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.