[knappe]

At that height you can see the curvature of the Earth. For perspective here is a picture:

http://imgur.com/I5qQA3D EDIT:

Fair point. The max height on cumulonimbus clouds are 60,000 feet[0]. Effectively there is zero weather.

[0] - http://en.wikipedia.org/wiki/Cumulonimbus_cloud

[tfe]

To be fair, in a lot of those "curvature of the earth" pictures, the apparent curvature comes from the wide-angle lens more than actual curvature of the earth.