Visible extinction measurements in rain and snow using a forward-scatter meter

摘要

Abstract: A forward scatter meter is a convenient and accurate method to measure the local visible extinction coefficient in fog and haze. The device works by illuminating with near-IR light a sample volume of air and measuring the intensity scattered in the angular range of 27-42 degrees. The scattered intensity is well correlated to the extinction coefficient of fog regardless of the fog droplet size distribution. The forward scatter meter also gives meaningful measurements during rain and snowfall. A comparison of extinction measurements made with a narrow beam transmissometer and the forward scatter meter have been made. The results show that during rain, the forward scatter meter extinction coefficient is from 25-50% greater than that measured with the transmissometer. During snowfall the forward scatter meter extinction coefficient is about 10-40% less than that measured with the transmissometer. These results can be used to define correction factors so that the forward scatter meter can be used to estimate extinction during rain and snow as well as during fog.!