hardness vs severity

hardness and severity both are nouns.

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

As nouns, hardness and severity are synonyms defined as:

Other synonyms of hardness include harshness, inclemency, rigor, rigorousness, rigour, rigourousness, severeness, stiffness.

hardness (noun)	severity (noun)
a quality of water that contains dissolved mineral salts that prevent soap from lathering	excessive sternness
the property of being rigid and resistant to pressure; not easily scratched; measured on Mohs scale	used of the degree of something undesirable e.g. pain or weather
the quality of being difficult to do	something hard to endure
devoid of passion or feeling; hardheartedness	extreme plainness
excessive sternness