severity vs rigourousness

severity and rigourousness both are nouns.

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

As nouns, severity and rigourousness are synonyms defined as:

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

As nouns, severity and rigourousness are synonyms defined as:

Other synonyms of severity include asperity, grimness, hardship, rigor, rigorousness, rigour, severeness.

severity (noun)	rigourousness (noun)
excessive sternness	something hard to endure
used of the degree of something undesirable e.g. pain or weather	excessive sternness
something hard to endure
extreme plainness