126. The jet aircrafts fly very easily and smoothly in the lower stratosphere. What could be the appropriate explanation? (2011)

1. There are no clouds or water vapour in the lower stratosphere. 
2. There are no vertical winds in the lower stratosphere. 

Which of the statements given above is/are correct in this context? 

(a) 1 only 

(b) 2 only 

(c) Both 1 and 2 

(d) Neither 1 nor 2 

Answer: (c) 

Statement 1 is correct: The amount of water vapour in the stratosphere is very low, only 5 out of one million air molecules are water molecules. This means that under normal conditions there are no clouds in the stratosphere. Clouds are found almost exclusively in the troposphere.

Statement 2 is correct: As one moves upward through the stratosphere Temperatures rise opposite of the behaviour in the troposphere in which we live, where temperatures drop with increasing altitude. Because of this temperature stratification, there is little convection and mixing in the stratosphere, so the layers of air there are quite stable. Jet aircraft fly in the lower stratosphere to avoid the turbulence which is common in the troposphere below.