somberness vs melancholy

somberness and melancholy both are nouns.

Word	Noun	Adjective	Verb	Adverb
somberness	Yes	No	No	No
melancholy	Yes	Yes	No	No

As nouns, melancholy is a hypernym of somberness; that is, melancholy is a word with a broader meaning than somberness:

Other hypernyms of somberness include apprehension, apprehensiveness, dread.

somberness (noun)	melancholy (noun)
a manner that is serious and solemn	a humor that was once believed to be secreted by the kidneys or spleen and to cause sadness and melancholy
a feeling of melancholy apprehension	a feeling of thoughtful sadness
a state of partial or total darkness	a constitutional tendency to be gloomy and depressed

somberness (adjective)	melancholy (adjective)
	grave or even gloomy in character
	characterized by or causing or expressing sadness