Excluding the exosphere, the atmosphere has four primary layers, which are the troposphere, stratosphere, mesosphere, and thermosphere.The exosphere is the outermost layer of Earth's atmosphere (i.e. It extends from the exobase, which is located at the top of the thermosphere at an altitude of about 700 km above sea level, to about 10,000 km (6,200 mi; 33,000,000 ft) where it merges into the solar wind.Because the general pattern of the temperature/altitude profile is constant and measurable by means of instrumented balloon soundings, the temperature behavior provides a useful metric to distinguish atmospheric layers.In this way, Earth's atmosphere can be divided (called atmospheric stratification) into five main layers.Filtered air includes trace amounts of many other chemical compounds.Many substances of natural origin may be present in locally and seasonally variable small amounts as aerosols in an unfiltered air sample, including dust of mineral and organic composition, pollen and spores, sea spray, and volcanic ash.Atmospheric effects become noticeable during atmospheric reentry of spacecraft at an altitude of around 120 km (75 mi).
Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire 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 atoms and molecules are so far apart that they can travel hundreds of kilometers without colliding with one another.
In general, air pressure and density decrease with altitude in the atmosphere.
However, temperature has a more complicated profile with altitude, and may remain relatively constant or even increase with altitude in some regions (see the temperature section, below).