clouding vs clouding up

clouding and clouding up both are nouns.

Word	Noun	Adjective	Verb	Adverb
clouding	Yes	No	No	No
clouding up	Yes	No	No	No

As nouns, clouding and clouding up are synonyms defined as:

clouding and clouding up: the process whereby water particles become visible in the sky

clouding (noun)	clouding up (noun)
the process whereby water particles become visible in the sky	the process whereby water particles become visible in the sky