lenticularis species is discussed in a following section on "Orographic Clouds." ALTOCUMULUS    CASTELLANUS.— Altocumulus castellanus cloud elements build upward from the base to form towers, tufts, or "turrets." The tops and edges of the buildup may appear ragged, and not   have   the   smoother   rounded   appearance   or cauliflower-like  tops.  The  ragged  tops  are  sometimes described as looking similar to the turrets on a medieval castle,  which  gives  some  reference  for  the  name “castellanus.”    Usually originating in the lower portion of the mid-etage, these clouds may build upwards to moderate or great extent, and are similar to towering cumulus, except with high bases and smaller appearing elements.   Continued   development   may,   in   rare situations,  produce  cumulonimbus  clouds.  Virga  is common, and even light precipitation reaching the ground is not uncommon with altocumulus castellanus. ALTOCUMULUS FLOCCUS.—Related to the altocumulus  castellanus  is  the  less  developed altocumulus floccus cloud form.  Altocumulus floccus resemble small, ragged cumulus humilis and typically appear as small tufts of white cloud with rounded or bulging tops. They often have small fibrous trails of virga extending from ragged bases. Both altocumulus castellanus   and   altocumulus   floccus   indicate approaching frontal systems with conditions favorable for  thunderstorm  activity. ALTOCUMULUS    STRATIFORMIS.— Altocumulus stratiformis is by far the most common form of altocumulus. In this form we typically see an extensive layer of cloud with smooth, evenly spaced rounded cells or just a cell-like pattern in a generally stratiform  layer.  Figure  1-22,  a  typical  altocumulus cloud, is a good example of the species altocumulus stratiformis. Cirrus (CI) Cirrus  clouds,  a  high-etage  cloud  type,  form generally between 16,500 feet and 45,000 feet in the mid-latitudes.  Cirrus  clouds  are  composed  of  ice crystals. CIRRUS UNCINUS.—The most common form of cirrus is the thin strand-like wisps of cloud filaments, often curved at on end and described as hook-shaped and called cirrus uncinus (fig. 1-24). These cirrus clouds are popularly called "mare’s tails" because of their resemblance to the tail of a galloping horse. CIRRUS  SPISSATUS.—The  dense  blow-off from the top of a cumulonimbus, which looks similar to stratus or altostratus clouds, is called cirrus spissatus, but is often referred to as dense cirrus (refer to figure 1-13). Although this cloud typically presents a strati- form appearance, it is not called cirrostratus, because, by definition, cirrostratus is never dense enough to hide the sun. Cirrus spissatus usually forms in a single large patch with a distinct edge. This cloud is reclassified as altostratus when the base lowers to less than 23,000 feet. A different variety of cirrus spissatus also forms from  phenomena  that  have  nothing  to  do  with cumulonimbus  blow-off  or  dissipating  cumulonimbus cells. When dense cirrus is formed by other means than by  cumulonimbus  blow-off  or  dissipating cumulonimbus clouds, it will frequently be seen as many dense patches at different levels (cirrus spissatus duplicatus), often mixed with thin cirrus filaments. Another variety, cirrus spissatus intortus, is sometimes described as looking like "entangled sheaves" of cirrus clouds.  When  viewed  toward  the  sun,  the  denser patches often have gray bases. CIRRUS   FLOCCUS   AND   CIRRUS CASTELLANUS.—Patches of dense cirrus may take on the form of cirrus floccus, with the upper portion of the patch forming rounded tufts, and the base portion becoming ragged. Dense cirrus patches may also grow turrets  or  battlements  and  become  cirrus castellanus. Both cirrus floccus and cirrus castellanus may have ice crystal virga trails showing from the base of the cloud patch, and may be slightly larger than the standard 1°. (Also  see  cirrocumulus  floccus  and  cirrocumulus castellanus.) Cirrostratus  (CS) Cirrostratus clouds usually appear as a thin white veil over the sky.  If the cirrostratus clouds are very thin and of uniform thickness, the only indication of their presence may be a faint halo, or a whitish tint to the sky. As long as the sun is higher than 30° above the horizon, cirrostratus clouds should not be able to block the sun; shadows should be apparent from sunlight shining through this cloud. When cirrostratus is low on the horizon, it tends to block the blue color of the sky more thoroughly because it is viewed on an angle, and it is commonly mistaken for denser altostratus. This also happens near sunrise and sunset with low sun angles. Cirrostratus cloud layers appear to move very slowly, and change shape very slowly. Typically, the edge of a cirrostratus layer is so indistinct that it is difficult to detect where the blue sky ends and the cloud begins. If movement or changes in shape are detectable during the observation period, the cloud near the horizon may well be  altostratus. 1-22