Dark rain clouds are called stratus clouds. They are rainy, dense, dark or snowy clouds that sometimes appear as white cottony clumps surrounded by blue sky.
The word stratus is derived from a Latin word meaning “layers.” These are low clouds that look like shapeless gray sheets extending in every direction. Stratus clouds are approximately 1,093.6 yards thick and can spread to over 621.5 miles wide. These rain clouds build when moist, warm air rises above colder air; they are gloomy and dark and related to drizzle and rain. When stratus clouds rest on the sea or the ground, they are known as fog.