pea-souper vs pea soup

pea-souper and pea soup both are nouns.

Word	Noun	Adjective	Verb	Adverb
pea-souper	Yes	No	No	No
pea soup	Yes	No	No	No

As nouns, pea-souper and pea soup are synonyms defined as:

pea-souper (noun)	pea soup (noun)
a heavy thick yellow fog	a thick soup made of dried peas (usually made into a puree)
	a heavy thick yellow fog