Before aircraft and space travel it was not known how thick the atmosphere is. It was known clouds were closer to the surface than the sun, moon, planets and stars since clouds will always cover up (come in-between) the surface and those objects. Once the atmosphere was probed it was discovered just how very thin the atmosphere is. The atmosphere is just a thin shell of air around the earth that stays in place due to gravity. The atmosphere does not have an exact top like a lid would be on a jar. What happens is that the air gets thinner and thinner and eventually merges into and then becomes outer space. Pressure and the density of the air decreases with height. One way we can think about heights in the atmosphere is to give the amount of the atmosphere that is below a certain height: 5.5 kilometers- about half the atmosphere is below this height 9.0 kilometers- about 70% of the atmosphere is below this height 16.0 kilometers- about 90% of the atmosphere is below this height 36.0 kilometers- about 99% of the atmosphere is below this height 100.0 kilometers- atmosphere is so thin that it is virtually the vacuum of space above 600 kilometers- atmosphere is so thin that it is considered outer space Compare how small these distances above are compared to how far these objects are from the earth: Moon: about 384,400 kilometers Sun: about 149,597,890 kilometers Nearest star: about 42,000,000,000,000 kilometers |