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 gas pipelines. The North American market, consisting of Canada, Mexico, and the United States, all connected by a common pipeline network, has had much lower gas prices in recent years than some other major world gas markets, such as Europe (since 2010), Japan (since 2008), and Korea. The United States is connected by pipeline to Canada and Mexico. The US has long imported large quantities of gas from Canada, and exported smaller quantities to some parts of eastern Canada. In 2014, the US imported 2,634 BCF from Canada, and exported 769 BCF, so that net imports from Canada totaled 1,865