CLOUD, a mass of condensed vapour, either liquid or solid, hanging in the air at some height from the earth (from the same root and probably the same word, as "clod," common to Teutonic languages in various forms meaning a mass or lump; it is first applied in the more usual sense in the late 13th century; the Anglo-Saxon chid is used only for "a mass of rock," wolcen being the A.S. word for "cloud") . This accepted definition rules out fogs and creeping mists from any classification.
Classification of Clouds.—It is reasonable to suppose that the shapes and general appearance of clouds must have been a matter of interest and speculation from earliest times. The clas sical writers make use of a variety of expressions for cloud forms but as no systematic early classification is known it becomes neces sary to translate such expressions in general terms. Theophrastus of Erusus (c. 373-286 B.c.) in his Book of Signs (trans. J. G. Wood, 1894) certainly distinguishes "streaks of clouds" from "clouds like fleeces of wool" and uses the distinction in his weather prognostications, but the classification does not appear to have been developed. The field remained open until the i 9th century when the famous French biologist Jean Baptiste, Chevalier de Lamarck, in the course of his pioneer work in weather-mapping (1801-04), made use of a scheme of cloud classification. La marck's climatological work was ill-appreciated in France and but little known elsewhere. In 1803 Luke Howard published his "On the modifications of clouds" in Tilloch's Philosophical Magazine. This was an entirely new and independent scheme in which the terms were all Latin, and were applied with such excellent judg ment that his system remains as the broad basis of those in use to-day. He named three primary types of cloud—Cirrus, Cumu lus and Stratus—and proposed four additional compound forms Cirro-cumulus, Cirrostratus, Cumulo-stratus and Cumulo-cirro stratus or Nimbus. Thomas Forster in his Researches about at mospheric Phenomena (1813) gives Howard's classification and suggests English names for the various types, e.g. cirrus becomes curlcloud ; cumulus, stackencloud ; stratus, fallcloud, cirrocumu lus, sondercloud ; cirro-stratus, wanecloud ; cumulo-stratus, twain cloud and nimbus becomes raincloud. Howard's scheme was uni versally adopted, in fact, his cirrus and cumulus are the clouds still known by the same names, though his stratus which seems to have been fog, has now been given a different meaning. His compounds were modified subsequently and further subdivisions were made, which involved the use of new terms. Many new terms failed to establish themselves either because the need for the new term was not widely accepted or because the author's definition of the name was not precise enough to give his exact cloud type. Other names such as A. Poey's palliurn to signify a uniform sheet have been fairly generally adopted. Notwithstanding the obvious importance of Howard's original paper it was either disregarded or not known to several later writers, for example, The Rev. W. Clement Ley does not refer to it though in his remarkable book Cloudland (1894) he describes an entirely new scheme. Instead of a classifi cation based on the appearance of clouds, four different groupings are made which depend upon the processes of cloud formation, viz.: (1) Clouds of Radiation, chiefly fogs. (2) Clouds of Inter fret, chiefly stratus. (3) Clouds of Inversion, cumulo and nimbus types. (4) Clouds of Inclination, chiefly cirro forms. Ley's defini tions of his terms are that interfret comprises the interaction of two currents on one another, which currents differ in velocity or direction or both. Inversion is merely vertical convection while inclination supposes that the ice particles are falling through cur rents of air which have a progressive change of velocity and so draw out the crystals into the characteristic wisps and bands of the cirrus types. Modern research has proved that Ley's expla nations of his interfret and inclination types are inadequate, while his radiation types are best grouped as fogs. Nevertheless his in genious scheme possesses elements more in accordance with some desirable future scientific classification than any which preceded it, but such a scheme cannot be adopted until meteorologists are in more general agreement as to the complicated processes involved in cloud formation.
For the purpose of other than local weather charts it was soon realized that some internationally accepted scheme had to be evolved. For many years after the institution of the International Meteorological Organization considerable attention was given to the matter and constructive proposals were contributed by a host of workers including Ley, Rotch, Mohn, Riggenbach, Teisserenc de Bort, Abercromby and Hildebrandsson. The last two meteor ologists had suggested a modification of Howard's classification which differed chiefly in the introduction of a new class, which they distinguished by the use of the prefix Alto. Their scheme was formally adopted at the Munich International Meteorological Con ference of 1891, and a committee was appointed to draw up an atlas showing the exact forms typical of each variety considered. Finally in Aug. 1894 a small sub-committee consisting of Hilde brandsson, Riggenbach and Teisserenc de Bort was charged with the task of producing the atlas. This was issued in 1896 and a sec ond edition by Hildebrandsson and Teisserenc de Bort was pub lished in 191o. The International Cloud Atlas has ten principal forms of cloud which are included in the code of telegraphic reports on cloud forms.
Shaw's definitions of these forms in the Manual of Meteorology' (1926), differing slightly from those given in the second edition of the Atlas, may be summarized as follows: 1. Cirrus (Ci.).—Detached clouds of delicate appearance, fibrous (thread-like) structure and feather-like form, generally white in colour.
2. Cirro-stratus (Ci. thin sheet of whitish cloud some times covering the sky completely and merely giving it a milky appearance ; it is then called cirro-nebula or cirrus haze ; at other times presenting more or less distinctly a fibrous structure like a tangled web. This sheet often produces halos around the sun and moon.
3. Cirro-cumulus (Ci. Cu.) .—A mackerel sky with small rounded masses or white flakes without shadows, or showing very slight shadow; arranged in groups and often in lines. (See Plate II. fig. 3.) 4. Alto-cumulus (A. Cu.) .—Larger rounded masses, white or greyish, partially shaded, arranged in groups or lines, and often so crowded together in the middle region that the cloudlets join. (See Plate II. fig. 4.) 5. Alto-stratus (A. S.) .—A dense sheet of a grey or bluish colour sometimes forming a compact mass of dull-grey colour and fibrous structure ; at other times thin like the denser forms of No. 2. (See Plate I. fig. 1.) 6. Strato-cutnulus (S. Cu.).—Large lumpy masses or rolls of dull grey cloud, frequently covering the whole sky, especially in winter. It may be distinguished from No. 7 by its lumpy or roll ing appearance, and by the fact that it does not tend to rain. (See Plate I. fig. I.) 7. Nimbus (N.) (Raincloud).—A dense layer of dark shapeless cloud, with ragged edges, from which steady rain or snow usually falls. If there are any openings in the cloud an upper layer of cirrostratus or alto-stratus may almost invariably be seen through them. Nimbus breaking into ragged cloud or into detached frag ments underneath a large nimbus (the "Scud" of Sailors) is termed Fracto-Nimbus (Fr. Nb.) . (See Plate I. fig. 2.) 8. Cumulus (Cu.) (Woolpack or Cauliflower Cloud).—Thick cloud of which the upper surface is dome-shaped and exhibits protuberances while the base is generally horizontal. (See Plate I. fig. 5.) 9. Cuinulo-nimbus (Cu. N.) (The Thunder-cloud; shower cloud).—Great masses of cloud rising in the form of mountains or towers or anvils, generally having a veil or screen of fibrous tex ture (false cirrus) at the top and at its base a cloud-mass similar to nimbus. From the base local showers of rain or of snow, occa sionally of hail or soft hail usually fall. (See Plate I. fig. 3.) The front of a thunderstorm of wide extent is frequently in the form of a large arch above a region of uniformly lighter sky.
1 o. Stratus (S.) .—A uniform layer of cloud, like fog, but not lying on the ground.
The international scheme also provides that where certain clouds take a lumpy form, the adjective cumuliformis shall be used, and if the base shows downward projecting bosses the word mammato shall be prefixed, while for clouds whose shapes approx imate to that of a doubly convex lens the term lenticularis shall be affixed. In consequence of such modifications as these twelve other cloud forms are added to the ten principal types. Issued as it has been with the authority of an international congress this scheme has been generally accepted, and for the great majority of observations is quite detailed enough, but some specialists con sidered that for the minuter study of cloud forms a more elaborate scheme was needed. In 1896 H. H. Clayton of the Blue Hill Ob servatory, Massachusetts, published a highly detailed scheme in which the international types and a number of subdivisions were grouped under four classes—stratiforms or sheet clouds; cumuli forms or woolpack clouds ; flocci f orms, including stratocumulus, alto-cumulus and cirrocumulus; and cirri f orms or hairy clouds. In _ 19o4 F. L. Obenbach (Cleveland Observatory) devised a sys tem, in which the International types are sub-divided into a num ber of species, but in the absence of any definitive atlas neither of these American schemes has come into general use.
In general, during the loth century the tendency of independent workers has been to increase the number of forms. sometimes to an unwieldy number. The cirrus type exhibits so many diverse forms that it has been found specially prolific in sub-types, e.g., Clayden in Cloud Studies (1905 and 1925) names nine varieties. Vincent in Atlas des Nuages (Brussels, 1909) gives 13 varieties, as also does Besson in AperFu historique sur la classification des Nuages (1923). Improvements in photographic methods both from the ground and by aviators from above the cloud layers has considerably stimulated interest in the subject and in this con nection Clayden's Cloud Studies is notable. Not only does it con tain beautiful photographs but much information is given on camera technique. For photographs of "clouds as seen from an aeroplane" reference should be made to Douglas's article with this title in the Q. J. R. Met. Soc. (192o, pp. 233-242), while for an extended bibliography on the subject, which is now becoming a formidable list, see Manual of Meteorology, vol. i., by Shaw and Austen (1926).