fog vs pea soup

fog and pea soup both are nouns.

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

As nouns, pea soup is a hyponym of fog; that is, pea soup is a word with a more specific, narrower meaning than fog:

Other hyponyms of fog include fogbank, ice fog, pogonip, mist, pea-souper.

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

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