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. [Illustration: Copyright O. P. Anderson A LENTICULAR CLOUD OVER MT. RAINIER] 7. NIMBUS (NB.)--_A dense layer of dark, shapeless cloud with ragged edges from which steady rain or snow usually falls. If there are openings in the cloud an upper layer of cirro-stratus may almost invariably be seen through them._ If a layer of nimbus separates in strong wind into ragged cloud, or if small detached clouds are seen drifting underneath a large nimbus (the “scud” of sailors), either may be specified as _fracto-nimbus_ (FR.-NB.). 8. CUMULUS (CU.) (_Wool-pack cloud_)--_Thick cloud of which the upper surface is dome-shaped and exhibits protuberances, while the base is generally horizontal._ These clouds appear to be formed by ascensional movement of air in the daytime, which is almost always observable. When the cloud and the sun are on opposite sides of the observer, the surfaces facing the observer are more brilliant than the margins of the protuberances. When, on the contrary, it is on the same side of the observer as the sun, it appears dark with bright edges. When the light falls sideways, as is usually the case, cumulus clouds show deep shadows. True cumulus has well-defined upper and lower margins; but one may sometimes see ragged clouds, like cumulus torn by strong wind, of which the detached portions are continually changing; to this form of cloud the name fracto-cumulus may be given. 9. CUMULO-NIMBUS (CU.-NB.)(_The thundercloud_)--_Great masses of cloud rising in the form of mountains or towers or anvils, generally having a veil or screen of fibrous texture (“false cirrus”) at the top, and at its base a cloud mass similar to nimbus._ From the base local showers of rain or snow, occasionally of hail or graupel, usually fall. Sometimes the upper margins have the compact shape of cumulus, or form massive heaps round which floats delicate “false cirrus.” At other times the margins themselves are fringed with filaments similar to cirrus clouds. This last form is particularly common with spring showers. The front of a thunderstorm of wide extent is frequently in the form of a large low arch above a region of uniformly lighter sky. 10. STRATUS (ST.)--_A uniform layer of cloud, like fog, but not lying on the ground._