(1) _Cirrus._--Parallel, flexuous or diverging fibres, extensible in
any or all directions.
(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. Poey'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) Poey, published in the _Annuaire de la societe meteorologique de
France_, 1865, (2) M. l'Abbe Maze, published in the _Memoires du congres
meteorologique 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.