The layer of the Earth’s atmosphere where most planes fly is the TROPOSPHERE.

The TROPOSPHERE is the layer of the atmosphere closest to the Earth. It extends approximately 7 miles, or 11 km above sea level, but is thicker at the equator and thinner at the poles. 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.