harshness vs severity

harshness and severity both are nouns.

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

As nouns, harshness and severity are synonyms defined as:

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

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