Chapter 5 - Human Perception of Haze and Landscape Features

Abstract

The chapter starts with an outline of the perceptibility issues associated with the seeing of haze as well as viewing landscape features through haze. The underlying question is one of how various visibility metrics relate to the threshold and suprathreshold problems and to various indicators of judgments of visual air quality, such as scenic beauty and levels of visibility impairment, that are found to be unacceptable. A number of studies have been carried out to address these questions and many of these efforts are reviewed. The review starts with the more fundamental colorimetric measurements and then progresses through other studies on judged visual air quality. Then, various threshold studies are discussed that were directed at detecting the level of contrast required to just see an industrial plume, such as those associated with coal-fired power plants. Finally, two just-noticeable-change calculations are presented that are based on colorimetric and visual air quality judgment studies. Important conclusions are that it would take about a 10–15% change in extinction (1.0–1.5 dv) to be noticed, the threshold contrast of large, dark objects as seen against the horizon sky is about −0.02, and a full-length plume with a Gaussian profile subtending approximately 0.33° with a contrast of −0.01 could be noticed. When viewing geometry is such that a plume is viewed “end on,” contrast threshold increases to about 0.04.