harshness vs rigor

harshness and rigor both are nouns.

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

As nouns, harshness and rigor are synonyms defined as:

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

harshness (noun)	rigor (noun)
the quality of being harsh or rough or grating to the senses	something hard to endure
excessive sternness	the quality of being valid and rigorous
the roughness of a substance that causes abrasions	excessive sternness
the quality of being cruel and causing tension or annoyance