Answers:
1. It is because each layer has different characteristics, and it has also different altitude and density that make it separated.
2. The temperature increases in the Stratosphere because of the high concentration of ozone in the Stratosphere. Ozone absorbs ultraviolet radiation from the sun and converts the energy into heat, causing air temperature increase.
3. Airplanes cannot fly in the Mesosphere because air pressure and density in the Mesosphere are extremely low.
4. The temperature in the Thermosphere depends on how the oxygen absorbs ultraviolet radiation from the sun.