Stratus is capable of producing only very light
precipitation, such as drizzle or snow grains; or during
extremely cold temperatures, ice crystals. Heavier
precipitation, such as showers, accompanied by a very
dark portion of the stratus layer, indicates the presence
of embedded or higher level cumuliform clouds. Rain
or snow would be a strong indication that the cloud layer
is NOT stratus, but actually the mid-level cloud,
nimbostratus. Another factor that indicates that the
cloud layer is nimbostratus rather than stratus is the
presence of stratus fractus clouds.
The appearance of the sun through the cloud layer is
another indication of the cloud type. If a sharp, well-
defined outline of the sun can be seen through a cloud
layer, the cloud layer may be stratus. If, however, the
outline of the sun is blurred, fuzzy, or appears to be
viewed through ground glass, the cloud layer may be
One of the key indicators of stratus clouds lies in the
previous observation record. Stratiform cloud layers
normally do not suddenly develop or move over an area.
They are associated with stable layers in the atmosphere
and evolve slowly. A hazy layer aloft or at the surface
will gradually thicken with time to form stratus or fog.
When moving over an area, a very thin or hazy layer will
gradually become denser as the stratiform layer
movement progresses. The exception to this is the fog
banks found over large bays and coastal waters. These
fog banks may have very distinct boundaries. As they
move overland, they may stay on the ground as fog or
may lift slightly, forming a low stratus layer. The
transition from clear skies and unrestricted visibility to
low overcast stratus and poor visibility in fog may be
very sudden. The observation record, however, would
note the presence of the moving fog bank.
STRATUS NEBULOSUS.When stratus forms
in a layer with no distinct features or denser portions, it
is termed stratus nebulosus. Stratus nebulosus is the
most common form of stratus (fig 1-21).
STRATUS FRACTUS. Strutus fractus clouds
form in more or less continuous layers. They present a
ragged appearance, as if shredded by the wind. Stratus
fractus clouds are generally indicators of bad weather
and are usually found below layers of nimbostratus
clouds. As with cumulus fractus clouds, a layer of
clouds of the species stratus fractus is called "pannus."
This mid-etage cloud has features similar to stratus,
as we have previously discussed. Although the height
Figure 1-21.Stratus cloud.
of the cloud base is the primary difference between
stratus and altostratus, the cloud composition is another
Altostratus clouds frequently form
above the freezing level. In North America, the freezing
level during the winter may be at the surface; but during
summer the freezing level may range from 4,000 feet to
16,000 feet, depending on location and weather
patterns. The range between 8,000 to 10,000 feet is a
fair mean freezing level for the continental United
States. When altostratus clouds form above the freezing
level, they consist of ice crystals and super-cooled water
droplets (water at or below freezing that has not
crystallized). Ice crystals give the altostratus clouds
their grayish or bluish color and the customary fibrous
appearance. The ice crystals also diffuse light more,
such that the sun will appear as though viewed through
Another indicator used to differentiate between
altostratus, stratus, and the higher cirrostratus clouds is
the presence and type of optical phenomena. We must
consider two types of optical phenomena at this point:
the corona and the halo.
A corona is a reddish or brownish ring of small
diameter seen around the sun or the moon when viewed
through clouds. It is often easier to see in a reflection off
calm water than by direct observation. A corona may
occasionally display very pale rainbow colors, but red
will normally predominate and show in the outermost
ring. The corona is produced by refraction of light in
liquid water droplets, such as the super-cooled droplets
found in altocumulus clouds. It is rare for even super-
cooled water droplets to exist at too high an altitude, so a
corona usually indicates a low or mid-etage cloud form.
Large droplets produce a small corona, while smaller
droplets produce a larger corona.