云朵是可數(shù)名詞嗎

　　例句：

　　The plane disappeared behind a cloud.

　　飛機消失在云層里。

　　"Cloud Street" is already in its third printing.

　　《云街》已經是第3次印刷了。

　　Fog and low cloud cover are expected this afternoon.

　　預計今天下午有霧和低空云層。