<P> The atmosphere has a mass of about 5.15 × 10 kg, three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface . The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space . The Kármán line, at 100 km (62 mi), or 1.57% of Earth's radius, is often used as the border between the atmosphere and outer space . Atmospheric effects become noticeable during atmospheric reentry of spacecraft at an altitude of around 120 km (75 mi). Several layers can be distinguished in the atmosphere, based on characteristics such as temperature and composition . </P> <P> 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 . </P> <P> The three major constituents of air, and therefore 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 10 ppm by volume in the coldest portions of the atmosphere to as much as 5% by volume in hot, humid air masses, and concentrations of other atmospheric gases are typically quoted in terms of dry air (without water vapor). The remaining gases are often referred to as trace gases, among which are the greenhouse gases, principally carbon dioxide, methane, nitrous oxide, and ozone . Filtered air includes trace amounts of many other chemical compounds . Many substances of natural origin may be present in locally and seasonally variable small amounts as aerosols in an unfiltered air sample, including dust of mineral and organic composition, pollen and spores, sea spray, and volcanic ash . Various industrial pollutants also may be present as gases or aerosols, such as chlorine (elemental or in compounds), fluorine compounds and elemental mercury vapor . Sulfur compounds such as hydrogen sulfide and sulfur dioxide (SO) may be derived from natural sources or from industrial air pollution . </P> <Table> Major constituents of dry air, by volume <Tr> <Th_colspan="2"> Gas </Th> <Th_colspan="2"> Volume </Th> </Tr> <Tr> <Th> Name </Th> <Th> Formula </Th> <Th> in ppmv </Th> <Th> in% </Th> </Tr> <Tr> <Td> Nitrogen </Td> <Td> </Td> <Td> 780,840 </Td> <Td> 78.084 </Td> </Tr> <Tr> <Td> Oxygen </Td> <Td> O </Td> <Td> 209,460 </Td> <Td> 20.946 </Td> </Tr> <Tr> <Td> Argon </Td> <Td> Ar </Td> <Td> 9,340 </Td> <Td> 0.9340 </Td> </Tr> <Tr> <Td> Carbon dioxide </Td> <Td> CO </Td> <Td> 400 </Td> <Td> 0.04 </Td> </Tr> <Tr> <Td> Neon </Td> <Td> Ne </Td> <Td> 18.18 </Td> <Td> 0.001818 </Td> </Tr> <Tr> <Td> Helium </Td> <Td> He </Td> <Td> 5.24 </Td> <Td> 0.000524 </Td> </Tr> <Tr> <Td> Methane </Td> <Td> CH </Td> <Td> 1.79 </Td> <Td> 0.000179 </Td> </Tr> <Tr> <Th_colspan="4"> Not included in above dry atmosphere: </Th> </Tr> <Tr> <Td> Water vapor </Td> <Td> H O </Td> <Td> 10--50,000 </Td> <Td> 0.001%--5% </Td> </Tr> <Tr> <Th_colspan="4"> notes: <P> volume fraction is equal to mole fraction for ideal gas only, also see volume (thermodynamics) ppmv: parts per million by volume Water vapor is about 0.25% by mass over full atmosphere Water vapor strongly varies locally </P> </Th> </Tr> </Table>

What are the three main parts of the atmosphere