UPSC Civil Services 2011 Prelims General Studies Paper I

The idea is to optimize fuel burn reason is the low air density that reduces parasitic drag on the airframe. It also allows them to stay above any hard weather (extreme turbulence). Air flow in the stratosphere is much lesser turbulent than in the troposphere and this is the reason that jet aircraft like to cruise at stratospheric altitudes for, the flight is less “bumpy.”
So statement 2 is correct. Majority of clouds form in the Earth’s troposphere, but there are occasions where clouds in the stratosphere and mesosphere are formed and have been observed. The statement 1 which says that There are no clouds or water vapour in the lower stratosphere, is not a correct statement, to the extent that it does not explain the flight of jets in lower stratosphere.