4. Alto-stratus (A.-St.)—A dense sheet of a gray or bluish color, sometimes forming a compact mass of dull gray color and fibrous structure.
At other times the sheet is thin, like the denser forms of cirro-stratus, and through it the sun and moon may be seen dimly gleaming as through ground glass. This form exhibits all stages of transition between alto-stratus and cirro-stratus, but, according to measurements, its normal altitude is about one-half that of cirro-stratus.
5. Alto-cumulus (A.-Cu.)—Larger rounded masses, white or grayish, partially shaded, arranged in groups or lines, and often so crowded together in the middle region that the cloudlets join.
A Cloud Banner Over Mt. Assiniboine, Canadian Rockies. (Photographed by Dr. C. D. Walcott.)
Cirrus (with a few patches of lower clouds in the foreground). This is cirrus, but not of the “mare’s tail” variety. There are many distinct types of cirrus, which have sometimes been given separate names. (Photographed at the Observatory of Trappes, France.)
The separate masses are generally larger and more compact (resembling strato-cumulus) in the middle region of the group, but the denseness of the layer varies and sometimes is so attenuated that the individual masses assume the appearance of sheets or thin flakes of considerable extent with hardly any shading. At the margin of the group they form smaller cloudlets resembling those of cirro-cumulus. The cloudlets often group themselves in parallel lines, arranged in one or more directions.
6. Strato-cumulus (St.-Cu.)—Large lumpy masses or rolls of dull gray cloud, frequently covering the whole sky, especially in winter.
Generally strato-cumulus presents the appearance of a gray layer broken up into irregular masses and having on the margin smaller masses grouped in flocks, like alto-cumulus. Sometimes this cloud form has the characteristic appearance of great rolls of cloud arranged in parallel lines close together (“roll cumulus”). The rolls themselves are dense and dark, but in the intervening spaces the cloud is much lighter and blue sky may sometimes be seen through them. Strato-cumulus may be distinguished from nimbus by its lumpy or rolling appearance, and by the fact that it does not tend to bring rain.