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The gaseous envelope surrounding the Earth. The dry atmosphere consists almost entirely of Nitrogen (78.1% volume mixing ratio) and Oxygen (20.9% volume mixing ratio), together with a number of trace gases, such as Argon (0.93% volume mixing ratio), Helium, radiatively active greenhouse gases such as Carbon dioxide (0.035% volume mixing ratio), and Ozone. In addition, the atmosphere contains water vapor, whose amount is highly variable but typically 1% volume mixing ratio. The atmosphere also contains clouds and aerosols.