Appendix A, Cloud Types A-9
system. These elements are generally flat-topped, smooth, and large; observed at an angle of more than 30
degrees above the horizon, the individual stratocumulus element subtends an angle of greater than five
degrees. When a layer is continuous, the elemental structure is revealed in true relief on its undersurface.
Stratocumulus is composed of small water droplets, sometimes accompanied by larger droplets, soft hail,
and (rarely) snowflakes. Under ordinary conditions, ice crystals are too sparse even to give the cloud a
fibrous aspect; however, in extremely cold weat her, ice crystals may be numerous enough to produce
abundant virga and sometimes even halo phenomena. The highest liquid water contents are in the tops of
these clouds where the icing threat is the greatest, if cold enough. Virga may form under the cloud,
particularly at very low temperatures. Precipitation rarely occurs with stratocumulus.
Pilots can expect some turbulence and possible icing at subfreezing temperatures. Ceiling and visibility are
usually better than with low stratus.
Figure A-10. Stratocumulus (Sc)
The stratocumulus in Figure A-10 occurs in patches or layers, composed of rounded masses or rolls, at one
or more levels. The clouds are gray or whitish and always have dark parts. Sometimes the elements lie in
parallel bands. Due to perspective, these may appear to converge towards the horizon. It may also occur in
the shape of lenses or almonds, although this is fairly rare.
A.3.3 Stratus (St)
Stratus (St) (see Figure A-11 and Figure A-12) is a cloud type in the form of a gray layer with a fairly
uniform base. Stratus does not usually produce precipitation, but when it does occur, it is in the form of
minute particles, such as drizzle, ice crystals, or snow grains. Stratus often occurs in the form of ragged
patches or cloud fragments, in which case rapid transformation is a common characteristic. When the Sun
is seen through the cloud, its outline is clearly discernible. In the immediate area of the solar disk, stratus
may appear very white. Away from the Sun, and at times when the cloud is sufficiently thick to obscure it,
stratus gives off a weak, uniform luminance.
The particulate composition of stratus is quite uniform, usually of fairly widely dispersed water droplets
and, at lower temperatures, of ice crystals (although this is much less common). Halo phenomena may occur
with this latter composition.
Appendix A, Cloud Types A-10
Stratus produces little or no turbulence, but temperatures near or below freezing can create hazardous icing
conditions. When stratus is associated with fog or precipitation, the combination can become troublesome
for visual flying.
Stratus most commonly occurs as a gray, fairly uniform, and featureless single layer of low cloud.
Occasionally, it can be dark or threatening, although at most it can only produce weak precipitation. This
feature makes it fairly easy to distinguish it from nimbostratus (Ns), which nearly always produces rain,
snow, or ice pellets. Fog will often lift into a layer of stratus by an increase in wind or a rise in temperature.
Stratus is sometimes comparatively thin, and the disk of the Sun or Moon may be seen with a clear outline.
Figure A-11. Stratus (St)
Stratus fractus (StFra) and/or cumulus fractus (CuFra) (see Figure A-12) are ragged shreds of low cloud s
that always appear in association with other clouds for a short time before, during, and a short time after
precipitation. They often form beneath lowering altostratus (As) or nimbostratus (Ns). They also occur
beneath cumulonimbus (Cb) and precipitating cumulus (Cu) and are collectively known as “scud clouds.”
Figure A-12. Stratus Fractus (StFra) and/or Cumulus Fractus (CuFra) of Bad Weather
Appendix A, Cloud Types A-11
A.3.4 Cumulonimbus (Cb)
Cumulonimbus (Cb) (see Figure A-13 and Figure A-14) is a cloud type, exceptionally dense and vertically
developed, occurring either as isolated clouds or as a line or wall of clouds with separated upper portions.
These clouds appear as mountains or huge towers, at least a part of the upper portions of which are usually
smooth, fibrous, or striated, and almost flattened as it approaches the tropopause. This part often spreads
out in the form of an anvil or vast plume. Under the base of cumulonimbus, which is often very dark, there
frequently exist virga, precipitation, and low, ragged clouds, either merged with it or not. Its precipitation
is often heavy and always of a showery nature. The usual occurrence of lightning and thunder within or
from this cloud leads to its popul ar appellations: thundercloud, thunderhead (the latter usually refers only
to the upper portion of the cloud), and thunderstorm.
Cumulonimbus is composed of water droplets and ice crystals, the latter almost entirely in its upper
portions. It also contains large water drops, snowflakes, snow pellets, and sometimes hail. The liquid water
forms may be substantially supercooled. Cumulonimbus contains nearly the entire spectrum of flying
hazards, including extreme turbulence.
In Figure A-13, no part of the cloud top has acquired a fibrous appearance or any anvil development. The
protuberances tend to form a whitish mass without striations. Showers or thunderstorms may occur. The
presence of lightning/thunder differentiate between this cloud and towering cumulus. Cumulus,
stratocumulus, or stratus may also be present.
Figure A-13. Cumulonimbus (Cb) Without Anvil
In Figure A-14, the characteristic shape of these clouds can only be seen as a whole when viewed from a
distance. The tops of these massive clouds show a fibrous or striated structure that frequently resembles an
anvil, plume, or huge mass of hair. They may occur as an isolated cloud or an extensive wall and squalls,
hail, and/or thunder often accompany them. Underneath the base, which is often very dark, stratus fractus
frequently form and, in storms, these may be only a few hundred feet above the Earth ’s surface, and they
can merge to form a continuous layer. Mammatus (a cellular pattern of pouches ) may form, especially on
the underside of the projecting anvil, and may appear particularly prominent when the Sun is low in the
sky. A whole variety of other clouds, such as dense cirrus, altocumulus, altostratus, stratocumulus, cumulus,
and stratus may also be present.
Appendix A, Cloud Types A-12
Figure A-14. Cumulonimbus (Cb) with Anvil