The atmosphere is the space around the Earth which is filled by a mixture of gasses held against the Earth by the force of gravity. This mixture of gasses we call air.
Because the Earth spins on its axis, and because the surface temperature is greater at the equator than at the poles, the atmosphere extends further out into space at the equator than at the poles. Air Density decreases with increasing altitude.
The boundary between the troposphere and the stratosphere is known as the Tropopause and is covered by a separate article.
Most light aircraft and turboprop aircraft fly within the troposphere and this is where most of the water vapour and therefore cloud formation exists. Many types of jet aircraft are able to cruise in the Stratosphere, especially in high latitudes where the Tropopause is lower, thereby avoiding almost all weather - although some particular vigorous thunderstorms may penetrate the stratosphere.
Temperature falls with height in the troposphere but is generally constant at about -57°C in the stratosphere.
Considerable vertical movement of air occurs in the troposphere - warm air rising and cool air descending. This vertical movement is a direct consequence of Solar Heating.