lowness vs sadness

lowness and sadness both are nouns.

Word	Noun	Adjective	Verb	Adverb
lowness	Yes	No	No	No
sadness	Yes	No	No	No

As nouns, sadness is a hypernym of lowness; that is, sadness is a word with a broader meaning than lowness:

Other hypernyms of lowness include unhappiness.

lowness (noun)	sadness (noun)
the quality of being low; lacking height	the quality of excessive mournfulness and uncheerfulness
a low or small degree of any quality (amount or force or temperature etc.)	emotions experienced when not in a state of well-being
a feeling of low spirits	the state of being sad
a position of inferior status; low in station or rank or fortune or estimation