Cumulonimbus are massive, convective, rain-producing clouds with substantial vertical extent and storm potential. They form in unstable, moist conditions and were formalised in cloud atlases; they appear frequently in art and storm literature.
Nimbostratus are thick, opaque, precipitating layers of cloud that produce steady rain or snow. They form in broad lift regions and may span much vertical depth. The name comes from later refinements to Howard’s schema and serves as a core genus in modern meteorology.