continuously forms, while the trailing or leeward edge
These clouds usually form downwind from a
mountain range or over a mountain in a small portion of
the sky at one level, but may form in different levels and
appear stacked on top of the lower level cloud elements.
They may also form in a layer downwind from a
mountain range with individual cloud elements well
separated from each other. Lenticularis clouds need not
form in conjunction with other orographic clouds, the
rotor clouds, and cap clouds.
The process that forms altocumulus lenticularis
clouds also on occasion forms the same clouds closer to
the ground in the low-etage, and frequently forms
similar clouds in the high-etage. The difference in the
classification of stratocumulus lenticularis,
altocumulus lenticularis, and cirrocumulus lenticularis
stems solely from the height at which they form, and
should not be based on the apparent size of the cloud,
which, in this case, may be very misleading. Studies
have shown that although lenticularis clouds usually
form with bases in the mid-etage range, they can form
downstream of larger mountain ranges, with the base of
the lowest lenticularis in the 20,000- to 30,000-foot
range. Bases of the higher elements of stacked
lenticularis may be as high as 35,000 feet, with cloud
tops near 40,000 feet.
Describe two potentially dangerous wind
phenomena associated with an outflow
A wall cloud will usually form in what location of
a CB cell?
Stratus fractus clouds generally form in
conjunction with what other type of cloud?
When must altostratus clouds be reclassified as
What might the formation of altocumulus
castellanus or altocumulus floccus indicate?
What are cirrus clouds composed of?
Explain the formation of a rotor cloud.
Explain the formation of a cap cloud.
The amount of cloud cover is the second
determination you must make in observing the overall
condition of the sky. Clouds of the types we have just
discussed may form at various levels in the atmosphere.
It is not uncommon to have different layers of low-etage
clouds, along with several layers of mid- and high-etage
Estimation by the observer is the primary method
used to determine cloud amounts. However, automatic
weather systems can measure the amount of clouds at
each level, and these inputs may be used as a
supplemental tool by observers for cloud layer coverage
and total sky cover.
Generally, there are two different types of cloud
amount measurements necessary for an observation.
The more difficult measurement is cloud layer
The easier measurement is the total sky
cover. In cloud layer coverage, the amount of cloud in
each layer must be estimated. Both are estimated in
eighths (oktas) of the celestial dome (the total area of
sky or the dome of the sky). Layer coverage is used to
determine the cloud ceiling, which is the lowest layer or
layers that block 5/8 or more of the celestial dome from
A cloud layer is defined as "clouds and/or
obscuring phenomena aloft, either continuous or
composed of detached elements, that have bases at
approximately the same level." Both continuous and
detached elements may combine to form a layer, and all
layers and obscuring phenomena are to be considered
opaque. The essential requirement is that bases be at
approximately the same level. The upper portions of
cumulonimbus clouds are often spread horizontally by
the wind, and form a layer of cirrus spissatus, dense
altostratus, or dense altocumulus clouds, Velum may
also be present.
These horizontal extensions are
regarded as separate layers if their bases appear
horizontal, and they cover 1/8 of the sky or more. A
layer may be a combination of cloud types or a
combination of obscuring phenomena as long as the
bases are all at approximately the same level. For
example, cumulus mediocris and cumulus congestus
may be considered as the same layer if their bases are
the same height.
When observing layer coverage, you must not only
estimate the amount of clouds in each layer, but also
consider phenomena that hide 1/8 or more of the sky as a
layer. (A partial obscuration hiding less than 1/8 of the
sky is ignored.) Obscurations may be surface-based or
aloft and include phenomena such as rain, snow, fog,
smoke, or haze.
However, liquid or frozen water