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Air content and atmospheric pressure vary at different layers, and air suitable for use in photosynthesis by terrestrial plants and breathing of terrestrial animals is found only in Earth's troposphere and in artificial atmospheres.The atmosphere has a mass of about 5.15 three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface.The exosphere contains most of the satellites orbiting Earth.The thermosphere is the second-highest layer of Earth's atmosphere.This layer is mainly composed of extremely low densities of hydrogen, helium and several heavier molecules including nitrogen, oxygen and carbon dioxide closer to the exobase.
The height of the thermopause varies considerably due to changes in solar activity.
The study of Earth's atmosphere and its processes is called atmospheric science (aerology).
Early pioneers in the field include Léon Teisserenc de Bort and Richard Assmann.
Atmospheric effects become noticeable during atmospheric reentry of spacecraft at an altitude of around 120 km (75 mi).
Several layers can be distinguished in the atmosphere, based on characteristics such as temperature and composition.
In general, air pressure and density decrease with altitude in the atmosphere.