Orographic Clouds

floccus species name is used. When the virga trails make the entire element larger than 1°, the cloud must be classified cirrus floccus. CIRROCUMULUS CASTELLANUS.—Cirro- cumulus castellanus identifies the cirrocumulus cloud layers where each element shows a cumuliform buildup such that the height of the element is greater than the width of its base, and each element is smaller than 1°. If the vertical development of the cloud elements progresses such that the element becomes larger than 1°, the cloud is reclassified cirrus castellanus. Now let’s consider some cloud types we haven’t covered in table 1-1. Orographic Clouds Several species of low-, mid-, and high-etage clouds are associated only with moist airflow over mountainous areas. These clouds usually form during Mountain Wave conditions, when strong winds blow across mountain ranges. The presence of these clouds is significant in that they may be associated with dangerous turbulence. All of the orographic cloud forms are unique in that they are stationary over a particular area and do not move with the wind flow. Slow changes in the arrangement of elements or the cloud pattern may be noted as the upper wind direction or intensity changes. The significant orographic cloud forms are the rotor cloud, the cap cloud, and the lenticular clouds. The rotor cloud (fig. 1-25) is formed downwind from the mountain range. The rotor cloud is formed as the strong winds moving across the mountains set up a wavelike action in the winds downstream from the mountain. The waves or eddies in the air may extend in a series downstream from the mountain for hundreds of miles. The upward moving air in the waves or eddies, if moist enough, is brought to saturation as it rises, forming the rotor clouds. Rotor clouds are cap-shaped, with smooth rounded tops, flattened or concave bases, ragged up-wind edges, and very ragged downstream edges. The up-wind edge continuously forms, while the downstream edge continuously dissipates. Rotor clouds may form as a few isolated elongated elements, or in succeeding bands elongated parallel to the mountain range. When you are observing the sky condition, rotor clouds are usually classified as cumulus clouds, and a note about the presence of rotor clouds may be added in the remarks section. A special type of stratocumulus cloud, called the Foehnwall or cap cloud, may form on the top of mountain ranges, resembling a "cap" on the mountain. It is formed as moist air is forced upward by the mountain top, and dissipates on the leeward side of the mountain as the moving air descends. Cloud particles and denser patches of cloud may be seen moving over the mountain and occasionally may be seen flowing down the leeward side of the mountain, giving the appearance of a waterfall. The cloud itself is stationary on the mountain top. Altocumulus lenticularis (abbreviated ACSL) clouds are typically described as lens-shaped, almond- shaped, or cat-eye shaped, and usually have a windswept appearance (fig. 1-26). Although the cloud elements may grow or shrink in size, they are usually stationary. The size of the individual cloud elements is usually quite large. The leading or windward edge Figure 1-25.—Rotor cloud-an orographic form of cumulus cloud. Figure 1-26.—Standing lenticular cloud. 1-24