4. Chicago Sky (62 miles)
What exactly is the sky? Us non-scientist people would probably call it everything high above us right up until outer space begins. In other words, it’s our atmosphere. But there’s a heck of a lot of it and it starts right above us and extends really far. Wondering where the sky ends is a common question, and Hungarian-American engineer Theodor von Karman gave us one answer when he calculated the Karman Line at about 62 miles above the surface. From my limited understanding, it seems that above this line the atmosphere is so thin that any aircraft would have to travel faster than orbital velocity in order to stay up.