1
general::
Also, cloud up. 1. Become overcast with clouds, as in It's clouding over now, so it may rain soon, or It was too hot and sunny, but after a while the sky clouded up and we ventured outside. [Mid-1700s] 2. Become opaque, misty, or dim, as in I'm sweating so much that my eyeglasses are clouded over.
American Heritage Idioms
2
general::
Phrase(s): cloud over
1. Lit. [for the sky] to fill with clouds; [for the sun] to be obscured by clouds. • It was beginning to cloud over, so we went inside. • The sky clouded over and it began to get chilly. 2. Fig. [for something once clear] to become opaque; to become clouded. • My vision seemed to cloud over, and I could see very little. • The mirror clouded over as the steam rose from the hot water in the sink.
McGrawhill's American Idioms And Phrasal Verbs