floccus species name is used. When the virga trails
make the entire element larger than 1°, the cloud must
be classified cirrus floccus.
CIRROCUMULUS CASTELLANUS.—Cirro-
cumulus castellanus identifies the cirrocumulus cloud
layers where each element shows a cumuliform buildup
such that the height of the element is greater than the
width of its base, and each element is smaller than 1°. If
the vertical development of the cloud elements
progresses such that the element becomes larger than
1°, the cloud is reclassified cirrus castellanus. Now
let’s consider some cloud types we haven’t covered in
table 1-1.
Orographic Clouds
Several species of low-, mid-, and high-etage
clouds are associated only with moist airflow over
mountainous areas.
These clouds usually form during Mountain Wave
conditions, when strong winds blow across mountain
ranges. The presence of these clouds is significant in
that they may be associated with dangerous turbulence.
All of the orographic cloud forms are unique in that they
are stationary over a particular area and do not move
with the wind flow. Slow changes in the arrangement of
elements or the cloud pattern may be noted as the upper
wind direction or intensity changes. The significant
orographic cloud forms are the rotor cloud, the cap
cloud, and the lenticular clouds.
The rotor cloud (fig. 1-25) is formed downwind
from the mountain range. The rotor cloud is formed as
the strong winds moving across the mountains set up a
wavelike action in the winds downstream from the
mountain. The waves or eddies in the air may extend in
a series downstream from the mountain for hundreds of
miles. The upward moving air in the waves or eddies, if
moist enough, is brought to saturation as it rises,
forming the rotor clouds.
Rotor clouds are cap-shaped, with smooth rounded
tops, flattened or concave bases, ragged up-wind edges,
and very ragged downstream edges. The up-wind edge
continuously forms, while the downstream edge
continuously dissipates. Rotor clouds may form as a
few isolated elongated elements, or in succeeding bands
elongated parallel to the mountain range. When you are
observing the sky condition, rotor clouds are usually
classified as cumulus clouds, and a note about the
presence of rotor clouds may be added in the remarks
section.
A special type of stratocumulus cloud, called the
Foehnwall or cap cloud, may form on the top of
mountain ranges, resembling a "cap" on the mountain.
It is formed as moist air is forced upward by the
mountain top, and dissipates on the leeward side of the
mountain as the moving air descends. Cloud particles
and denser patches of cloud may be seen moving over
the mountain and occasionally may be seen flowing
down the leeward side of the mountain, giving the
appearance of a waterfall. The cloud itself is stationary
on the mountain top.
Altocumulus lenticularis (abbreviated ACSL)
clouds are typically described as lens-shaped, almond-
shaped, or cat-eye shaped, and usually have a
windswept appearance (fig. 1-26). Although the cloud
elements may grow or shrink in size, they are usually
stationary. The size of the individual cloud elements is
usually quite large. The leading or windward edge
Figure 1-25.—Rotor cloud-an orographic form of cumulus
cloud. Figure 1-26.—Standing lenticular cloud.
1-24
