Thank you all for the comments!
The main goal of this picture was to create a technical good looking planet and not to make the picture atmospherical itself. It should look good even the planet is in the center of the picture. Maybe I will will make a new picture which looks more dramatically.
From time to time I play around with 3D programs but never make very big projects due to lack of time.
On my homepage you can see more pictures of planets I’ve made (here). The first tries were made with the Linux program FracPlanet and POV-Ray. Then I tried something with my copy of Cinema4D R6 XL (there are more planets made with it, but they are currently used in a project, which is still in development, so they are not added to the page yet). Then I tried Blender to make some banner pictures for a browser-game clan.
I also tried to add shadows for the clouds, but the results looked weird…
Maybe I try the thing to fake them on the color sphere. Thank you for the hint!