somberness vs seriousness

somberness and seriousness both are nouns.

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

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

Other hypernyms of somberness include earnestness, serious-mindedness, sincerity.

somberness (noun)	seriousness (noun)
a manner that is serious and solemn	the trait of being serious
a feeling of melancholy apprehension	the quality of arousing fear or distress
a state of partial or total darkness	an earnest and sincere feeling