Example : 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.
A sour temper.
A sullen crowd.

Synonyms : dark, dour, glowering, glum, moody, morose, saturnine, sour

Example : A heavy sky.

Synonyms : heavy, lowering, threatening