"The stratosphere is a layer of Earth's atmosphere that is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler higher up and warmer farther down. The stratosphere is situated between about 17 km and 50 km altitude above the surface at moderate latitudes, while at the poles it starts at about 8 km altitude. The stratosphere sits directly above the troposphere.
Commercial airliners typically cruise at an altitude near 10 km in temperate latitudes, in the lower reaches of the stratosphere. This is to avoid atmospheric turbulence from the convection in the troposphere." (Wikipedia)
The stratosphere is a very dry layer with practically no clouds which allows jet passengers to have a spectacular and fantastic sight.

The flight from Barcelona to Munich lasts roundabout two and a half hours and follows the route along the shores of the Mediterranian Sea to Marseille, then heads north to Lake Geneva, crosses the Alpes and descends slowly over the rural region of southern Bavaria.