High correlation but high scale-dependent variance between satellite measured night lights and terrestrial exposure

Exposure to artificial light at night (ALAN) is a significant factor in ecological and epidemiological research. The levels of exposure used in such studies are frequently estimated from satellite-based measurements of upward radiance. Although the correlation between upward radiance and zenith sky brightness is established, the correlation between light escaping upward and the light experienced from all directions on the ground has not been investigated. Because ground-based exposure to light at night can depend on local glare sources in addition to scattered light in the atmosphere, ecological and epidemiological studies using upward radiance have relied on an untested relationship. To establish the nature of the relationship between upward radiance and hemispherical (scalar) illuminance on the ground and to calibrate future experimental and laboratory studies of ALAN, we used hemispheric digital photography to measure scalar illuminance (SI) at 515 locations along the coast of southern California, and compared those values to geographically co-located satellite-based measures of upward radiance as described by the Visible Infrared Imaging Radiometer Suite (VIIRS) satellite and zenith downwards radiance as described by the World Atlas of Artificial Night Sky Brightness (WAANSB). We found significant variations in SI within the geographic scale defined by the resolutions of both VIIRS and the WAANSB (742 m), as well as both luminance and color correlated temperature (CCT) across individual image hemispheres. Despite these limitations, we were able to construct a simple linear model of log(SI), a measure of brightness as experienced in the field, and two parameters: the percent of the image hemisphere obscured by structures along the horizon (percent horizon) and log(WAANSB). This model accounts for 76% of observed variation in log(SI), but with up to two or more orders of magnitude variation in ALAN exposure within any given satellite-measured unit.