Visibility and Light Attenuation in Falling Snow

The attenuation of visible light by failing snow was studied by making simultaneous determinations of attenuation coefficients and snow concentration. The attenuation coefficient was calculated from photo-metric measurements and from visual observations. Snow concentration in the air was evaluated by two methods. Formvar replicas collected during the snowfall, and mass accumulation of snow in collecting pans. The snowflakes were arbitrarily classified by crystal types according to their estimated fall velocity. It was found that the correlation between extinction coefficient and snow concentration was generally much higher by types than when all snowflakes were considered together regardless of crystal components and degree of riming. When no fog is present during the snowfall, the experimental results for some snow types coincide well with attenuation theory if a reasonable correction is applied to the values obtained in the measurement of snow-flake diameters. Measurements of mass flux indicate that for a given intensity the attenuation caused by snow is an order of magnitude greater than that caused by the same mass flux of rain.