<P> If the entire mass of the atmosphere had a uniform density from sea level, it would terminate abruptly at an altitude of 8.50 km (27,900 ft). It actually decreases exponentially with altitude, dropping by half every 5.6 km (18,000 ft) or by a factor of 1 / e every 7.64 km (25,100 ft), the average scale height of the atmosphere below 70 km (43 mi; 230,000 ft). However, the atmosphere is more accurately modeled with a customized equation for each layer that takes gradients of temperature, molecular composition, solar radiation and gravity into account . </P> <P> In summary, the mass of Earth's atmosphere is distributed approximately as follows: </P> <Ul> <Li> 50% is below 5.6 km (18,000 ft). </Li> <Li> 90% is below 16 km (52,000 ft). </Li> <Li> 99.99997% is below 100 km (62 mi; 330,000 ft), the Kármán line . By international convention, this marks the beginning of space where human travelers are considered astronauts . </Li> </Ul> <Li> 50% is below 5.6 km (18,000 ft). </Li>

What are the three layers of the earth's atmosphere