On long flights, airplanes generally travel somewhere near the border of the

On long flights, airplanes generally travel somewhere near the border of the stratosphere and mesosphere.
The stratosphere is very dry, with little watervapor and few clouds.
One benefit of the stratosphere is that it shows very little temperature differences and the air density is very low, which means that jets and/or planes burn a lot less fuel resulting in further travel ranges. 

Because some people pollute water ans waste it