Primary Forms of Clouds: Nimbus and Stratus
A Nimbus, or Storm Cloud, is any cloud from which rain falls. The Stratus, or Layer Clouds, form in long, horizontal sheets or bands. These clouds are most common in the early morning or evening, when the ascending currents are weak.
Edwin J. Houston, The Elements of Physical Geography, for the use of Schools, Academies, and Colleges. (Philadelphia: Eldredge & Brother, 1891) 103