<P> The relative concentration of gasses remains constant until about 10,000 m (33,000 ft). </P> <P> In general, air pressure and density decrease with altitude in the atmosphere . However, temperature has a more complicated profile with altitude, and may remain relatively constant or even increase with altitude in some regions (see the temperature section, below). Because the general pattern of the temperature / altitude profile is constant and measurable by means of instrumented balloon soundings, the temperature behavior provides a useful metric to distinguish atmospheric layers . In this way, Earth's atmosphere can be divided (called atmospheric stratification) into five main layers . Excluding the exosphere, the atmosphere has four primary layers, which are the troposphere, stratosphere, mesosphere, and thermosphere . From highest to lowest, the five main layers are: </P> <Ul> <Li> Exosphere: 700 to 10,000 km (440 to 6,200 miles) </Li> <Li> Thermosphere: 80 to 700 km (50 to 440 miles) </Li> <Li> Mesosphere: 50 to 80 km (31 to 50 miles) </Li> <Li> Stratosphere: 12 to 50 km (7 to 31 miles) </Li> <Li> Troposphere: 0 to 12 km (0 to 7 miles) </Li> </Ul> <Li> Exosphere: 700 to 10,000 km (440 to 6,200 miles) </Li>

What is the top layer of earth's atmosphere