Atmospheric Gases The atmosphere's constituent gases
The atmosphere is composed of many different gases. These can be divided into two categories: permanent gases which have a constant concentraion,evenly mixed throughout the atmosphere; variable gases who's concentration varies in space and time. The table following shows the different constituents of the atmosphere (dry air).

Composition of the atmosphere below 25km
Component Symbol Volume % (dry air) Molecular weight
Permanent Gases
Nitrogen N2 78.08 28.02
Oxygen O2 20.98 32.00
Argon Ar 0.93 39.88
Neon Ne 0.0018 20.18
Helium He 0.0005 4.00
Hydrogen H2 0.00005 2.02
Krypton Kr 0.0011 83.80
Xenon Xe 0.00009 131.30
Variable Gases
Carbon Dioxide CO2 Variable 44.00
Ozone O3 Variable 48
Methane CH4 Variable 16
Sulphur Dioxide SO2 Variable 64.06
Water Vapour H2O Variable 18

In reality, however, the atmosphere contains a large amount of water vapour, resulting from evaporation from surface water and transpiration by plants. Near the surface, this is typically about 4% (by volume) but only 3-6 parts per million (ppm) above 10 km.