Original artwork by Windows to the Universe staff (Randy Russell).
Air gets ‘thinner’ with increasing altitude. That’s why mountain climbers sometimes need bottled oxygen to breathe, and why it is so easy to get ‘winded’ while hiking in high mountains or even visiting someplace at elevation. Atmospheric pressure is around 1,014 millibars (14.7 pounds/inch2) at sea level. At an elevation of 10 km (6 miles or about 30,000 feet), roughly the height of Mt. Everest, pressure drops to 265 millibars. That’s less than 30% of the pressure at sea level! Both atmospheric pressure and the density of air decrease with altitude.
source: windows to the universe