The homosphere is the layer of Earth's atmosphere where chemical constituents are well-mixed. The main reason for this well-mixed condition is that the mean free path between molecules is small compared to the scale of bulk motions of the air. Thus, gases do not separate according to their molecular weight, as bulk air motions keep mixing the atmosphere.
The homosphere begins at the Earth's surface and extends to an altitude of about 80 km. The homosphere is composed of the troposphere, the stratosphere, and the mesosphere. Above the homosphere is the heterosphere, which is stratified according to the molecular weight of its constituent gases.