stratus
noun [ U ] environment specialized uk/ˈstreɪ.təs/ us/ˈstreɪ.t̬əs/
层云（一种扁平的灰色云，低于所有其他类型的云，可引起阴沉的天气或小雨）
a type of flat, grey cloud found at the lowest level, below all other cloud types, and causing dull weather or light rain
Compare: cirruscumulusnimbus