| Section C | C index | 541-549 of 1157 terms |
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cloud-top entrainment instability—A condition whereby entrainment of dry air into a cloud top causes even more entrainment, leading to the dissipation of the cloud. When the entrained air mixes with cloudy air, evaporation of the cloud drops into the mixture causes the mixture to cool. As this cool mixture sinks, it generates a turbulent circulation that can cause more entrainment, thereby continuing the process until the cloud dissipates. While there is still debate about the requirements for such a process to occur, one of the first suggestions was that this instability will occur when Δθe < Δθe critical, where Δθe is the difference of equivalent potential temperature from just above to just below cloud top, and Δθe critical is a critical value that is near zero. While the exact value of Δθe critical has yet to be determined, it depends on the buoyancy of the air (i.e., on virtual potential temperature) and on the temperature change possible due to latent heat changes when cloud droplets evaporate.
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cloud tracer—Cloud movement that is used as a measure of air motion.
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cloud winds—Wind estimates based on the observed movements of identifiable cloud elements over short time intervals.
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cloud—1. A visible aggregate of minute water droplets and/or ice particles in the atmosphere above the earth's surface. Cloud differs from fog only in that the latter is, by definition, close (a few meters) to the earth's surface. Clouds form in the free atmosphere as a result of condensation of water vapor in rising currents of air, or by the evaporation of the lowest stratum of fog. For condensation to occur at a low degree of supersaturation, there must be an abundance of cloud condensation nuclei for water clouds, or ice nuclei for ice-crystal clouds, at temperatures substantially above −40°C. The size of cloud drops varies from one cloud type to another, and within any given cloud there always exists a finite range of sizes. Generally, cloud drops (droplets) range from 1–100 μm in diameter, and hence are very much smaller than raindrops. See cloud classification. 2. Any collection of particulate matter in the atmosphere dense enough to be perceptible to the eye, as a dust cloud or smoke cloud.
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cloudbow—(Also called fogbow, mistbow, white rainbow.) A large, faintly colored, circular arc formed by light (usually sunlight) falling on cloud or fog. The apparent center of the cloudbow is the antisolar point (the shadow of the observer's head or possibly the shadow of the plane in which the observer is flying). The term rainbow is a general term for this phenomenon, a cloudbow merely being a rainbow formed in the smaller cloud or fog droplets. The broad whitish appearance of the cloudbow and slightly smaller angular radius certainly demonstrate that an explanation that treats light as a series of rays does not work very well for small droplets.
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cloudburst—(Also called rain gush, rain gust.) In popular terminology, any sudden and heavy fall of rain, almost always of the shower type. An unofficial criterion sometimes used specifies a rate of fall equal to or greater than 100 mm (3.94 inches) per hour. See excessive precipitation.
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