"It will be cloudy tomorrow."
Translation:Il y aura des nuages demain.
It is, but the subject has to be "le temps".
- Il y aura des nuages
- Le temps sera nuageux
Note that "il fera nuageux" is incorrect. The verbe "faire" is used with ordinary adjectives that have other meanings (il fera beau/gris/froid/humide...), whereas "nuageux" is specifically related to the weather.