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