Synonyms for Sullen (same or very similar meaning)
WordNet sense 1 (showing a brooding ill humor):
dark, dour, glowering, glum, moody, morose, saturnine, sourWordNet sense 2 (having an irritable and unpleasant disposition):
ill-naturedWordNet sense 3 (darkened by clouds):
heavy, threatening, loweringWordNet sense 4 (full of or covered with clouds):
cloudyFrom the ODE community, based on WordNetadd/edit