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By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases.
Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere.
The three major constituents of air, and therefore of Earth's atmosphere, are nitrogen, oxygen, and argon.
Water vapor accounts for roughly 0.25% of the atmosphere by mass.
Layers drawn to scale, objects within the layers are not to scale.
Aurorae shown here at the bottom of the thermosphere can actually form at any altitude in this atmospheric layer.
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.
However, the aurora borealis and aurora australis sometimes occur in the lower part of the exosphere, where they overlap into the thermosphere.
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.
It extends from the mesopause (which separates it from the mesosphere) at an altitude of about 80 km (50 mi; 260,000 ft) up to the thermopause at an altitude range of 500–1000 km (310–620 mi; 1,600,000–3,300,000 ft).
The height of the thermopause varies considerably due to changes in solar activity.
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.