There was much low cloud in the sky, steadily thickening.

The cloud might well thicken by morning, even bring rain.

As they went by, the cloud over the road thickened.

The clouds were thickening again, and the moon had gone.

The clouds thickened and grew black as the day wore on.

Before all movement was completed, the clouds had thickened, and we were plunged into total darkness.

The high clouds had thickened and it was darker than ever when they came at last to the point where the rapids made themselves felt.

The clouds are thickening, but the storm will not actually break for some time yet.

There was still blue up there, but the clouds were thickening in the west.

The clouds thickened, and soon a fine drizzle wet her face.