I don’t think it’s the quantity of atmosphere (troposhere or what not), but the quality. Near the ground, the air is more dense: more molecules of gas per square inch. The air gets “thinner” as you go up. With less molecules blocking the sunshine and reflecting it away, more rays get through to burn you faster.
A hearty thanks to all of you. I just happened to be thinking about this while on a plane the other week, and in fact was discussing my theories on this matter over lunch today.
I’m glad to know that I was on the money!
Thanks!