The Earth's atmosphere differs in density and composition as the altitude increases above the surface. The lowest part of the atmosphereis called the trosposphere and extends from the surface to 10 km .
All mountains are within the troposhpere and all of our daily activities occur here as well.
Above the troposphere is the stratosphere, which extends from 10 km to about 50 km above the Earth's surface. The gas is still dense enough for hot air balloons to ascend 15-20 km. It is within the stratophere that incoming solar radiation breaks up the molecular O 2 into separate individual oxygen atoms. At heights above 80 km, the gas is so thin that free electrons can exist for short periods of time before they are captured by a positive ion. The location of these charged particles indicates the beginning of the ionosphere.