Atrmwpheric Environment Vol. I&No. lO/lI, pp. 2031-2042. 1981 ooo4~9981/81!102031-12$02.00/O Printed in Gnat Britain. 0 1981 Perpmon Press Ltd. zyxwvutsrqpon VISIBILITY IN THE SOUTHWESTERN UNITED STATES FROM SUMMER 1978 TO SPRING 1979* WILLIAM C. MALM Advanced Monitoring and Systems Branch, Integrated Monitoring Systems Division, U.S. Environmen~l Protection Agency, Las Vegas, NV 89114, U.S.A. ERIC G. WALTHER Visibility Research Center of the John Muir Institute, Department of Physics, University of Nevada, Las Vegas, NV 89154, U.S.A. KENNETH O’DELL Northern Arizona University, Flagstaff, AZ 86001, U.S.A. and MARVIN KLEINE Visibility Research Center of the John Muir Institute, Department of Physics, University of Nevada, Las Vegas NV 89354, zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCB U.S.A. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFE (First received 3 September 1980 and injnolform 9 February 1981) Abstract-The Environmental Monitoring Systems Laboratory, U.S. Environmental Protection Agency, recognizing the importance of visibility to the experience of a visitor to Class I areas in the United States, entered into an agreement with the National Park Service to initially deploy an experimental research teleradiometer network at 13 national parks and monuments in the Southwest. Objectives of the network are to: (1) evaluate the ability of multiwavelen~h teleradiometers to measure visibility, (2) evaluate the ability of several physical variables to characterize visibility and (3) determine the temporal and spatial dynamics of visibility deterioration on a regional scale. Visibility is more than the ability to see an object at the distance at which it just disappears. Visibility includes the effects ofatmospheric constituents on the ability of an observer to see color, texture, and form of both near and distant vistas. Target apparent contrast and delta contrast are variables that can represent atmospheric visibility, but they are vista specific. Hence, standard visual range, a parameter which tends to normalize differences between targets, remains a useful interpretive parameter for comparing data from different vistas. Each park vista is also monitored with standardized photography, allowing a pictorial description of visibility. Analysis ofdata from summer 1978through spring 1979 shows that winter had the highest standard visual range and spring the lowest. Capitol Reef National Park had the best visibility, while Wupatki National Monument had the worst. Visibility on cloudless days was usually better than on cloudless and cloudy days taken together. In March of 1978, the United States Environmental Protection Agency’s Environmental Monitoring Sys- tems Laboratory, Las Vegas, Nevada, in conjunction with the National Park Service, established a south- west regional experimental visibility monito~ng network. Muitiwavelength teleradiometers, measuring visibility related variables, were initially operated by National Park Service personnel at 13 national parks or monuments. Additional locations were added later. The initial locations (see Fig. 10, Tables 1 and 3) extend from west Texas through View Mexico, northern Arizona, western Colorado, and into Utah. Major objectives of this program are to: (1) evaluate the ability of a teleradiometer to measure visibility; (2) determine which visibility-related variables * Paper presented at the Symposium on Plumes and Vi~b~jty: Measurements and Model Components. Grand Canyon, Arizona, U.S.A. 10-14 November 1980. effectively characterize the ability to “see” and (3) determine the existing regional and local character of visibility deterioration. THEORY Visibility is more than the farthest distance that a human observer can see a black target; it also involves the ability to see line, texture, and color of geologic structures located in near and middle distance vistas (U.S. Forest Service, 1973). Vista refers to the total view of sky, moun~ins and other features, while target refers to a specific feature in the vista, usually the top of a distant mountain. The ability to ‘*see” is determined by the ability of gas and particulate matter to scatter or absorb light as it passes through the atmosphere. The ability of the atmosphere to scatter light is pro- portional to the scattering coefficient, b_, while the absorption coefficient, bab,, is used to represent the atmosphere’s light absorbing capabilities. The sum of 2031