Characterized as rounded balls of semi-transparent cloud, but ill-defined and hazy. No shadows. Ci. Cu. Neb. 3. _Cirro-cumulus_ (Plates 18 and 19). Characterized as opaque rounded balls clearly defined, but showing no shadows on their under sides. Ci. Cu. Com. Wave forms again are indicated by the addition of the word undatus. GROUP ALTO CLOUDS. Divisible into nine species. 1. _Alto-stratus._ High stratus. A uniform veil of cloud showing no details of structure except local variation in density in patches. Rarely dense enough to completely hide the sun, or even the full moon. A. S. 2. _Alto-stratus maculosus_ (Plate 30). Mackerel sky. Characterized as numerous nearly equal and small lenticular patches ranged on a level and about equi-distant from each other. A. S. Mac. 3. _Alto-stratus fractus_ (Plate 34). Patches and bits of cloud of irregular shape, but resembling broken bits of a level sheet. A. S. Fr. 4. _Alto-strato-cumulus_ (Plate 32). Intermediate between alto-stratus and alto-cumulus. A. S. Cu. 5. _Alto-cumulus informis_ (Plate 25). Characterized as more or less rounded cloudlets interspersed with ragged bits of cloud and occasionally with streaks of cirrus, the cloudlets showing no clear-cut outlines, but having distinct shadows. A. Cu. In. 6. _Alto-cumulus nebulosus_ (Plate 26). Hazy alto-cumulus. A. Cu. Neb. 7. _Alto-cumulus castellatus_ (Plate 28). Turret cloud. A high cloud resembling a number of tall narrow cumulus clouds on a very diminutive scale. The cloudlets show distinct shadows, are very opaque, and their upper margins are sharply defined. Vertical axes longer than the horizontal ones. A. Cu. Ca. 8. _Alto-cumulus glomeratus_ (Plate 29). Characterized by the roundness and regularity of the cloudlets, which have sharp margins, cast distinct shadows, and have their axes about equal in all directions. A. Cu. Gl. 9. _Alto-cumulus communis._ Small high cumulus of the ordinary pyramidal pattern. A. Cu. Com. 10. _Alto-cumulus stratiformis_ (Plate 27). Flattened cloudlets gathering into small detached sheets. A. Cu. S. Lower clouds. GROUP STRATUS. 1. _Stratus communis_ (Plates 37 and 41). In its most typical state, stratus consists of a sheet of cloud of approximately uniform thickness. The most common form, however, does vary considerably, though usually dense enough to hide the sun. Portions of such a sheet would take the same specific name, unless the portions are very small and ragged, which would be expressed by adding the word fractus. S. Com. 2. _Stratus maculosus_ (Plate 40). Formed either by the appearance of cloud in lumps, which are always lenticular in shape, and ultimately join together to form a stratus, or by the break up of the typical stratus. S. Mac. 3. _Stratus radius_ (Plate 42). Roll cloud. Formed during the break up of a low stratus, which separates up into a number of parallel lines of cloud. S. R. 4. _Stratus lenticularis_ (Plate 47). Fall cloud.