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This system was universally adopted, and apart from some ambiguity in the definitions of cumulo-stratus and nimbus, it was sufficiently detailed for many purposes, such as the general relations between clouds and the movements of the barometer. When, however, such questions as the mode of origin of particular forms of cloud came to be investigated, it was at once felt that Howard’s classes were too wide, and something much more detailed was required. The result has been the promulgation from time to time of revised schemes, most of these being based on Howard’s work, and differing from him by the introduction of new terms or of subdivisions of his types. Some of these new terms have come more or less into use, such as A. Poëy’s pallium to signify a uniform sheet, but as a general rule the proposals were not accompanied by a clear enough exposition of their precise meaning for others to be quite sure of the author’s intention. Other writers not appreciating how fully Howard’s names had become established, boldly struck out on entirely new lines. The most important of these were probably those due respectively to (1) Poëy, published in the Annuaire de la société météorologique de France, 1865, (2) M. l’Abbé Maze, published in the Mémoires du congrès météorologique international, 1889, and (3) Frederic Gaster, ''Quart. Jour. R. Meteorological Society'', 1893. In all of these Howard’s terms are used, but the systems were much more elaborate, and the verbal descriptions sometimes difficult to follow.

In his book Cloudland (1894) Clement Ley published a novel system. He grouped all clouds under four heads, in accordance with the mode in which he believed them to be formed.

It will be seen that Ley’s scheme is really an amplification of Howard’s. The term “Interfret” is defined as the interaction of horizontal currents of different velocities. Inversion is a synonym for vertical convection, and Inclination is used to imply that such clouds consist of sloping lines of falling ice particles.

While Ley had been finishing his work and seeing it through the press, H. Hildebrand-Hildebrandsson and R. Abercromby had devised another modification which differed from Howard’s chiefly by the introduction of a new class, which they distinguished by the use of the prefix Alto. This scheme was formally adopted by the International Meteorological Conference held at Munich in 1891, and a committee was appointed to draw up an atlas showing the exact forms typical of each variety considered. Finally in August 1894 a small sub-committee consisting of Messrs H. Hildebrand-Hildebrandsson, A. Riggenbach-Burckhardt and Teisserenc de Bort was charged with the task of producing the atlas. Their task was completed in 1896, and meteorologists were at last supplied with a fairly detailed scheme, and one which was adequately illustrated, so that there could be no doubt of the authors’ meaning. It is as follows:—

The International Classification.

(a) Separate or globular masses (most frequently seen in dry weather).

(b) Forms which are widely extended, or completely cover the sky (in wet weather).

A. Upper clouds, average altitude 9000 metres. &emsp;a. 1. Cirrus. &emsp;b. 2. Cirro-stratus.

B. Intermediate clouds, between 3000 m. and 7000 m.

&emsp;a. 3. Cirro-cumulus. 4. Alto-cumulus. &emsp;b. 5. Alto-stratus.

C. Lower clouds, 2000 m.

&emsp;a. 6. Strato-cumulus. &emsp;b. 7. Nimbus.

D. Clouds of Diurnal Ascending Currents.

&emsp;a. 8. Cumulus, apex 1800 m., base 1400 m. &emsp;b. 9. Cumulo-nimbus, apex 3000 m. to 8000 m., base 1400 m.

E. High Fogs, under 1000 m. &emsp;&ensp;10. Stratus.

Explanations.

1. Cirrus (Ci.).—Detached clouds, delicate and fibrous-looking, taking the form of feathers, generally of a white colour, sometimes arranged in belts which cross a portion of the sky in great circles and by an effect of perspective, converge towards one or two points of the horizon (the Ci.-S. and the Ci.-Cu. often contribute to the formation of these belts). See Plate, fig. 1.

2. Cirro-stratus (Ci.-S.).—A thin, whitish sheet, at times completely covering the sky, and only giving it a whitish appearance (it is then sometimes called cirro-nebula), or at others presenting, more or less distinctly, a formation like a tangled web. This sheet often produces halos around the sun and moon. See fig. 2.

3. Cirro-cumulus (Ci.-Cu.).—Small globular masses, or white flakes without shadows, or having very slight shadows, arranged in groups and often in lines. See fig. 3.

4. Alto-cumulus (A.-Cu.).—Largish globular masses, white or greyish, 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 (changing to S.-Cu.) at the centre of the group; at the margin they form into finer flakes (changing to Ci.-Cu.). They often spread themselves out in lines in one or two directions. See fig. 4.

5. Alto-stratus (A.-S.).—A thick sheet of a grey or bluish colour, showing a brilliant patch in the neighbourhood of the sun or moon, and without causing halos, sometimes giving rise to coronae. This form goes through all the changes like Cirro-stratus, but according to measurements made at Upsala, its altitude is one-half as great. See fig. 5.

6. Strato-cumulus (S.-Cu.).—Large globular masses or rolls of dark cloud, frequently covering the whole sky, especially in winter, and occasionally giving it a wavy appearance. The layer is not, as a rule, very thick, and patches of blue sky are often seen through intervening spaces. All sorts of transitions between this form and Alto-cumulus are seen. It may be distinguished from nimbus by its globular or rolled appearance, and also because it does not bring rain. See fig. 6.