Propagation of Visible and Infrared Radiation in Snow and Rain

As part of a program to develop improved models for infrared and visible extinction in adverse weather, a one-kilometer transmission range has been established near Houghton, Michigan, where annual rainfall is about twenty inches, fog is common, and annual snowfall often exceeds 200 inches. A Barnes Engineering Transmissometer is used to record transmission in the 3-5 and 8-14 micrometer bands, a Helium-Neon laser transmissometer is used to record transmission at 0.63 micrometers, and a Nd:YAG laser transmissometer is used to measure transmission at 1.06 micrometers. During Transmission measurements, an automated weather station periodically records standard weather parameters. Results are presented as plots of the logarithm of the infrared extinction coefficient versus the logarithm of the visible extinction coefficient (the GAP model format). Extinction coefficients for rain tend to lie along straight lines, in fairly good agreement with the GAP model, when the atmosphere contains rain drops but not suspended droplets. Measured extinction coefficients in snow tend to lie along straight lines on the log-log plots, with slight differences from one snow storm to another, which are attributed to differences in types of snow-flakes.