Tropopause
upper boundary of troposphere
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.
![](http://up.wiki.x.io/wikipedia/commons/thumb/7/78/Earth_Atmosphere.svg/200px-Earth_Atmosphere.svg.png)
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.