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.--