Nimbostratus clouds are the thick, dark, solid rain (or snow) clouds produced by many fronts and low pressure areas. These are different from Cumulonimbus clouds which create thunderstorms. Nimbostratus clouds cause those long, dark days of constant rain, with cast grey, featureless skies. The word literally means 'stormy layers'. Nimbostratus clouds are often formed when altostratus clouds lower due to an approaching front.