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Why do modern jet airliners fly above the clouds?

Clouds and rain form only where there is water vapour, and water vapour forms only in the lowest layer of the atmosphere called the troposphere.

To avoid a rough flight caused by bad weather, modern jet airliners fly above the clouds, in the next layer of the atmosphere called the stratosphere.

The air in the stratosphere has no water vapour, and so it is still and clear. This means no bumps, so that you have a comfortable flight.

It is also somehow related to efficiency and speed. The turbine engines used on modern passenger jets are at their most efficient at higher altitudes because the air is thinner and generates less drag so a turbine aircraft will burn a lot less fuel and/or go much faster at higher altitudes.