I got to thinking that, if I were to go 20 miles straight up instead of to the west, I would die from lack of oxygen. I'd be in the middle of the stratosphere, and also in the midst of the ozone layer. The temperature would only be about -20 Centigrade (-4 Fahrenheit). The air pressure is only 0.01 atmospheres (it is 1 atmosphere at the Earth's surface) -- I would be above 99% of the atmosphere!
This gave me some appreciation for how thin Earth's atmosphere really is.