<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> <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>

The layer that comprises approximately 80 of earth by volume is the