Reasons of efficiency.
not the least of which is that the pressure generated in the combustion chamber has to fight less "back pressure" (think of it as pressure resistance) at high altitude because the air-pressure is less.
Jet planes fly in the stratosphere, which is the second layer of Earth's atmosphere. This layer provides a stable environment for jet propulsion and allows for efficient flying at high altitudes.
Jet streams can occur at various altitudes, but the strongest and most well-known jet streams, like the polar jet stream and subtropical jet stream, typically occur at altitudes between 20,000 to 40,000 feet in the upper troposphere. These high-altitude, fast-flowing air currents play a significant role in influencing weather patterns.
The jet stream is located in the tropopause, which is the boundary between the troposphere and the stratosphere. It is a fast-flowing, narrow air current that moves from west to east at high altitudes.
The fact that jet airplanes' contrails form cirrus clouds suggests that they fly at altitudes where the temperature and humidity are conducive to contrail formation, typically above 20,000 feet. This indicates that jet airplanes generally fly at high altitudes, where the air is colder and less dense, allowing contrails to persist and spread out into cirrus clouds.
Planes fly at high altitudes because the thinner air reduces air resistance, allowing the aircraft to fly more efficiently. Flying at high altitudes also provides smoother air currents, less turbulence, and shorter flight times due to the ability to take advantage of jet streams. Additionally, being at a higher altitude allows planes to avoid obstacles on the ground and have more flexibility in choosing their flight path.
For thrust. Engines are used to overcome the drag of the airplane.
Jet powered aircraft have a number of advantages over conventional internal combustion propeller driven aircraft. To start with, jet engines can operate in a much wider range of conditions. Propellers lose their effectiveness at high altitudes and at high speeds while jet engines are capable of operating at much higher altitudes and a much much higher speeds with little or no loss of power. Jet engines have fewer moving parts than internal combustion engines which makes them easier and simpler to design, build, maintain and repair. Jet engines run on fuel that is very similar to diesel fuel, and is much safer to handle and store than gasoline. Jet engines can produce much more thrust power for their weight than conventional reciprocating engines do.
Jet airplanes fly at high altitudes because it is more fuel efficient. Flying at higher altitudes uses less fuel than flying at lower altitudes.
Jet streams
Jet fuel is injected into high pressure air in jet engines.
Most planes use a type of aviation fuel known as jet fuel. Jet fuel is a specialized type of fuel designed for use in jet engines, which power most commercial airplanes. It is a type of kerosene that is refined for its high energy content and stability at high altitudes.
jet stream
Jet stream
On the standard version the Blackbird has its j58 engines on its wingtips and if you are referring to an older model the j58's are of lower quality but still located in the same place.on the other side of the jet engines there is the outer wing. that means that the j58's have extra stability while at high altitudes.
Polar jet streams
jet streams
It is condensated air from engines, due to extremely low temperatures at high altitudes the hot jet stream comming out of turbines get transformed in a white cloud. shivering for several mkinutes