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R. H. Garstang (JILA, University of Colorado and NIST)
Several people have suggested that in cloudy climates the brightness of the clouds over a city at night might be measured repeatedly over many years and used as a monitor of light pollution. With this in mind I have extended my earlier calculations on the brightness of clouds at night. I have used my light pollution model, adapted to an observer at the center of a circular city observing at the zenith. The city is assumed to have circular symmetry, but the brightness of the city lights may be a function of the distance from the city center. I assumed a normal brightness of 1200 lumens per capita. The doubling method of Wiscombe was used in calculating the reflectivity of the clouds. The brightness was calculated as a function of the optical depth of the cloud layer. Calculations were performed for two “test” cities, one with population 100,000 and radius 5 km, the other with population 1 million and radius 15.81 km (i.e. same population density, and hence same luminous output per unit area). The average brightness was calculated for a distribution of optical depths using data reported by observers in Russia. For the smaller “test” city the average brightness is 0.061\rm\,cd/m2. The effects on the brightness were also calculated when increased luminous output was present in various zones. Additional light near the center of the smaller city has about 200 times the effect of the same additional amount of light at the periphery. The principal conclusions were (1) the brightness seen by the observer only increases modestly for a city population 10 times larger, (2) for optical depths of 20 or more the brightness is not strongly dependent on the optical depth of the clouds, and (3) the brightness is relatively sensitive to the luminous output near the observer. Other factors are being examined.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.