Parallel-Beam Attenuation of Light, Particularly by Falling Snow

Abstract

A conventional optical system has been used for obtaining a nearly parallel beam of light, with a divergence less than 0.1°. Attenuation measurements over a path of 123 m were made in Montreal during the first part of 1964. The largest attenuation by falling snow was 100 db km?1. The attenuation by ordinary dry snow was nearly proportional to the snowfall rate, and with a rate of 1 mm (water) per hour it was 18 db km?1. The attenuation by wet snow and snow pellets was typically 10 db km?1 when normalized to unit precipitation rate. Transmission measurements seem to be promising for determining the snowfall rate as well as the visual range. On a few occasions city pollution reduced the received signal by as much as 8 db km?1; for an average day 1.5 db km?1 was representative. With a temperature difference between transmitter shelter and outside air the signal was found to fluctuate. These fluctuations were evidently due to light deflection by eddies associated with the temperature difference. The deflection also reduced the mean signal level, in this experiment by as much as 1 db. Some implications relevant to further work, including laser applications in atmospheric studies, are pointed out.