The sky is about 1,000 miles high, since the earth's atmosphere extends almost 1,000 miles above the surface and then becomes space. (The moon is about 250,000 miles away from earth, although this distance varies because the moon's orbit is elliptical.)
However, 90 percent of the earth's atmosphere is contained within the lowest 10 miles.
Above 18,000 feet, air pressure is half of what it is at sea level, reducing available oxygen by half.
