The phrase "clouds up" typically refers to the darkening of the sky before a storm or the gathering of clouds before rain. Antonyms for this phrase would include "clears up" or "brightens up," which suggest that the weather is improving and the sky is becoming clearer. Other antonyms might include "disperse," "scatter," or "evaporate," which describe the dissipation or disappearance of clouds altogether. These antonyms suggest a brightening or clearing of the sky, while "clouds up" implies a darkening or worsening of the weather. Ultimately, the choice of words will depend on the context and the mood or tone being conveyed.