mammillate in the manner of mammato-cumulus clouds. The fibrous alto-stratus is composed of snow crystals. It does not cause halos. The compact form is composed of water droplets and may cause coronas. Alto-stratus clouds indicate varying conditions of moisture and quiet air, rather than definite weather conditions. Nevertheless rain and snow may fall from them.[1]
5. Alto-cumulus (A-Cu).—Large globular masses, white or grayish, partly shaded, in groups or lines, and often so closely packed that their edges appear confused. The detached masses are generally larger and more compact (resembling St-Cu) at the center of the group, but the thickness of the layer varies. At times the masses spread themselves out and assume the appearance of small waves, or thin, slightly curved plates. At the margin they form into finer flakes (resembling Ci-Cu). They often spread themselves out in lines in one or two directions.
It is evident that the observer will record alto-cumulus as cirro-cumulus and vice versa; many times a description of either will fit the other. Fortunately such an error is harmless.
6. Strato-cumulus (St-Cu).—Large globular masses or rolls of dark clouds often covering the whole sky, especially in winter. Generally St-Cu presents the appearance of a gray layer irregularly broken up into masses of which the edge is often formed of smaller masses, often of wavy appearance resembling A-Cu. Sometimes this cloud-form presents the characteristic appearance of great rolls arranged in parallel lines, and pressed close against one another. In their centers these rolls are dark in color. Blue sky may be seen through the intervening spaces, which are much lighter in color. (Roll-cumulus in England, Wulst-cumulus in Germany.) Strato-cumulus clouds may be distinguished from Nb by their globular, or their roll appearance and by the fact that they are not generally associated with rain.
Strato-cumulus clouds usually follow a winter storm, covering the sky during the filling of a low barometer. The foregoing description is sufficiently plain and clear to indicate the character and appearance of strato-cumulus clouds. If they are high enough, however, they may be mistaken for alto-cumulus. In such a case it might be correct to call them alto-cumuli. Close to the horizon, strato-cumulus clouds resemble the normal stratus clouds at times, but they are much higher. Pretty nearly every transition between strato-cumulus and alto-cumulus clouds may be observed.
7. Nimbus (Nb).—Rain Clouds.— A thick layer of dark clouds, without shape and with ragged edges, from which steady rain or snow usually falls.
- ↑ At Blue Hill Observatory thay are classed as alto-nimbus when rain or snow is falling from them.
