The Earth is shrouded in a thin veil of gas called the atmosphere. At sea level it provides the air we breathe, the wind and the weather, but with increasing height, it becomes more and more rarefied, slowly blending into the virtual vacuum of space. The atmosphere has no easily defined top; technically the height at which space shuttles orbit is within the atmosphere. Our atmosphere has evolved so that there is both carbon dioxide and oxygen held in balance by photosynthesis and respiration. This combination supports life on Earth, making it unique amongst the planets of the Solar System.
Above about 450 Km any gas molecules are on their way out into space, so this is called the exosphere. Below that, down to about 80 Km
The mesosphere down to 50 Km, is too low to be warmed by direct radiation and too thin to be warmed b y convection. It includes most of the ionosphere, consisting of variable layers of atoms with one or more electrons stripped off to give them and electrical charge. Certain parts of the ionosphere can reflect shortwave radio signals and filter out X rays from space.
From about 15-50 Km above us is the stratosphere, at temperatures below 0 degree C. violent volcanic eruptions can propel gas and dust to this level, where they slowly spread around the globe. But generally, the stratosphere remains horizontally stratified, with little vertical mixing. This layer screens out most solar ultraviolet radiation.
The bottom 15Km of the atmosphere, the layer in which weather condition occur, is the troposphere. It is warmed by radiation from the Earth and is therefore normally warmed from the bottom. The boundary with the stratosphere varies from about 10 Km at the poles to 20 Km at the equator.