Thanks again, guys. Well, the reason I used a backdrop is, I had the scene completely lit by the background shader and the sun at a low level. The background looked evenly grey at the time, and I only wanted some visible clouds without having to change the lighting. I used a hollow hemisphere with shadows off for that.
I have to say, that I was so focused on the street scene, that I didn't realize the bad quality of the cloud image. I'll see, if I can improve the background clouds, or if I might use real ones.