Table 105. 



INTERNATIONAL CLOUD CLASSIFICATION. 



The International Conference of Meteorologists held at Munich in 1891 recommended the 

 following classification of clouds, elaborated by Messrs. Abercromby and Hildebrandsson : 



a. Detached clouds with rounded upper outlines (most frequent in dry weather). 



b. Clouds of great horizontal extent suggesting a layer or sheet (wet weather). 



A. Upper Clouds, average altitude oooo" 1 . 



a. i. Cirrus. 



b. 2. Cirro-stratus. 



B. Intermediate Clouds, between 3000"' and 7000™. 



( 3. Cirro-cumulus. 

 a ' \ 4. Alto-cumulus, 

 b. 5. Alto-stratus. 



C. Lower Clouds, below 2000™. 



a. 6. Stralo-cumulus. 



b. 7. Nimbus. 



D. Clouds of diurnal ascending currents. 



a. 8. Cumulus; top i8oo">; base 1400" 1 . 



b. q. Cumulonimbus; top 3000™ to 8ooo m ; base 1400"*. 



E. High Fogs, under iooo m . 



10. Stratus. 



DEFINITIONS AND DESCRIPTIONS OF CLOUD FORMS. 



1. Cirrus (Ci.). — Detached clouds 0} delicate and fibrous appearance, often shoiving a feather- 

 like structure, generally of a whitish color. Cirrus clouds take the most varied shapes, such as 

 isolated tufts, thin filaments on a blue sky, threads spreading out in the form of feathers, 

 curved filaments ending in tufts, sometimes called Cirrus uncinus, etc.; they are sometimes 

 arranged in parallel belts which cross a portion of the sky in a great circle, and by an effect of 

 perspective appear to converge towards a point on the horizon, or, if sufficiently extended, 

 towards the opposite point also. (Ci.-St. and Ci.-Cu., etc., are also sometimes arranged in 

 similar bands.) 



2. Cirro-stratus (Ci.-St.). — A thin, whitish sheet of clouds sometimes covering the sky 

 completely and giving it only a milky appearance (it is then called Cirro-ncbula), at other 

 times presenting, more or less distinctly, a formation like a tangled web. This sheet often 

 produces halos around the Sun and Moon. 



3. Cirro-cumulus (Ci.-Cu.). Mackerel sky. — Small globular masses or white flakes 

 without shadows, or showing very slight shadows, arranged in groups and often in lines. 



4. Alto-stratus (A. -St.). — A thick sheet of a gray or bluish color, sometimes forming a 

 compact mass of dark gray color and fibrous structure. At other times the sheet is thin, 

 resembling thick Ci.-St., and through it the Sun or the Moon may be seen dimly gleaming as 

 through ground glass. This form exhibits all changes peculiar to Ci.-St., but from measure- 

 "ments its average altitude is found to be about one half that of Ci.-St. 



5. Alto-cumulus (A.-Cu.). — Largish globular masses, white or grayish, partially shaded, 

 arranged 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. 



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 character- 

 istic appearance of great rolls arranged in parallel lines and pressed close up against one 

 another. In their centers these rolls are of a dark color. Blue sky may be seen through the 

 intervening spaces which are of a much lighter color. (Roll-cumulus in England, Wulst- 

 cumulus in Germany.) St.-Cu. clouds may be distinguished from Nb. by their globular or 

 rolled appearance, and by the fact that they are not generally associated with rain. 



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. Through the openings in these 

 clouds an upper layer of Ci.-St. or A.-St. may be seen almost invariably. If a layer of Nb. 



Smithsonian Tables. 



234 



