Rangno, in Encyclopedia of Atmospheric Sciences, 2003 Middle-Level CloudsĪltocumulus, altostratus, and nimbostratus clouds ( Figures 4, 5, and 6, respectively) are considered ‘middle-level’ clouds because their bases are located between about 2 and 7 km AGL (see discussion concerning the variable bases of nimbostratus clouds below.) These clouds are generally the product of slow upcurrents (centimeters per second) taking place in the middle troposphere over an area of thousands of square kilometers. As background for those discussions, the present chapter isolates and discusses certain generic properties of the nimbostratus clouds that occur within these cloud systems.Ī.L. Chapters 9–12 Chapter 9 Chapter 10 Chapter 11 Chapter 12 will discuss the structure and dynamics of the clouds of these large and complex types of storm systems individually in detail. 1 Sometimes they are produced or enhanced by widespread upslope flow over mountains or other terrain features.
They usually do not occur independently but rather are spawned by organized storms, namely, mesoscale convective systems, tropical cyclones, and frontal cloud systems (e.g., Figures 1.30–1.33 Figure 1.30 Figure 1.31 Figure 1.32 Figure 1.33). Nimbostratus clouds (defined and discussed briefly in Section 1.2.2.2) occur extensively in both midlatitudes and tropics. In this chapter, we consider some basic characteristics of the deeper nimbostratus clouds that produce larger particles and substantial amounts of rain or snow. The few particles that grow large enough to precipitate from stratus and stratocumulus usually do so before the particles exceed the size of small drizzle or tiny snow crystals. Their depth and robust precipitation production distinguish them from the shallow stratus and stratocumulus clouds considered in Chapter 5, which do not have sufficient vertical extent to produce much precipitation. Nimbostratus clouds are produced by nearly thermodynamically stable air motions and are deep enough to allow precipitation particles to grow to the sizes of raindrops and snowflakes. Houze Jr., in International Geophysics, 2014
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