What type of clouds bring long and steady rain?

The National Weather Service explains that nimbostratus clouds, which are thick and possess dense stratus, are responsible for steady rain. They are found under 6,500 feet. Nimbostratus clouds are usually dark grey in color due to their high-moisture content.

Identifying the base of a nimbostratus cloud sometimes proves difficult because it may extend high into the atmosphere. A nimbostratus cloud typically covers most of the sky, as visible from the ground. It often hides the sun behind its dense stratus. This type of cloud brings about poor visibility for pilots during rain or snow, and its high-moisture content can interfere with in-flight de-icing when temperatures fall below freezing.