Blurring the boundaries between Standard General Sky types due to multiple scattering of light

Abstract

The diffuse light reaching the ground varies with instantaneous physical state of the atmosphere, specifically with atmospheric turbidity, vertical stratification of attenuating and scattering constituents, cloud coverage and many others. Despite this complexity, a set of empirical models based on so-called homogeneous sky types is widely used to simulate the real lighting conditions. However, the sky type classification can be influenced by multiple scattering of light. Even if multiple scattering is essentially important under low elevation angles and high turbidity conditions, it is quite often ignored -e.g. when modelling the luminance patterns. As a consequence, the interpretation of experimental data can suffer from the omission of multiple scattering effects. Since multiple scattering smoothes the luminance patterns, the neighbouring sky types may be subject to blurring, and thus it might be difficult to distinguish them in automated data processing. In this paper we introduce a model that lowers the uncertainties caused by multiple scattering, so the subsequent numerical routines will definitely profit from more accurate identification of sky types.

Estimating linear radiance indicators from the zenith night-sky brightness: on the Posch ratio for natural and light-polluted skies

“…The scattered term can be calculated from the extra-atmospheric spectral radiance of the Moon by using theKocifaj-Kránicz (2011) model for the spectral radiance of the sky, applied here to the Moon, not to the Sun as in the original authors' work. These type of sky models, based on a two-parameter continuous function defined on the upper hemisphere(Kocifaj 2009), provide a rigorous physical basis to other commonly used daylight sky radiance distributions(Kittler 1998;Kocifaj 2013;Kittler & Darula 2021;Kocifaj & Kómar 2021), and have also proven to be particularly well adapted to compute the sky radiance due to ground-based artificial light sources(Kocifaj & Bará 2019).…”