An Assessment of Night Sky Visual Quality in U.S. National Parks

Background: Artificial light at night affects locations distant from its source through light scattered back to earth from the sky (sky glow). Sky glow is visible in protected and undeveloped areas hundreds of kilometers from large cities. It degrades the scenic quality of the night sky, and may alter nocturnal environments and the visual performance in all organisms. A new world atlas of artificial sky brightness recently (July 2015) has provided an updated estimate of its geographic extent and severity. The U.S. National Park Service has collected night sky brightness data at hundreds of sites in public protected areas over the past decade. The integration of these two data sources allows a landscape scale assessment of the potential impact of sky glow on national park lands. Methods: All-sky photometry was obtained from 170 field locations throughout the United States, resulting in image mosaics of the celestial hemisphere. Numeric indicators of artificial sky glow, including zenith brightness and average all-sky brightness, were derived from these images. These field measures were used in the calibration of a continuous gradient map of artificial sky brightness (the new world atlas). The atlas is based upon recent information from the Suomi NPP satellite's VIIRS day/night band senor and a physical model of the scattering of upward emitted light at night by the atmosphere. A digital overlay of the protected area boundaries with the predicted sky luminance map resulted in the extraction of area-based descriptive statistics of the potential impact to these lands. Results: The majority of U. S. National Parks exhibit visible alterations to the night sky and ambient illumination of the ground. There are a few remaining locations in the western U.S., Alaska, and Hawaii where outstanding examples of nearly pristine skies exist in several park units. The impacts are most severe near cites; Lake Mead National Recreation Area is an example of a park area with a very large gradient of impact because of the proximity of its western boundary to the Las Vegas, Nevada, metropolitan area, while the eastern boundary abuts a large unpopulated area. Some park areas, such as Walnut Canyon National Monument, near Flagstaff, Arizona, demonstrate that excellent visual sky quality is possible near cities of moderate size. Conclusions: Light pollution is degrading the integrity of protected areas throughout the continental United States. Emergent opportunities to improve the electrical efficiency of outdoor lighting hold great promise for reducing light pollution, though realization of these benefits will require careful attention to the spectra of new light sources and a conservative approach to the amount of light required to meet human needs.  Present and future availability of VIIRS day/night band satellite data along with continued field data collection of all-sky photometry will allow the tracking of trends in light pollution geographically, and point to specific areas where restoration of sky quality is taking place.