Stratus (ST)

mid-etage (fig. 1-20). Because of their greater height, the elements of altocumulus clouds appear smaller than stratocumulus cloud elements. The best way to judge whether a cloud is altocumulus or stratocumulus is by obtaining the height from the ceilometer or other cloud- height sensors. Alternatively, an estimate of the size of the individual elements is a reliable indicator. Individual elements of stratocumulus clouds are wider than 5°, while the individual elements of altocumulus are narrower than 5°. The width of your three middle fingers held at arm’s length is about 5°. Precipitation rarely occurs with stratocumulus clouds. When it does occur, it tends to be very light and intermittent. Light snow showers (snow flurries) are the most common form ofprecipitation from stratocumulus clouds. Due to the light intensity of stratocumulus- produced precipitation, more often than not any precipitation will be in the form of virga. The major species of stratocumulus clouds are stratocumulus floccus, stratocumulus castellanus, and stratocumulus stratiformis. STRATOCUMULUS FLOCCUS.— Sirato- cumulus floccus clouds appear as tightly-packed, flattened cloud cells in a layer, with higher clouds or blue sky visible in the area between the cells. Individual cells have very indistinct, feathery edges. STRATOCUMULUS CASTELLANUS.—In the species stratocumulus castellanus, the individual cloud cells develop in a stratiform layer. These cells usually have rather indistinct edges and scattered cumuliform buildups protruding upward from the generally flatter tops of the stratiform cloud layer. These buildups are commonly in the form of small towers or tufts. STRATOCUMULUS STRATIFORMIS.— When a layer of cumulus humilis or cumulus mediocris cloud cells begins to lose the clear definition on the edges, such as is seen beginning at point C in figure 1-7, the layer is classified stratocumulus stratiformis. As development continues, these cells may merge, completely tilling in the clear gaps between the cloud cells. Stratus (ST) Occasionally, a layer of stratocumulus is mistaken for a stratus cloud. The base of a stratus cloud has a very smooth and uniform appearance, often appearing fuzzy or indistinct. A stratus cloud base may have slightly darker areas, but the darker patches are not arranged in any regular pattern. Stratus usually forms very close to earth's surface, and is called fog when it is in contact with or within 50 feet of the surface. Stratus clouds may form in conjunction with other higher clouds. Figure 1-20.—Lower mid-etage altocumulus cloud. 1-18