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 was still increasing. The most commonly quoted producer price for natural gas is the Louisiana-based Henry Hub price, which is futures-traded on NYMEX. A barrel of oil releases about 5.8 million BTU when burned, so that 5.8 MCF of gas (at the standard one thousand BTU per cubic foot) releases about the same energy as a barrel of oil. Sometimes gas containing 5.8 million BTU is defined as a "barrel of oil equivalent for energy calculation purposes. When describing reserves or production, however, the oil and gas industry more commonly uses the rounded number of 6 MCF of gas (or