Wiktionary
cloud up
vb. To become cloudy
WordNet
cloud up
v. become covered with clouds; "The sky clouded over" [syn: overcloud, cloud over]
Usage examples of "cloud up".
The voice of Bosco Bob when ole Bosco was getting ready to cloud up and rain all over you.
Jonathan started to cloud up, but Sam made a silly face at him, so he decided to laugh instead.