Atmosphere of Earth the border between the atmosphere and outer space. Atmospheric effects become noticeable during atmospheric reentry of spacecraft at an altitude of around . Several layers can be distinguished in the atmosphere, based on characteristics such as temperature and composition. The study of Earth's atmosphere and its processes is called atmospheric science (aerology). Early pioneers in the field include Léon Teisserenc de Bort and Richard Assmann. The three major constituents of Earth's atmosphere are nitrogen, oxygen, and argon. Water vapor accounts for roughly 0.25% of the atmosphere by mass. The concentration of water vapor (a greenhouse gas) varies significantly from around
Natural gas in the United States 25.7 TCF in 2014, an increase of 21 percent. Remaining proved reserves increased overall, from 301 TCF in 2013 to 338 TCF in 2013 (the last year for which reserves are available), an increase of 11 percent. The rise in gas production despite fewer rigs drilling has been explained by the greater efficiency in drilling, and the greater productivity of shale gas wells. Top producing gas fields in the United States, 2013 Most oil fields produce some gas, and vice versa, but the ratio of oil and gas varies considerably. Some gas fields, such as coalbed methane wells, and some