How the sky is high?

Answer 1

The atmosphere extends tenuously into space as the exosphere, which gradually merges into empty space and where the air molecules almost never interact. The upper region of the thermosphere is between 500 and 1000 kilometers above the Earth's surface. What we see as a blue sky is not at any particular altitude, and it is still transparent to objects as bright as or brighter than the Moon.

The lower atmosphere is a lot thinner than you think. A cubic meter of air at sea level only contains about 1.2 kilograms of air. Higher in the atmosphere, a cubic meter would contain less air. For example, at 9000 meters (the summit of Mount Everest) a cubic meter of air would contain only about 0.47 kilograms of air, less than half as much as at sea level. It is this decrease of mass per unit volume (density) that results in air becoming "thinner" at high elevations. There is about the same percentage of oxygen, but less air for a given inhaled volume. Breathable air exists to about 10 kilometers (33,000 feet) but no human can remain conscious for very long at that altitude. Above 5.5 kilometers (18,000 feet), most humans suffer debilitating effects from prolonged exposure.