The stratosphere is the second layer of Earth’s atmosphere, just above the troposphere. It extends from about 10 kilometers (6 miles) to 50 kilometers (31 miles) above Earth’s surface. The stratosphere is mostly free of clouds and other weather phenomena. The temperature in the stratosphere increases with height until it reaches a maximum at the stratopause, which marks the top of this layer.
The stratosphere contains most of Earth’s ozone, which absorbs ultraviolet radiation from the Sun and protects life on Earth from its harmful effects. Ozone levels are highest in the lower stratosphere and decrease higher up. This layer also contains very little water vapor, so it doesn’t experience much weathering.
Winds in the stratosphere are generally very weak and variable. However, there is a region near the equator called the jet stream where winds can reach speeds of over 100 kilometers per hour (60 miles per hour). The jet stream occurs where cold air from the poles meets warm air from the tropics.
Aircraft fly in the lower part of the stratosphere because it has less turbulence than lower layers and because it contains more oxygen for engines to burn. However, flying too high in the stratosphere can be dangerous because there is not enough oxygen for humans to breathe safely.
The upper limit of human habitability is around 7 kilometers (4 miles), just below what is known as Armstrong’s Line, named after astronaut Neil Armstrong who was one of first people to experience symptoms while flying at this altitude during a test mission in 1962.”