<P> This layer is completely cloudless and free of water vapor . However, non-hydrometeorological phenomena such as the aurora borealis and aurora australis are occasionally seen in the thermosphere . The International Space Station orbits in this layer, between 350 and 420 km (220 and 260 mi). </P> <P> The mesosphere is the third highest layer of Earth's atmosphere, occupying the region above the stratosphere and below the thermosphere . It extends from the stratopause at an altitude of about 50 km (31 mi; 160,000 ft) to the mesopause at 80--85 km (50--53 mi; 260,000--280,000 ft) above sea level . </P> <P> Temperatures drop with increasing altitude to the mesopause that marks the top of this middle layer of the atmosphere . It is the coldest place on Earth and has an average temperature around − 85 ° C (− 120 ° F; 190 K). </P> <P> Just below the mesopause, the air is so cold that even the very scarce water vapor at this altitude can be sublimated into polar - mesospheric noctilucent clouds . These are the highest clouds in the atmosphere and may be visible to the naked eye if sunlight reflects off them about an hour or two after sunset or a similar length of time before sunrise . They are most readily visible when the Sun is around 4 to 16 degrees below the horizon . A type of lightning referred to as either sprites or ELVES occasionally forms far above tropospheric thunderclouds . The mesosphere is also the layer where most meteors burn up upon atmospheric entrance . It is too high above Earth to be accessible to jet - powered aircraft and balloons, and too low to permit orbital spacecraft . The mesosphere is mainly accessed by sounding rockets and rocket - powered aircraft . </P>

Which layer of atmosphere has the highest density of gas molecules