and severity of these local climatic impacts appear to be largely a function of site-specific criteria such as local meteorology, the magnitude of heat release, and the means of heat rejection. A considerable amount of work has been done to characterize the atmospheric impacts of cooling towers. Most cooling towers are, and most likely will continue to be, "wet" towers, in which heat exchange occurs by evaporation from countless water droplets generated by splashing warm water over successive barriers. There are two major types of cooling towers in use today: mechanical-draft and natural-draft. In mechanical-draft towers, large fans force the vertical air flow, whereas in natural-draft towers, the great size of the tower causes vertical air flow to develop without a fan because of density gradients. Of the two types of cooling towers, the natural-draft tower appears to cause less serious local impacts. In the United Kingdom, it is felt that the impact of these towers on local climate has been negligible throughout 50 years of operational experience.29 Although visible plumes of water droplets frequently occur near natural-draft cooling towers, ground fog due to downwash occurs infrequently.30-32 fact, £n many cases, downwind measurements of ground-level relative humidity have shown no measurable increase.29 The output of heat and moisture from natural-draft cooling towers is believed to enhance development of cumulus clouds, particularly when the atmosphere is unstable or conditionally unstable. In several cases, anomalous precipitation events have been observed within a few kilometers of large heat and moisture releases from natural-draft towers.33-35 However, measurements from local weather stations taken over several years have not shown that statistically significant increases in precipitation occur near large cooling towers.36,37 Drops of cooling water can be carried out of the top of the tower as water splashes over the heat exchange surfaces. These drops can contain inpurities such as salts and fungicides and can damage local vegetation. This phenomenon, called "drift deposition," may be an important environmental impact but should not significantly change the local climate. Mechanical-draft cooling towers are much more likely to produce fog because they are not so tall as natural-draft towers. Fog is common within a few kilometers of these towers at wind speeds of 3-5 m/s or greater, due to
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