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.