From a long article at American Cinematographer:
The street set completely covered Stage 6 at 20th Century Fox, and Cundey’s lighting solution was to stretch a silk over the entire stage — about two square blocks.
“One thing that I would like to fool around with, if the same situation ever arises,” says Cundey, “is something we discovered while we were shooting the tests. We were always afraid of tilting up and seeing the silk, but we found that when we did tilt up and we were a little overexposed, the silk looked like a white sky. We mounted remote fog units along the top of the buildings and had them squirt fog on intervals so that it would hang along the silk, and the few times that we do tilt up, it looks like clouds, fog, and a realistic white sky.”