesosphere and thermosphere are still higher and are far too thin to host ordinary weather. A useful distinction is that the troposphere contains most atmospheric water vapor, not all of Earth’s water, which is overwhelmingly stored in oceans, ice, and groundwater.