汉语说的“乌云”不仅可以用 black cloud 和 grey cloud 表示，而且还可以用black cloud和storm cloud表示。下面的句子均来自语法书和词典，可供参考：

▲black cloud

Black clouds threatened rain. 天上乌云密布，要下雨了。

A large black cloud covered the moon. 一大片乌云遮住了月亮。

Look at those black clouds; there’s going to be a storm. 瞧那些乌云，眼看要有暴风雨了。

▲dark cloud

Do dark clouds signify rain? 有乌云是否显示要下雨?

Rain followed the dark clouds and lightning. 雨紧接着乌云和雷电而来。

Lightning flashed among the distant dark clouds. 一道道电光在远处的乌云中闪现。

The dark clouds spurred us to work faster before the rain started. 乌云激励我们加快进度抢在下雨之前完工。

▲grey cloud

Grey clouds obscure the sun. 乌云蔽日。

The gray clouds had begun to break up. 乌云已开始散去。

A thick grey cloud masked the sun. 一片厚厚的乌云遮住了太阳。

▲storm cloud

Storm clouds were gathering overhead. 乌云笼罩在上空。

The storm clouds gathered and the rain poured down. 乌云密布，下起了倾盆大雨。

The storm clouds threw the mountain peaks into dense shadow. 暴风雨来临时的乌云把山里笼罩在浓重的阴影之中。