乌云密布

The morning began fine, but now clouds are banking up.

早晨开始是晴天, 可是现在乌。

The sky blackened and soon it began to rain.

天乌, 很快便下起雨来。

The sky clouded over before the storm.

暴风雨来临前, 天乌。

The sky clouded over; we could see there was going to be rainstorm.

"天乌,我们看得出快来暴风雨了。"

The thick clouds passed overhead, but fortunately the rain kept off the whole day.

虽然乌, 但很幸运地一整天都没下雨。