Akademik

  layer of gases surrounding earth and held there by gravity. Nitrogen forms 78.09% by volume and Oxygen 20.95%. The remaining 0.96% is made up of a further 19 gases. The boundary of the atmosphere is 1000km above sea level but 99% of the gases are held in the lower 40km. This is split into three layers

  the troposphere and the stratosphere separated by the tropopause. Carbon dioxide in the troposphere allows photosynthesis and also holds long-wave radiation to give warmth. Temperatures fall with altitude at a rate of approximately 6.5°C per km to the tropopause where they are stable. Ozone in the stratosphere traps ultraviolet radiation and leads to an increase in temperature in this layer.