Which type of cloud, consistently dark due to being horizontally extensive and thick, often signals steady, prolonged rain or snow?

Answer

Nimbostratus clouds

Nimbostratus layers are described as horizontally extensive and quite thick, which causes their darkness and leads to prolonged precipitation.

Which type of cloud, consistently dark due to being horizontally extensive and thick, often signals steady, prolonged rain or snow?
weatheratmospherecloudsmeteorologyStorms