The adj sullen has 2 senses (first 2 from tagged texts)
1. (9) dark, dour, glowering, glum, moody, morose, saturnine, sour, sullen -- (showing a brooding ill humor; "a dark scowl"; "the proverbially dour New England Puritan"; "a glum, hopeless shrug"; "he sat in moody silence"; "a morose and unsociable manner"; "a saturnine, almost misanthropic young genius"- Bruce Bliven; "a sour temper"; "a sullen crowd")
2. (1) heavy, lowering, sullen, threatening -- (darkened by clouds; "a heavy sky")