Why do planes fly in the stratosphere?
Beginning from six to 11 miles above the earth is a layer of the atmosphere called the stratosphere.
In this layer the air temperatures, pressures and humidity are all much lower than in the troposphere
, which lies below it. In the colder seasons and at higher latitudes around the globe where jet aircraft can reach the stratosphere, operators and passengers benefit from the favorable conditions.
The type of turbulence caused by convective heating, the same heating that helps cause thuderstorms, does not occur in the stratosphere.
Jet engines burn fuel more efficiently at the lower temperatures in the stratosphere.
Airframe icing, a danger to aviation, occurs only below the stratosphere
and although thunderstorms may occasionally punch up into the stratosphere, they are easier to avoid and do not result in turbulence outside of their boundaries.
Aircraft move easier through the stratosphere than at lower altitudes because of the reduced friction and drag associated with lower atmospheric density.
Especially on the higher side of the tropopause (the transition area between the troposphere and stratosphere) upper level winds are at their strongest, helping aircraft to their destination faster when it is in the right direction.
Because sound travels poorly through lower density fluids and is disrupted by the sharp temperature changes at the tropopause, the noise of jet engines reaches the earth at a lower and almost imperceptible level.
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