severity vs harshness

severity and harshness both are nouns.

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

As nouns, severity and harshness are synonyms defined as:

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

severity (noun)	harshness (noun)
excessive sternness	the quality of being harsh or rough or grating to the senses
used of the degree of something undesirable e.g. pain or weather	excessive sternness
something hard to endure	the roughness of a substance that causes abrasions
extreme plainness	the quality of being cruel and causing tension or annoyance