If the sky clouds over, it becomes covered with clouds.阴云密布,乌云翻滚
If a person's face clouds over, they suddenly look unhappy or worried.(脸色)阴沉;愁云满面;忧郁
See also: cloud
👨🏻🏫 Mr. Ng 劍橋詞典 📚切換為繁體中文