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R. H. Garstang (University of Colorado)
I have started a program to calculate the brightness of a layer of clouds over a city at night (in spite of the fact that astronomers do not observe on cloudy nights!). There seems to be a lack of information on the subject in the literature. I am combining the scattering properties of clouds with my light pollution model of a city. I have adapted my light pollution model to work for an observer situated near the center of a city and observing near the zenith. For simplicity I have used my original flat Earth model (Garstang 1986), and I treat the cloud as if it were a reflecting layer at the height of the cloud base. There are a number of models of radiative transfer in clouds that can be used to give approximate downward radiative intensities in any direction from a patch of a cloud layer when light shines in many directions from a large city onto the patch of the cloud layer. I have used the simple Eddington radiative transfer approximation as developed by Shettle and Weinman (1970), and I have also used the scattering matrices given by Twomey, Jacobowitz and Howell (1976). I hope soon to try two other models. Preliminary results suggest that for a city with the population of metropolitan Denver, treated as a circle of radius 12 km, the zenith night sky brightness when there is no snow on the ground is about V=14.3 mag/sec2, equivalent to 0.2 cd/m2, roughly 1000 times the natural night sky brightness near sunspot minimum. When the ground is snow covered the brightness may be as much as V=12.3 mag/sec2, equivalent to 1.2 cd/m2.