We all live in a thick layer of air called the atmosphere. On average it is about fifty miles thick.
But as you go up, the atmosphere gets noticeably thinner. At the top of Mount Evans (14,130 feet or 4306.8 meters), there is 1/3 less air than at sea level. That means less air between me and the deep blue sky.
Amy on Mount Evans. Longs Peak, sixty miles away in Rocky Mountain National Park, is in the background.
Yes, the sky really was THAT blue.