stiffness vs harshness

stiffness and harshness both are nouns.

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

As nouns, stiffness and harshness are synonyms defined as:

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

stiffness (noun)	harshness (noun)
the physical property of being inflexible and hard to bend	the quality of being harsh or rough or grating to the senses
the property of moving with pain or difficulty	excessive sternness
the inelegance of someone stiff and unrelaxed (as by embarrassment)	the roughness of a substance that causes abrasions
excessive sternness	the quality of being cruel and causing tension or annoyance
firm resoluteness in purpose or opinion or action