At an average of 5,300 feet above sea level—5,352 at the Sunport, as the ABQ airport is adorably named—Albuquerque is officially a mile-high city. That means the atmosphere is pretty thin around here.

After all, it’s common knowledge that the higher you go, the thinner it gets: skiers get light-headed around Boulder (10,000 feet), climbers wear oxygen tanks up Mount Everest (29,000 feet), and airplane cabins are pressurized to keep us conscious (36,000 feet).

But can you explain why? Why isn’t the planet wrapped in a blanket of evenly-dense air from the ground to the edge of space?