From Glossary of Meteorology


Mixture of gases forming Earth's atmosphere, consisting of nitrogen (∼78%), oxygen (∼21%), water vapor, and other trace gases such as carbon dioxide, helium, argon, ozone, or various pollutants.

The concentration of water vapor is very variable, being a strong function of temperature and, hence, altitude in the atmosphere. Dry air is referred to as air from which measurable amounts of water vapor have been physically removed. Pure, dry air has a density of 1.293 kg m−3 at a temperature of 273 K and a pressure of 101.325 kPa. Apart from the variability of water vapor, the composition of air is essentially constant to an altitude of at least 50 km and is presently approximated as follows.

revised air table

The concentration of ozone is variable, between approximately 10 and 0.01 ppm. Carbon dioxide, methane, and nitrous oxide have all been increasing since the beginning of the industrial age. Carbon dioxide and methane concentrations vary by season and location.

Term revised 26 June 2017.

Copyright 2022 American Meteorological Society (AMS). For permission to reuse any portion of this work, please contact [email protected]. Any use of material in this work that is determined to be “fair use” under Section 107 of the U.S. Copyright Act (17 U.S. Code § 107) or that satisfies the conditions specified in Section 108 of the U.S.Copyright Act (17 USC § 108) does not require AMS’s permission. Republication, systematic reproduction, posting in electronic form, such as on a website or in a searchable database, or other uses of this material, except as exempted by the above statement, require written permission or a license from AMS. Additional details are provided in the AMS Copyright Policy statement.