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 altostratus. 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." Altostratus  (AS) 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 important  clue. 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 ground glass. 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. 1-19