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Related to cloud up: cloud over, cloudiness
1. To become overcast, as of the sky. Enjoy this sunshine while it lasts because the sky is supposed to cloud up this afternoon.
2. To become very sad or upset. Her face clouded up at the mention of her ex-girlfriend. When I saw the baby cloud up, I prepared myself for the wails to follow.
1. Lit. [for the sky] to get cloudy, as if it were going to rain. All of a sudden it clouded up and began to rain. It usually clouds up at the top of the mountain.
2. Fig. [for someone] to grow very sad, as if to cry. Whenever Mary got homesick, she'd cloud up. She really wanted to go home.