- noun a mixture of gases forming the Earth’s atmosphere, which cannot be seen
- The mixture of gases that comprise the earth's atmosphere. Its composition varies depending on various factors, such as altitude, water vapor present, and contaminants at the site where the sample is taken. The main components of dry air at sea level, and their approximate proportion by volume, are: nitrogen (78%), oxygen (21%), and argon (0.9%). Other components include carbon dioxide, neon, helium, methane, and krypton.
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