An Empirically Derived Adjustable Model for Particle Size Distributions in Advection Fog

Abstract

Realistically modelled atmospheric phenomena are a long-standing research topic in rendering. While significant progress has been made in modelling clear skies and clouds, fog has often been simplified as a medium that is homogeneous throughout, or as a simple density gradient. However, these approximations neglect the characteristic variations real advection fog shows throughout its vertical span, and do not provide the particle distribution data needed for accurate rendering. Based on data from meteorological literature, we developed an analytical model that yields the distribution of particle size as a function of altitude within an advection fog layer. The thickness of the fog layer is an additional input parameter, so that fog layers of varying thickness can be realistically represented. We also demonstrate that based on Mie scattering, one can easily integrate this model into a Monte Carlo renderer. Our model is the first ever non-trivial volumetric model for advection fog that is based on real measurement data, and that contains all the components needed for inclusion in a modern renderer. The model is provided as open source component, and can serve as reference for rendering problems that involve fog layers.