Of clouds, the sky, water, etc.: reflecting or transmitting little light; gloomy, sombre; murky; dull.

transferred. Of the clouds, sky, weather, etc.: Gloomy, dark, threatening. Sometimes figurative of attendant circumstances. Occasionally…

Of the sky, the weather, etc.: gloomy, dark, cloudy, troubled. Also figurative of prospects, a person's looks, etc.

Of the sky, etc.: Dull, gloomy, threatening.