nimbostratus

n.乱层云

A rain cloud,especially a low dark layer of clouds such as a nimbostratus. 雨云雨云，尤指象雨层云一样的深色低云层。

A rain cloud, especially a low dark layer of clouds such as a nimbostratus. 雨云雨云，尤指象雨层云一样的深色低云层

The lack of precipitation, other than fine drizzle, signals that it is a thin cloud, and not nimbostratus. 这种天气降雨较少，有时下点毛毛雨，也就是说这种云比较薄，不算是雨层云。