Through the openings in these clouds an upper layer of Ci-St or A-St may almost invariably be seen. If a layer of Nb separates into shreds in a strong wind, or if small loose clouds are visible floating under a large Nb, the cloud may be described as fracto-nimbus (Fr-Nb), the “scud” of sailors.
Inasmuch as the sky is almost always wholly overcast during a steady rain or snow, the ragged edges are rarely visible. The foregoing description is hardly true of tropical rain clouds with their sharp, greasy-appearing edges. The observer will not be in serious error in designating any low cloud from which rain is falling as nimbus. The flying scud, its top pointing with the wind, drops no rain. The breaking of a nimbus usually denotes the clearing of a storm; and although the scud is rainless, it is properly nimbus cloud matter though not “rain clouds.”
8. Cumulus (Cu), Wool-pack Clouds.—Thick clouds of which the upper surface is dome-shaped, and exhibits protuberances while the base is horizontal. These clouds appear to be formed by a diurnal ascensional movement which is almost always noticeable. When the cloud is opposite the sun the surfaces facing the observer have a greater brilliance than the margins of the protuberances. When the light falls aslant, as is usually the case, these clouds throw deep shadows; when, on the contrary, the clouds are on the same side of the observer as the sun, they appear with bright edges.
True cumulus has well-defined upper and lower limits, but in strong winds a broken cloud resembling cumulus is often seen, in which the detached portions undergo continual change. This form may be distinguished by the name fracto-cumulus (Fr-Cu).
The cumulus cloud with its flat base and rounded dome is so full of character that the foregoing description is ample. It is the summer cloud of the temperate zones and the shower cloud of the tropics. To the unaided eye the constant motion of the cloud matter is apparent; with a field glass the convectional motion is plainly visible in the larger clouds. The cumulus is an “ascensional” cloud, because the water vapor is carried upward until cooling brings about condensation. The condensed vapor sinks until it is again warmed to the temperature of vaporization.
9. Cumulo-nimbus (Cu-Nb), Thunder-cloud, Shower-cloud.—Heavy masses of cloud rising in the form of mountains, turrets, or anvils, generally surmounted by a sheet or screen of fibrous appearance (false cirrus) and having at its base a mass of cloud similar to nimbus. From the base local showers of rain or snow (occasionally of hail or soft hail) usually fall. Sometimes the upper edges assume the compact form of cumulus, and form massive peaks
