Back in the day, launched out of Lima, Peru back to the states, the great circle route back to our dest took us out over the ocean. The visibility had to be 300+ miles, as the Andes, which are in places 15k ft tall, were visible as they went over the horizon, but just the very snow covered tips. As we got closer, they became the full height as we went by. That was prob the cleanest air I ever saw.