Tropopause

The tropopause is a boundary region in Earth's atmosphere between the troposphere and the stratosphere. Here the air ceases to cool at -56.5°C (-70°F), and the air becomes almost completely dry.

The tropopause is between the troposphere and the stratosphere. The layers are not to scale.

Most commercial aircraft are flown in the tropopause. To put it into even simpler words, the tropopause is the interferance between the troposphere and the stratosphere.