Page:The American Cyclopædia (1879) Volume IV.djvu/723

 CLOUDS 711 observed from the base of the mountains. Their great elevation and feathery form suggest that the vapor exists in snowy flakes, and this is rendered more probable by the occurrence of those phenomena due to reflection and refrac- tion, as halos, parhelia, &c., most usually in these clouds and the cirro-cumulus. Traces of halos, such as are produced by the refraction of light from frozen particles, may almost al- ways be discovered in them by careful exami- nation with the aid of a blackened mirror. The long filamentous forms of cirrus are evi- dently connected with the action of currents in the air, maintaining uniform temperature along their lines, and drawing out in these shapes the residues of dissolving clouds. Va- riations in these currents of temperature or direction impress new forms upon the cirri. In diverging fibres, which is the true cirrus form, the cloud is supposed to indicate change- able weather, in summer rain andVwind, in winter frost or snow ; if the fibres have for some time continued to point in one direction, a gale of wind is looked for from that quarter. The cirro-cumulus is often produced by the cirrus descending to a lower level in the at- mosphere, and its parallel bands breaking up into the shape of small cumuli. It also appears independently of the previous existence of cirri. It floats at a high elevation, being often seen in the light of the moon through the scud and drift of the lower clouds, looking not un- like a flock at rest. Its spotted appearance has caused it to be commonly known as the "mackerel sky." The cirro-stratus is charac- terized by its arrangement in long parallel lines of cloud in close proximity, straight or waving, and lying in horizontal strata. It is also seen in a long and narrow horizontal sheet, taper- ing toward its extremities. The name of wane cloud has been given to it, from the fibres of the cirrus waning or subsiding to produce it. Its prevalence indicates wind, rain, or snow. In the form of rows of little clouds curved in a peculiar manner, it is called the cymoid cirro- stratus, and is regarded as a sure indication of approaching storms. When seen as a thin hazy veil spreading over the sky, particularly to- ward night, and obscuring the sun and moon, it is one of the most certain signs of approach- ing rain or snow. Virgil, in the Georgics, notices this obscuration of the sun behind a cloud soon after its rise, as sure to be fol- lowed by rain. The refractions of the light of the sun and moon, producing halos and mock suns, often appear in this cloud. The cumulo-stratus is formed by the passing of the cumulus into the nimbus, or rain cloud, though before the latter is perfected the cloud may be dispersed by evaporation, or turn back to the cumulus. It is also called the twain cloud, from the fact that two or more cumuli are often seen to join together to produce it. A single one is also sometimes observed to spread out laterally at top, till it overhangs the base in irregularly shaped protuberances. Such masses gather to- gether, presenting a most imposing spectacle, as a thunder storm is approaching. The mar- ginal protuberances are then seen, often shining with a strong silvery or golden light, contrast- ing finely with the darkness and density of the central portions of the cloud. Upon these the cirrus or cirro-stratus is sometimes formed by currents of air, causing the upper portion of the cloud to assume the curly forms of this variety, or spreading its long line horizontally across the summit. The nimbus is that stage of cloud from which the rain falls. It usually proceeds from the cumulo-stratus, which grows darker and denser, till its blackness and threat- ening aspect can no longer be mistaken as a sure sign of impending rain. -The blackness gives place to a gray obscurity, an evidence of a change in the disposition of the aqueous par- ticles, and the rain falls from the cloud, which is now a nimbus. The presence of its aqueous drops gives it the distinction among clouds of being the field upon which are displayed the beautiful colors of the rainbow. The nimbus having discharged its moisture, the various forms of cloud are again seen in their several places in the sky; the cirrus in some of its Fio. 4. Nimbus. modifications in the upper regions, while the fragments of the nimbus are converted into thin cumuli, which are borne along by the light winds near the surface. But if these gather again into the form of cnmulo-strati, rain is likely to return. The nomenclature of Howard is generally adopted by meteorologists. Prof. Loomis objects to the introduction of the nimbus as a distinct form, considering that it does not differ from some of the other varieties, except in the fall of rain, which is not sufficient to give it a peculiar character ; for clouds do not always undergo a decided change as rain begins to fall. He remarks upon the occur- rence, familiar to all who have lived in the vi- cinity of the great lakes, of a sheet of cloud continuing during a large part of the winter season to overcast the sky, from which snow is frequently emitted for days in succession, with- out change or apparent motion in the cloud it- self. This variety, including all clouds which cover the heavens with a nearly uniform and unbroken sheet, he places in the division stra- tus, thus supplying a deficiency in the European