Published online by Cambridge University Press: 19 June 2019
The Earth’s atmosphere is composed mostly of molecular nitrogen (N2, 78 % of dry air) and molecular oxygen (O2, 21 % of dry air). It holds also a fair amount of water vapor (H2O), which varies greatly in concentration (ranging from negligible in dry regions to a few % in humid regions) and leads to the formation of clouds and fogs in case of supersaturation. The Earth’s atmosphere also contains carbon dioxide (CO2), which has an average concentration of about 0.04 %. H2O and CO2 are gases that absorb infrared (IR) radiation, but let ultraviolet (UV) and visible solar radiation go through. Since they partially absorb IR radiation emitted by the Earth toward space, these species are called “greenhouse gases” (GHG).
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