INFERENCE OF FOG CHARACTERISTICS FROM ATTENUATION MEASUREMENTS AT MILLIMETER AND OPTICAL WAVELENGTHS

Measurements of fog characteristics are rare, and therefore models for predicting the attenuation caused by fog on the propagation of electr omagnetic radiation are scarce. Using experimental transmissions throu gh moderate radiation fogs on a 500-m path, a method is developed to i nfer the physical characteristics of fog from concurrent attenuation m easurements at millimeter, infrared, and visible wavelengths. It makes it possible to retrieve the fog drop size distribution, as well as it s time evolution, during the life cycle of a fog event. It yields real istic values for fog features, such as liquid water content and averag e drop diameter, which are not easily measured. Furthermore, on an exp erimental basis, simple relationships between inferred fog characteris tics and measured attenuation are proposed. It is found that while bot h millimetric and infrared attenuations are directly related to liquid water content, visible attenuation strongly depends on the size of th e fog droplets. Overall, using these relationships together with stati stical distributions of inferred fog characteristics enables predictio ns of fog attenuation at any wavelength from millimetric to visible.