i just found it fun to read this, dunno why, jolly lil article i must say
How high is the sky?
The sky starts as soon as the ground ends! There is also no distinct end to the sky. As you go higher and higher, the atmosphere just gets less and less dense very gradually and just fades away, and so there is no well-defined edge where it stops.
You can say however, that 99% of the atmosphere within an altitude of 31 km above sea level (31,000 meters, or 101,706 feet). For comparison, Mt. Everest, the tallest mountain in the world is only 8,846 meters or 29,028 feet. Because the atmosphere is dropping off slowly, you have to go all the way out to 42 km (137,795 ft) if you want to include 99.9% of the atmosphere.
Although it is in fact somewhat arbitrary, the boundary of the earth's atmosphere and outer space is said to be 100 kilometers. When a space craft is re-entering the Earth atmosphere, it is at an elevation of about 120 kilometers that atmospheric drag becomes noticeable.