The troposphere is the atmospheric layer of life, with 80% of the atmosphere’s mass, and 99% of its water vapor and aerosols. The troposphere averages 17 km, though its height variation is considerable. The tropic troposphere reaches 20 km. Near the poles in summer, one can go troppo for only 7 km up. In winter, the polar troposphere becomes indistinct.
The composition of the lower atmosphere is dynamically defined by the lithosphere – most dramatically by volcanoes, but most continuously by dust; most subtlety and constantly by evaporation from the hydrosphere; and constitutionally, by the most powerful of biota: plants, as well as by recently-descended hominids.
Plants have long poured their exhausts into the atmosphere, to the ultimate beneficence of all other life. Much more recently, humans have done the same, to quite the opposite effect.
The temperature of the troposphere drops with altitude, as does water vapor saturation, because saturation vapor pressure weakens with chill. Atmospheric pressure also decreases with altitude. The turbulent and thick portion of the troposphere is near the surface.