(2) Cumulus.—Convex or conical heaps, increasing upward from a horizontal base.

(3) Stratus.—A widely-extended continuous horizontal sheet, increasing from below.

(4) Cirro-cumulus.—Small, well-defined, roundish masses, in close horizontal arrangement.

(5) Cirro-stratus.—Horizontal or slightly inclined masses, attenuated towards a part or the whole of their circumferences, bent downward, or undulated, separate or in groups consisting of small clouds having these characters.

(6) Cumulo-stratus.—The cirro-stratus blended with the cumulus, and either appearing intermixed with the heaps of the latter or superadding a widespread structure to its base.

(7) Cumulo-cirro-stratus, or nimbus.—The rain-cloud: a cloud or system of clouds from which rain is falling. It is a horizontal sheet, above which the cirrus spreads, while the cumulus enters it laterally and from beneath.

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.

I. Clouds of Radiation.
Nebula Fog.
Nebula Stillans Wet fog.
Nebula Pulverea Dust fog.
II. Clouds of Interfret.
Nubes Informis Scud.
Stratus Quietus Quiet cloud.
Stratus Lenticularis Lenticular cloud.
Stratus Maculosus Mackerel cloud.
Stratus Castellatus Turret cloud.
Stratus Precipitans Plane shower.
III. Clouds of Inversion.
Cumulo-rudimentum Rudiment.
Cumulus Heap cloud.
Cumulo-stratus Anvil cloud.
Cumulo-stratus Mammatus Tubercled anvil cloud.
Cumulo-nimbus Shower cloud.
Cumulo-nimbus Nivosus Snow shower.
Cumulo-nimbus Grandineus Hail shower.
Cumulo-nimbus Mammatus Festooned shower cloud.
Nimbus Rainfall cloud.
Nimbus nivosus Snowfall.
Nimbus grandineus Hailfall.
IV. Clouds of Inclination.
Nubes Fulgens Luminous cloud.
Cirrus Curl cloud.
Cirro-filum Gossamer cloud.
Cirro-velum Veil cloud.
Cirro-macula Speckle cloud.
Cirro-velum Mammatum.[1] Draped veil cloud.

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.