Nimbostratus Clouds

Nimbostratus clouds bring small amounts of rain or other precipitation. They look like huge sheets of gray, dark clouds. These clouds form on warm fronts do not carry thunderstorms like the cumulonimbus clouds of cold fronts.

"Nimbostratus clouds form through the deepening and thickening of an altostratus cloud, often along warm or occluded fronts. These clouds extend through the lower and mid layers of the troposphere bringing rain to the surface below." (directly quoted from https://www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/clouds/mid-level-clouds/nimbostratus )