pea soup vs fog

pea soup and fog both are nouns.

Word	Noun	Adjective	Verb	Adverb
pea soup	Yes	No	No	No
fog	Yes	No	Yes	No

As nouns, fog is a hypernym of pea soup; that is, fog is a word with a broader meaning than pea soup:

pea soup (noun)	fog (noun)
a thick soup made of dried peas (usually made into a puree)	confusion characterized by lack of clarity
a heavy thick yellow fog	droplets of water vapor suspended in the air near the ground
	an atmosphere in which visibility is reduced because of a cloud of some substance

pea soup (verb)	fog (verb)
	make less visible or unclear