下午三点左右，天空开始乌云密布。

Around 3 PM, the sky began to cloud over.

天空乌云密布，看起来要下雨了。

The sky clouded over, it looks like it's going to rain.

乌云密布，雷声隆隆。

Dark clouds gathered, and thunder rumbled.

天空倏地变得乌云密布。

The sky suddenly became overcast.

天空顿时乌云密布。

The sky suddenly filled with dark clouds.

他们正在野餐时，天空突然乌云密布。

They were having a picnic when the sky suddenly clouded over.

当我们到达山顶时，天气突然乌云密布。

When we reached the top of the mountain, the weather suddenly clouded over.

海边的天气迅速乌云密布，我们不得不离开。

The weather at the beach quickly clouded over, and we had to leave.

天气不吸引人，乌云密布，大雨滂沱。

The weather was uninviting with dark clouds and heavy rain.

乌云密布，我预计要下雨。

Dark clouds massed, and I expected rain.