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 multiwavelength 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 of atmospheric 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 of data from summer 1978 through 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.