Synonyms for Somberness (same or very similar meaning)
WordNet sense 1 (a state of partial or total darkness):
gloom, sombreness
WordNet sense 2 (a feeling of melancholy apprehension):
gloominess
WordNet sense 3 (a manner that is serious and solemn):
graveness, gravity, soberness, sobriety
From the ODE community, based on WordNet
add/edit