<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> <P> By comparison, the summit of Mt . Everest is at 8,848 m (29,029 ft); commercial airliners typically cruise between 10 km (33,000 ft) and 13 km (43,000 ft) where the thinner air improves fuel economy; weather balloons reach 30.4 km (100,000 ft) and above; and the highest X-15 flight in 1963 reached 108.0 km (354,300 ft). </P> <P> Even above the Kármán line, significant atmospheric effects such as auroras still occur . Meteors begin to glow in this region, though the larger ones may not burn up until they penetrate more deeply . The various layers of Earth's ionosphere, important to HF radio propagation, begin below 100 km and extend beyond 500 km . By comparison, the International Space Station and Space Shuttle typically orbit at 350--400 km, within the F - layer of the ionosphere where they encounter enough atmospheric drag to require reboosts every few months . Depending on solar activity, satellites can experience noticeable atmospheric drag at altitudes as high as 700--800 km . </P>

Where are most of the gas molecules in the atmosphere found