Atmospheric pressure drops as you go higher up in the atmosphere. Pressure at sea level is around a thousand millibars. Five kilometers (about 16 thousand feet) up pressure is only about half as much.
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Original artwork by Windows to the Universe staff (Randy Russell).
How Pressure Changes with Altitude in Earth's Atmosphere
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.
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