Often the undersides of arc clouds are dark and turbulent-looking, while their tops are smooth. 14.4), usually associated with the flanking line. These cloud bands mark the leading edge of gust-front outflow from the rear-flank downdraft (Fig. 14.2b) or shelf clouds form near the ground in boundarylayer air that is forced upward by undercutting cold air flowing out from the thunderstorm. Not all thunderstorms have all these associated clouds.Īrc clouds (official name arcus, Fig. Sometimes you can see other clouds attached to thunderstorms, such as a funnel, wall, mammatus, arc, shelf, flanking line, scud, pileus, dome, and beaver tail (Fig. Figure 14.3 Photo of supercell thunderstorm.ġ4.1.2. The most severe, long-lasting, less-frequent thunderstorms are supercell thunderstorms (Figs. More complex thunderstorms can have one or more updraft and downdraft regions. However, in this book we will use the word thunderstorm to mean any cumulonimbus cloud, regardless of whether it has lightning. Such storms are technically not thunderstorms. Not all cumulonimbus clouds have lightning and thunder. When viewed from the ground under the storm, the main updraft often has a darker cloud base, while the rainy region often looks not as dark and does not have a well-defined cloud base. When the downdraft air hits the ground it spreads out, the leading edge of which is called the gust front. Also in the stem is a downdraft with precipitation. When this rising air hits the tropopause, it spreads to make the anvil. Within the stem of a mature thunderstorm is the cloudy main updraft tower topped by an updraft bubble (Fig. (c) Horizontal composite, showing the anvil at storm top (as viewed from above by satellite), the precipitation in the low-to-middle levels (as viewed by radar), and the gust front of spreading winds at the surface. Light shading indicates clouds, green and red shadings are moderate and heavy precipitation, and arrows show air motion. Figure 14.2 (a) Sketch of a basic (airmass) thunderstorm in its mature stage. For a storm with a larger anvil that looks strongly glaciated (i.e., has a fibrous appearance associated with ice-crystal clouds), then you would call the cloud a cumulonimbus capillatus. If the thunderstorm top is just starting to spread out into an anvil and does not yet have a fibrous or streaky appearance, then you identify the cloud as cumulonimbus calvus (see the Clouds Chapter). Figure 14.1 Airmass thunderstorm having a single mature cell. The anvil extends furthest in a direction as blown by the upper-tropospheric winds. The large top is called the anvil, anvil cloud, or thunderhead, and has the official name incus (Latin for anvil). 14.1
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