First, lets check out the layers of the atmosphere:.
1. Troposphere– It’s the part of the atmosphere where we live in. Most of the weather (cloud, snow, rain etc) is found here. Troposphere extends upto 18kms from the surface of the ground.
2. Stratosphere – It extends from the end of the troposphere to about 50kms, upto the ozone. It’s here where most of the aircraft’s fly.
3. Mesosphere– It extends upward to a height of about 85 km (53 miles) above our planet. Most meteors burn up in the mesosphere.
4. Thermosphere: The layer of very rare air above the mesosphere is called the thermosphere.
5. Exosphere: Its the layer above the thermosphere extending until space.
Now lets break it down into points, so that you understand why airplanes fly at high altitudes!!.
1. Increased fuel efficiency at high altitudes.
As we know, as the height increases, density of air decreases. In simple terms, air becomes lesser and lesser as higher we go. In a jet engine, a stoichiometric ratio has to be maintained between the fuel and air mixture. As the molecules of air is lesser at a high altitude, in-order to maintain this ratio, the fuel molecules needed will also be less. This results in reduction of fuel used for cruise.
2. Lack of weather
Nobody likes a bumpy flight. Most of the airlines fly within the first 10-15kms of the stratosphere. As we know, stratosphere is void of weather phenomenons, airplanes get to stay away from all the clouds, rains etc. Further it also saves time and fuel, as the pilots do not have to go around diverting avoiding weather.
3. More time
Incase of an emergency, the pilots have more time to respond and take an action. For example, at 35000ft if an engine fails, the pilot has more time and also the aircraft has more height to glide and hence covering further distance.
4. Reduction in drag
As discussed earlier, due to decrease in air molecules, resistance on the aircraft airframe is reduced. This causes further improvement of performance of the aircraft overall.
5. Free from bird activity.
There are only a few birds that fly above 30000 ft, and those that do are hardly to be found. So freedom from bird activity means alot to the pilots.
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