On the Light Attenuation and Visibility in Snow Drift

Abstract

Using transmissometers the study on the light attenuation and visibility in snow drift was made in the winter season of 1974 and 1975 in the Ishikari Plain, Hokkaido. The modulated visual light traversed through a path of 3 m long. Visibility observations by eye were carried out, using as targets 7 sheets of black boads installed at 150 m distant from observation point. The concentration of the snow particles was evaluated by the mass transportation of snow particles in drift traps at the same levels of the transmissometers.In general, the attenuation of light by airborne snow particles was explained by Mie's scattering theory. According to Mie's theory, it yields the scattering area ratio equal to 2. However, the size of airborne particles was large enough compared with the wavelength of light and the scattering area ratio of snow particles showed smaller than 1. Therefore, the light was attenuated not only by the scattering of snow particles but also mainly by their reflection.The relation between the concentration of airborne snow particles and the extinction coefficient of light was examined in the cases of drifting snow and falling snow. The result showed that when the extinction coefficient of light in drifting snow was about 5 times larger than that of falling snow under the condition of the same concentration of the particles, the particles of drifting snow are usually rounded and the size of them are smaller than the particles of falling snow. Therefore, it was considered that the extinction coefficient depended on the size and shape of the airborne snow particles.The visibilities in drifting snow and falling snow were measured under the condition of the same spatial mass concentration. The measurement showed that the visibility in drifting snow decreased 12 times less than that of falling snow and the visibility was determined by only the mass transportation of snow particles. Therefore, it was concluded that in the case of airborne particles having finite size such as snow particles was not explained by a homogeneous attenuation of light introduced by Koschmieder, but by the influence of visible particles themselves and their afterimages.