mourning vs sadness

mourning and sadness both are nouns.

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

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

Other hypernyms of mourning include sorrow, sorrowfulness.

mourning (noun)	sadness (noun)
the passionate and demonstrative activity of expressing grief	the quality of excessive mournfulness and uncheerfulness
state of sorrow over the death or departure of a loved one	emotions experienced when not in a state of well-being
	the state of being sad

mourning (adjective)	sadness (adjective)
sorrowful through loss or deprivation