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
Shale gas in the United States had all been vertical until 2008, when TXCO drilled and completed a number of horizontal wells. The upper Devonian shales of the Appalachian Basin, which are known by different names in different areas have produced gas since the early 20th century. The main producing area straddles the state lines of Virginia, West Virginia, and Kentucky, but extends through central Ohio and along Lake Erie into the panhandle of Pennsylvania. More than 20,000 wells produce gas from Devonian shales in the basin. The wells are commonly 3,000 to deep. The shale most commonly produced is the Chattanooga Shale, also called the