Appendix A, Cloud Types A-4
Figure A-4. Altocumulus (Ac)
A.2.1.1 Altocumulus Lenticularis
Altocumulus Lenticularis, commonly known as “Altocumulus Standing Lenticular ( ACSL)” (see Figure
A-5), are an orographic type of cloud. They often appear to be dissolving in some places and forming in
others. They also often form in patches in the shape of almonds or wave clouds. These formations are
caused by wave motions in the atmosphere and are frequently seen in mountainous or hilly areas. They may
be triggered off by hills only a few thousand feet high and may extend downwind for more than 60 mi
(100 km). The cloud elements form at the windward edge of the cloud and are carried to the downwind
edge where they evaporate. The cloud as a whole is usually stationary or slow moving. These clouds often
have very smooth outlines and show definite shading.
ACSL clouds indicate the position of the wave crests, but they do not necessarily give an indication on the
intensity of turbulence or strength of updrafts and downdrafts. This is because the clouds depend on both
lifting and moisture. A well-defined wave may be visible (i.e., ACSL cloud) in weak updrafts where there
is an adequate supply of moisture but may not be visible when the environment is very dry, even if the wave
is intense.
Appendix A, Cloud Types A-5
Figure A-5. Altocumulus Standing Lenticular (ACSL)
A.2.2 Altostratus (As)
Altostratus (As) (see Figure A-6) is a cloud type in the form of a gray or bluish (never white) sheet or layer
of striated, fibrous, or uniform appearance. Altostratus very often totally covers the sky and may, in fact,
cover an area of several thousand square miles. The layer has parts thin enough to reveal the position of the
Sun, and if gaps and rifts appear, they are irregularly shaped and spaced.
Within the rather large vertical extent of altostratus (from several hundred to thousands of feet), a very
heterogeneous particulate composition may exist. In this most complete case, there may be distinguished:
1. An upper part, mostly or entirely ice crystals;
2. A middle part, a mixture of ice crystals and/or snowflakes and supercooled water
droplets; and
3. A lower part, mostly or entirely supercooled or ordinary water droplets.
Note: A number of partial combinations of these composition types may occur but never an entire cloud
like item 3 above.
The particles are widely dispersed enough so as not to obscure the Sun, except by its thickest parts . They
impose a ground-glass effect upon the Sun’s image, and prevent sharply outlined shadows from being cast
by terrestrial objects. Halo phenomena do not occur. Pilots can expect little or no turbulence, but light to
moderate icing in the supercooled water regions.
Thin altostratus usually evolves from the gradual thickening of a veil of cirrostratus. This means that unlike
most clouds, which increase in height as they grow, altostratus (and nimbostratus) grow as the base of the
cloud lowers.
Appendix A, Cloud Types A-6
Figure A-6. Thin Altostratus (As)
A.2.3 Nimbostratus (Ns)
Nimbostratus (Ns) (see Figure A-7) is a gray cloud layer, often dark, rendered diffuse by more or less
continuously falling rain, snow, ice pellets, etc., which in most cases reaches the ground. It is not
accompanied by lightning, thunder, or hail.
Nimbostratus is composed of suspended water droplets, sometimes supercooled, and of falling raindrops
and/or snow crystals or snowflakes. It occupies a layer of large horizontal and vertical extent. The great
density and thickness (usually many thousands of feet) of this cloud prevent observation of the Sun. This,
plus the absence of small droplets in its lower portion, gives nimbostratus the appearance of dim and
uniform lighting from within. It also follows that nimbostratus has no well -defined base but rather a deep
zone of visibility attenuation. Frequently, a false base may appear at the level where snow melts into rain.
It is officially classified as a middle cloud , although it may merge into very low stratus or stratocumulus.
Other cloud classification systems may identify it as a low -level cloud. Nimbostratus produces very little
turbulence but can pose a serious icing problem if temperatures are near or below freezing.
Thick altostratus is darker or bluish gray and denser than thin altostratus, and the greater part is sufficiently
dense to completely mask the Sun or Moon. With further thickening of the altostratus and a lowering of its
base, the cloud may begin to produce precipitation, at which point it is called nimbostratus. Some cloud
charts will depict nimbostratus as a low-level cloud. This is because oftentimes, during continuously falling
precipitation, the base of nimbostratus clouds decrease s into the low level. But officially and historically,
nimbostratus is classified as a mid-level cloud.
Appendix A, Cloud Types A-7
Figure A-7. Thick Altostratus (As) or Nimbostratus (Ns)
A.3 Low Clouds
Cumulus (Cu), towering cumulus (TCu), stratocumulus (Sc), stratus (St), and cumulonimbus (Cb) are low
clouds composed of water droplets. However, they can also be composed of supercooled liquid water
droplets and/or ice crystals when temperatures are below freezing. Cumulus and cumulonimbus usually
have bases in the low level, but their vertical extent is often so great that their tops may reach into the
middle and high levels.
A.3.1 Cumulus (Cu) and Towering Cumulus (TCu)
Cumulus is a cloud type in the form of individual, detached elements that are generally dense and possess
sharp, nonfibrous outlines. These elements develop vertically, appearing as rising mounds, domes, or
towers, the upper parts of which often resemble a cauliflower. The sunlit pa rts of these clouds are mostly
brilliant white; their bases are relatively dark and nearly horizontal. Near the horizon, the vertical
development of cumulus often causes the individual clouds to appear merged. If precipitation occurs, it is
usually of a sh owery nature. Various effects of wind, illumination, etc., may modify many of the
aforementioned characteristics.
Cumulus is composed of a great density of small water droplets, frequently supercooled. Within the cloud,
larger water drops are formed that may, as the cloud develops, fall from the base as rain or virga. 13 Ice
crystal formation will occur within the cloud at sufficiently low temperatures, particularly in upper portions
as the cloud grows vertically.
For cumulus with little vertical development (see Figure A-8), pilots can expect some turbulence and no
significant icing. However, for towering cumulus (i.e., cumulus of moderate/strong development) , pilots
can expect very strong turbulence and some clear icing above the freezing level (where temperatures are
negative). Towering cumulus (see Figure A-9) is also referred to as the first stage of a thunderstorm.
13 Virga are wisps or streaks of water or ice particles falling out of a cloud but vaporizing before reaching the Earth’s
surface as precipitation.
Appendix A, Cloud Types A-8
Figure A-8. Cumulus (Cu) with Little Vertical Development
Cumulus formation is often preceded by hazy spots out of which the clouds evolve. When completely
formed, the clouds have clear -cut horizontal bases and flattened or slightly rounded tops. At this stage of
development, they are known as fair weather cumulu s. Over land, on clear mornings, cumulus may form
as the Sun rapidly heats the ground. Near coasts, cumulus may form over the land by day in a sea breeze
and over the sea during the night in a land breeze.
Figure A-9. Towering Cumulus (TCu)
A.3.2 Stratocumulus (Sc)
Stratocumulus (Sc) (see Figure A-10) is a cloud type, predominantly stratiform, in the form of a gray and/or
whitish layer or patch, that nearly always has dark parts and is nonfibrous (except for virga). Its elements
are tessellated, rounded, roll-shaped, etc., may or may not be merged, and are usually arranged in orderly
groups, lines, or undulations, giving the appearance of a simple (or occasionally a cross -pattern) wave