| Section C | C index | 521-529 of 1157 terms |
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cloud modification—Any process by which the natural course of development of a cloud in the earth's atmosphere is altered, for example, by the exhaust from an aircraft engine or smoke and heat from a forest fire.
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cloud motion vector—The speed and direction determined from tracking clouds in satellite imagery. If the clouds tracked are passive, that is, neither growing nor decaying, then the vector approximates the wind vector.
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cloud optical depth—The vertical optical thickness between the top and bottom of a cloud. Cloud optical depths are relatively independent of wavelength throughout the visible spectrum, but rise rapidly in the infrared due to absorption by water, and many clouds approximate blackbodies in the thermal infrared. In the visible portion of the spectrum, the cloud optical depth is almost entirely due to scattering by droplets or crystals, and ranges through orders of magnitude from low values less than 0.1 for thin cirrus to over 1000 for a large cumulonimbus. Cloud optical depths depend directly on the cloud thickness, the liquid or ice water content, and the size distribution of the water droplets or ice crystals.
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cloud physics—The body of knowledge concerned with physical properties of clouds in the atmosphere and the processes occurring therein. Cloud physics, broadly considered, embraces not only the study of condensation and precipitation processes in clouds, but also radiative transfer, optical phenomena, electrical phenomena, and a wide variety of hydrodynamic and thermodynamic processes peculiar to natural clouds. Cloud physics is a distinct subdivision of physical meteorology. Early interest in this subject was stimulated by the role of clouds in aircraft safety related to icing and turbulence (The Thunderstorm Project) and the discovery of cloud modification techniques by cloud seeding. The formation of precipitation and the influence of clouds in radiative processes in the atmosphere (both solar and thermal) determine the key role of the subject in global climate.
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cloud radar—A radar optimized to detect clouds. This is usually a short wavelength radar often called millimeter radar, since its wavelength is usually near 8 mm (Ka band) or 3 mm (W band). Such shorter wavelengths give radars an advantage in cloud detection because of the radar scattering cross section for small particles varying as the inverse radar wavelength to the fourth power. Because attenuation caused by atmospheric liquid and from water vapor is serious at these short wavelengths, such radars are usually operated at ranges closer than about 20 km. They can be either of the scanning type or fixed in the vertical. Their good spatial resolution allows them to depict finescale cloud features and, when used in combination with other active (e.g., lidar) and passive (microwave and IR radiometers) sensors, can, under some conditions, be used to quantitatively map cloud ice and liquid water contents, particle sizes, and concentrations.
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cloud radiative forcing—The difference between net irradiances measured for average atmospheric conditions and those measured in the absence of clouds for the same region and time period. Cloud radiative forcing depends jointly on the amount of cloud present and the sensitivity of radiation to cloud amount. It may be partitioned into longwave and shortwave forcing terms, the combination of which typically results in a negative net forcing when referenced to the top of the atmosphere (i.e., satellite measurements). That is, a clear region typically reflects less solar, and emits more terrestrial, radiation than does the average condition, and the difference in solar reflection is typically greater than the difference in terrestrial emission to space. While the definition of cloud radiative forcing in terms of average measured values is unambiguous, the relationship between cloud radiative forcing and the equilibrium effects of clouds on climate, especially on surface temperature, is a complicated topic. See also radiative forcing.
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