nimbostratus
noun [ U ] environment specialized uk/nɪm.bəʊˈstrɑː.təs/ us/nɪm.boʊˈstreɪt̬.əs/
乱层云（低空的一种平坦的暗灰色层云，云层厚、面积大，多携带雨水）
a type of stratus (= flat grey cloud) formed in a wide thick layer at a low level and usually carrying rain
Compare: altostratuscirrostratus