than other wavelengths, giving the Earth a blue halo when seen from space]] The Earth's atmosphere (or air) is a layer of gases surrounding the planet Earth that is retained by the Earth's gravity. Dry air contains roughly (by volume) 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% carbon dioxide, and trace amounts of other gases. Air also contains a variable amount of water vapor, on average around 1%. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night. There is no definite boundary between the atmosphere and outer space. It slowly becomes thinner and fades into space. An altitude of marks the boundary where atmospheric effects become noticeable during reentry. The Kármán line, at , is also frequently regarded as the boundary between atmosphere and outer space. Th

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