Darkening

“This, then,” he said, “is the ancient Castle of Wolf's Crag, often mentioned in the Scottish records,” looking to the old tower, then darkening under the influence of a stormy cloud, that formed its background.

The clouds are darkening, and it looks like rain.

The clouds are darkening, and it looks as though it's going to rain.

A full solar eclipse passed over Africa on Sunday, briefly darkening skies as it moved across the continent.

The sky was darkening and a storm was brewing in.

At last the first glimpse from a bridge of an open-top red bus, and a noticeable darkening of the atmosphere from the smoke of London: then the increasingly dingy stations with double-barrel names, set amid what has always been to me the outstanding feature of the "Premier Line" approach to London—the positively marvellous display of crazy chimney-pots on the grey inner suburban houses. As many as twenty, all of varying style, standing together like ranks of jagged teeth, and providing a Dickensian back-cloth which no other route can boast.

NEGATIVE FLASHES — Brief darkenings of a light that is otherwise steadily on.