CLOUD. 52 CLOUD. The descriptive ablircviations and classifica- tion introduced by tho International Meteorolog- ical C'ongress, licld at Jlunich in 1891, is that which is now adopted more or less completely bj' all the national weather bureaus. Sj'stematic observations upon the heights and movements of all clouds were made at many stations through- out tlie world in concert from May 1, ISItU, to July 1, 1897. From these observations a general idea of the heiglits of the respective kinds of clouds may be gatliered, as shown in the preced- ing list, compiled from Professor Bigelow's Re- port on International Cloudn:(yrk of the United fitnfes Weather liurcaii, page 20. The titles and descriptions of the clouds are as used by the International Committee. CiRRr.s ((';.). Isolated feathery clouds of line fibrous texture, generally brilliant white, frequently arranged in bands which spread like the meridians on a celestial globe over a part of the sky, and converge in perspective toward one or two opposite points of the horizon. (In the formation of such bands, cirro-stratus and cirro- cumulus often take part.) C'irro-Stratus ( ('(. .S'. ) . Fine whitish veil, sometimes quite dift'use. giving a whitish ap- pearance to the sky, and called by many 'cirrus haze,' and sometimes of more or less distinct structure, exhiljiting tangled fibres. The veil often produces halos around the sun and moon when seen through it. CiRRO-CuMUtrs (f'i. Cu.). Fleecy cloud. Small white balls and wisps, without shadows, Oi' with very faint shadows, which are arranged in groups and often in rows. Alto-Cumulus (A. Cu.). Dense fleecy cloud. Larger whitish or grayish balls, with shaded portions, grouped in flocks or rows, frequently ■so close together that their edges meet. The different balls are generally larger and more compact (passing into strato-cumuhis) toward the centre of the group, and more delicate and wispy (passing into cirro-cumulus) on its edges. They are very frequently arranged in lines in one or two directions. Alto-Str.ti:s {A. .S'. ). Thick veil of a giay or bluish color, exhibiting in the vicinity of the sun and moon a brighter portion, which, with- out causing halos, may produce corona?. This form shows gradual transitions to cirro-stratus; but, ' according to the measurements made at Cpsala, was of only half the altitude. Strato-Cumulus (j8. Cu.). Large balls or rolls of dark cloud, which frequently cover the whole sky, especially in winter, and give it at times an undulated appearance. The stratum of strato-cumulus is usually not very thick, and blue sky often appears in the breaks through it. Between this form and the alto-cumulus all possible gradations are found. It is distin- guished from nimbus by the ball-like or rolled form, and because it does not tend to bring rain. Nimbus CX.). Eain-clouds. Dense masses of dark, formless clouds, with ragged edges, from which generally continuous rain or snow is fall- ing. Through the breaks in these clouds there is almost always seen a higher sheet of cirro- stratus or alto-stratus. If the mass of nimbus is torn up into small patches, or if low frag- ments of cloud are floating much below a great nimbus, they may be called fraeto-nimbtis (the 'scud' of the sailors). Cumulus {Cu.). Woolpack clouds. Thick clouds, whose summits are domes with pro- tuberances, but whose bases are flat. These clouds appear to form in a diurnal ascensional movement, which is almost always apparent. When the cloud is opposite tlie sun, the surfaces v/hich are usually seen by the observer are more brilliant than the edges of the protuberances. When the illumination comes from the side, this cloud shows a strong actual shadow; on the sunny side of the sky, however, it appears dark, with bright edges. The true cumulus shows a sharp border above and below. It often torn by strong winds, the detached parts {fracto- cumulus) present continual changes. CuMULO-XiMBUS (Cm. A".). Thunder-cloud; shower-cloud. Heavy masses of clouds, rising like mountains, towers, or anvils, generally sur- rounded at the top by a veil or screen of fibrous texture ( 'false cirrus' ) , and below by nimbus- like masses of cloud. From their base generally fall local showers of rain or snow, and some- times hail or sleet. The upper edges are either of compact, cumulus-like outline, and form mas- sive summits, surrounded by delicate false cir- rus, or the edges themselves are drawn out into cirrus-like filaments. This last form is most common in "spring showers.' The front of thunder-storm clouds of wide extent sometimes shows a great arch stretching across a portion of the sky, which is uniformly lighter in color. Stk.vtus {S). 'Lifted fog' in a horizontal stratum. When this stratum is torn by the wind or by mountain sunnnits into irregular fragments, they may be called fracto-stratus. In general, the cirrus, cirro-stratus, and cirro- cumulus are the highest and swiftest ; the alto- stratus, alto - cumulus, strato - cumulus, and cumulo-nimbus are median: the nimbus, cumu- lus, and stratus are lowest and slowest. These three groups are also generally distinctive as to their appearances and methods of formation. There are some rarer forms of clouds that have received special or local names, such as the following: Phosphorescent, sometimes called iridescent, opalescent, or luminous night clouds. These are seen even at midniglit in Europe as distant, pure white clouds, near the horizon. . Measurements appear to show that they are from 10 to 20 miles above sea-level ; they may possibly be self-luminous or pliosphorescent, but it is more likely that they shine by reflecting the light from a distant twilight. Hoods, or false cirri, enveloping a mountain-top, or the summit of a cumulus dome. These are formed in the air that is pushing upward over the obstructing mountain. The tablecloth of Table Mountain at Cape Town is formed somewhat like the hood, but covers the whole flat top of the mountain and hangs down a little way over the leeward cliff's with frayed edges as the cloud- matter evaporates back into invisible moisture. The helm-cloud and helm-bar are stationary clouds, formed at the summits of standing waves of air. WTien an east wind blows over the Crossfell Eange, in Cumberland, England, there is formed not merely a cloud or hood or helmet with rain at the summit of the range, but a series of undulations to the leeward, one or more of which may rise high enough as a standing wave to form a cloud at its summit; through this cloud, in fact, the wind is blowing, and the cloud-particles formed on the windward side are
