Brightness of Clouds at Night over a City
Abstract
I have completed my program of calculations on the brightness of clouds at night over a city. To simplify the calculations I took the simple model of a uniformly illuminated circular city. I took my light pollution model, which was originally designed to calculate the sky brightness at a distant observatory caused by a city, and modified the program to calculate the sky brightness at the zenith as seen from the center of a circular city. I assumed a uniform horizontal cloud layer, described by the height of the cloudbase above the ground and the total optical depth of the cloud layer. Three methods of calculating the reflectivity and transmissivity of the clouds were tried; most of the results were obtained by using the doubling method as implemented by Wiscombe. The reflectivity and transmissivity were incorporated into the light pollution model. The brightness of the clouds at the zenith as seen from the city center was calculated, along with the brightness seen from a satellite in the nadir direction towards the city center. Calculations were performed for various parameter choices to get a feel for the importance of various parameters. The city population, city radius, ground reflectivity, light emission from the city per capita, the extent of light escaping from inadequately shielded lamps, height of the cloudbase, the single scattering coefficient in the clouds, and the total optical depth of the clouds were varies and the brightness calculated. The most critical parameters were the population density of the city and the total optical depth of the cloud layer. The paper has been accepted for publication in The Observatory for February 2007.
- Publication:
-
American Astronomical Society Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AAS...20915407G