Stratosphere

The video tutorial explains the stratosphere, the second layer of Earth's atmosphere, located above the troposphere and below the mesosphere. It extends from about 10 to 50 kilometers in altitude and is slightly lower at the poles. Unlike other atmospheric layers, the stratosphere's temperature increases with height. This layer is significant for aviation as airliners cruise at its lower edge, where the thin, cold air enhances engine efficiency.

Why do airliners cruise in the lower edge of the stratosphere?

What is the characteristic of the air in the stratosphere that aids engine efficiency?