Most planes fly at an altitude of around 10–12 Km. At this height the conditions are optimal: The air is dense enough to generate sufficient lift, as lift is generated due to difference in air pressure between the top and the bottom of the wing. read more
Jet planes fly through the troposphere to reach the stratosphere, where planes fly more smoothly and experience less weather interference. The height of the troposphere varies with altitude and season. The stratosphere is dry, with little water vapor. Therefore, a small amount of clouds are found in this layer compared to the troposphere. read more
This layer contains 75% of our atmosphere's mass and 99% of atmospheric water vapor. It is also the layer that contains all of our weather. To avoid turbulence and bad weather, larger planes will sometimes fly in the upper boundary of the Troposphere, called the Tropopause, or even into the lower boundary of the next layer, called the Stratosphere. read more