severity vs foulness

severity and foulness both are nouns.

Word	Noun	Adjective	Verb	Adverb
severity	Yes	No	No	No
foulness	Yes	No	No	No

As nouns, foulness is a hyponym of severity; that is, foulness is a word with a more specific, narrower meaning than severity:

Other hyponyms of severity include raininess, distressfulness, seriousness.

severity (noun)	foulness (noun)
excessive sternness	the attribute of having a strong offensive smell
used of the degree of something undesirable e.g. pain or weather	(of weather) the badness of the weather
something hard to endure	a state characterized by foul or disgusting dirt and refuse
extreme plainness	disgusting wickedness and immorality