Effect of cloud micro-physics on zenith brightness in urban environment

Abstract

Cloud cover is amplifying light pollution for observers located within light-emitting cities. In addition to light scattering and absorption below the cloud layer, artificial light from the ground is reflected, transmitted, and absorbed by the cloud layer itself. The sky brightness on a cloudy night is determined by cloud attributes such as cloud base height and reflectance. Satellite observations provide enough information about cloud micro-physical properties to compute cloud reflectance accurately.These characteristics include cloud optical thickness, particle phase, effective particle size, water path, and cloud base height. They can be obtained from satellites through direct measurements at various wavelengths of the visible and infrared spectrum or acquired empirically using other measured quantities. The method for obtaining cloud properties, as well as calculating cloud reflectance for numerical modeling of zenith brightness, are provided in the paper. The modeled zenith sky brightness is compared to measurements for selected sites with known cloud features.As a result, the zenith brightness is explicitly dependent on two co-playing parameters: cloud base height and optical thickness. While the higher the optical thickness, the brighter the zenith, the cloud base altitude has the reverse effect. The higher the cloud ceiling, the darker the zenith.