_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.
[Illustration: FIG. 1.--CIRRUS.]
[Illustration: FIG. 2.--CIRRO-STRATUS.]
[Illustration: FIG. 3.--CIRRO-CUMULUS.]
[Illustration: FIG. 4.--ALTO-CUMULUS.]
[Illustration: FIG. 5.--ALTO-STRATUS.]
[Illustration: FIG. 6.--STRATO-CUMULUS.]
[Illustration: FIG. 7.--CUMULUS.]
[Illustration: FIG. 8.--STRATUS.]
[Illustration: FIG. 9.--NIMBUS.]
[Illustration: FIG. 10.--CUMULO-NIMBUS.]
7. _Nimbus_ (N.), _Rain Cloud._--A thick layer of dark clouds, without
shape and with ragged edges, from which continued rain or snow
generally falls. Through openings in these clouds an upper layer of
cirro-stratus or alto-stratus may almost invariably be seen. If the
layer of nimbus separates up into shreds, or if small loose clouds are
visible floating at a low level, underneath a large nimbus they may be
described as _fracto-nimbus_ (Scud of sailors). See fig. 9.
8. _Cumulus_ (Cu.) _(Wool-pack Clouds)._--Thick clouds of which the
upper surface is dome-shaped and exhibits protuberances while the base
is horizontal. These clouds appear to be formed by a diurnal
ascensional movement which is almost always observable. When the cloud
is opposite the sun, the surfaces usually presented to the observer
have a greater brilliance than the margins of the protuberances. When
the light falls aslant, these clouds give deep shadows, but if they
are on the same side as the sun they appear dark, with bright edges.
See fig. 7.
The true cumulus has clear superior and inferior limits. It is often
broken up by strong winds, and the detached portions undergo continual
changes. These altered forms may be distinguished by the name of
_Fracto-cumulus_.
9. _Cumulo-nimbus_ (Cu.-N.); _The Thunder-cloud; Shower-cloud._--Heavy
masses of clouds, rising in the form of mountains, turrets or anvils,
generally having a sheet or screen of fibrous appearance above (false
cirrus) and underneath, a mass of cloud similar to nimbus. From the
base there generally fall local showers of rain or snow (occasionally
hail or soft hail). Sometimes the upper edges have the compact form of
cumulus, rising into massive peaks round which the delicate false
cirrus floats, and sometimes the edges themselves separate into a
fringe of filaments similar to that of cirrus. This last form is
particularly common in spring showers. See fig. 10.
The front of thunderclouds of wide extent frequently presents the form
of a large bow spread over a portion of the sky which is uniformly
brighter in colour.
10. _Stratus_ (S.).--A horizontal sheet of lifted fog. When this sheet
is broken up into irregular shreds by the wind, or by the summits of
mountains, it may be distinguished by the name of Fracto-stratus. See
fig. 8.
The scheme also provides that where a stratus or nimbus takes a lumpy
form, this fact shall be described by the adjective _cumuliformis_,
and if its base shows downward projecting bosses the word _mammato_ is
prefixed.
Issued as it has been with the authority of an international congress of
specialists, this scheme has been generally accepted, and must be
regarded as the orthodox system, and for the great majority of
observations it is quite detailed enough. But it does not give universal
satisfaction. Cirrus clouds, for instance, exhibit many forms, and these
so diverse that they must be due to very different causes. Hence for the
minuter study of cloud forms a more elaborate scheme is still needed.
Hence in 1896 H. H. Clayton of the Blue Hill observatory, Massachusetts,
published in the _Annals_ of the astronomical observatory of Harvard
College a highly detailed scheme in which the International types and a
number of subdivisions were grouped under four classes--_stratiforms_ or
sheet clouds; _cumuliforms_ or woolpack clouds; _flocciforms_, including
strato-cumulus, alto-cumulus and cirro-cumulus; and _cirriforms_ or
hairy clouds. The International terms are embodied and the special
varieties are distinguished by the use of prefixes such as tracto-cirrus
or cirrus bands, grano-cirro-cumulus or granular cirrus, &c.
Again in 1904 F. L. Obenbach of the Cleveland observatory devised a
different system, published in the annual report, in which the
International types are preserved, but each is subdivided into a number
of species. In the absence of any atlas to define the precise meaning of
the descriptions given, neither of these American schemes has come into
general use.
Further proposals were put forward by A. W. Clayden in _Cloud Studies_
(1905). His scheme accepts the whole of the International names which he
regards as the cloud genera, and suggests specific Latin names for the
chief varieties, accompanying the descriptions by photographs. The
proposed scheme is as follows.
_Genus._ _Species._
Cirrus Cirro-nebula Cirrus haze.
Cirro-filum Thread cirrus.
Cirrus Excelsus High "
" Ventosus Windy "
" Nebulosus Hazy "
" Caudatus Tailed "
" Vittatus Ribbon "
" Inconstans Change "
" Communis Common "
Cirro-stratus Communis Common Ci. S.
Nebulosus Hazy "
Vittatus Ribbon "
Cumulosus Flocculent Ci.-S.
Cirro-cumulus Cirro-macula Speckle cloud.
Nebulosus Hazy Ci. cu.
Alto-clouds Alto-stratus
" " maculosus Mackerel sky.
" " fractus
Alto-strato-cumulus
Alto-cumulus informis
" " nebulosus
Alto-clouds Alto-cumulus castellatus Turret cloud.
" " glomeratus High ball cumulus.
" " communis
" " stratiformis Flat alto-cum.
Stratus Stratus maculosus
" " radius Roll cloud.
" " lenticularis Fall cloud.
Strato-cumulus
Cumulus Cumulus minor Small cumulus.
" major Large cumulus.
Cumulo-nimbus Storm cloud.
The term nimbus is to be applied to any cloud from which rain is
falling, but if the true form of the cloud is visible the term should be
used as a qualifying adjective. The prefix fracto- or the adjective
fractus should be used when the cloud is undergoing disintegration or
appears ragged or broken. Mammato- is used in the ordinary sense, and
finally undatus or waved is to be added to the name of any cloud showing
a wave-like or rippled structure. (A. W. C.)