[Dog Gone]

While previous calculations assumed Earth's temperature at the top of the atmosphere to be well over 1,500 degrees F several billion years ago, the new mathematical models show temperatures would have been twice as cool back then.

I would never have imagined that the top of the atmosphere would ever have been even 750 degrees. The ground temperatures must have been incredible.

[doc30]

It's the similar as today. The extreme upper atmosphere is very hot, but the air pressure there is extremely low. It's near the atmosphere/space boundary. It's only in the lower atmosphere that the temperature vs altitude rule that we directly experience holds.