A halo is a 22° diameter ring seen encircling the sun or moon when viewed through clouds. The ring may show pale colors of the spectrum. Occasionally a secondary  ring  of  46°  diameter  may  be  visible encircling the 22° ring. Bright spots, which are called mock suns, may appear on the halo in a horizontal plane with the sun. A bright horizontal line may appear to connect the mock suns and the actual sun, which is called  the  parhelic  circle.  Occasionally,  vertical columns of light, or pillars, may appear above and below the sun or moon at low elevation angles, which intersect and form bright spots on the 22° halo. In the mid latitudes, the corona usually indicates that the cloud is a mid-etage cloud. It is only rarely observed  in  higher  low-etage  stratus  clouds  and  the lowest high-etage stratiform clouds. The presence of a halo, on the other hand, will indicate that the cloud is a high-etage cloud form, most often cirrostratus, and it may occasionally occur with cirrus. It does not occur with  altostratus  clouds. During the day, an indicator of altostratus is the absence of shadows on the ground. If the sun is seen through a stratiform cloud and shadows are present on the ground, the cloud could be either altostratus or cirrostratus. However, if the cloud is dense enough to prevent shadows from forming, it should be classified as  altostratus.  Cirrostratus  is  never  dense  enough  to prevent  shadows  during  the  daylight  hours. The height of the base of the altostratus clouds may range from 6,500 feet to 23,000 feet. The density of the stratiform cloud is the primary determining factor of stratiform cloud typing in the 18,500 to 23,000 foot range, while the presence of the corona and halo may be used as reliable secondary indicators. There  are  no  species  associated  with  altostratus clouds,  although  there  are  several  different  varieties. Nimbostratus  (NS) Usually formed from altostratus clouds thickening downward, nimbostratus, commonly called "the rain cloud," ranges in color from medium to very dark gray, with a diffuse, indefinite base. It is always thick enough to obscure the sun and is almost exclusively found near frontal zones. Stratus fractus clouds are commonly found under nimbostratus cloud layers, especially just prior to the start of precipitation and during light precipitation. The stratus fractus tend to dissipate during heavier precipitation. Nimbostratus clouds may also form from dissipating cumulonimbus clouds. Although nimbostratus is classified as a mid-etage cloud, its base often lowers well into the low-etage. With approaching occluded and warm frontal systems, nimbostratus may lower to within several hundred feet of the ground. Nimbostratus bases with stationary fronts  tend  to  be  slightly  higher. Normally,   altostratus   is   reclassified   as nimbostratus when the cloud base becomes very dark or stratus fractus clouds are observed under the base of the layer.  Altostratus  clouds  must  be  reclassified  as nimbostratus when precipitation begins or when bases drop to less than 6,500 feet. Nimbostratus clouds are usually  distinguished  from  opaque  altostratus  clouds  by the more diffuse, but denser and darker appearance of the base, which is often described as appearing "wetter" than  altostratus. The genus nimbostratus has no distinct species or varieties. Altocumulus  (AC) Altocumulus clouds are composed of super-cooled water droplets and ice crystals when located above the freezing level. Altocumulus clouds look very similar to stratocumulus clouds; the primary difference in their classification is by height, which may be inferred by the size of the elements in the cloud. We have already discussed  how  to  differentiate  between  stratocumulus and  altocumulus  clouds  based  on  the  size  of  the elements. Unfortunately, the altocumulus clouds in the middle to upper portion of the mid-etage (fig. 1-22) and the still higher cirrocumulus clouds of the high etage (fig. 1-23) also look very similar. If the cloud elements are larger than the width of one finger held at arm’s length, the cloud should be classified as altocumulus. If the individual cloud elements are smaller than the width of a finger held at arm’s length, the cloud should be classified as cirrocumulus. Do not use this method unless the cloud in question is more than 30° above the horizon. Altocumulus clouds appear white to light gray, or mottled with shadings between white and light gray. When  altocumulus  clouds  do  not  present  a  uniform appearance, you should consider other identifying features. Virga may occur from altocumulus clouds, but the trails appear shorter than those associated with stratocumulus. Based on the known height of the freezing level, the bent virga trails associated with frozen precipitation may indicate whether the cloud is high  enough  to  be  altocumulus. The  presence  of  a  corona  is  most  frequently associated with altocumulus clouds, even more so than 1-20