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 barrels per day in 2014. The US has been the world's leading producer of natural gas liquids since 2010, when US NGL production passed that of Saudi Arabia. Although the United States leads the world in natural gas production, it is only fifth in proved reserves of natural gas, behind Russia, Iran, Qatar, and Turkmenistan. The United States oil and gas industry is often informally divided into "upstream" (exploration and production), "midstream" (transportation and refining), and "downstream" (distribution and marketing). Petroleum and natural gas share a common upstream (exploration and production) sector, but the midstream and downstream sectors are largely