Direct Photographic Method for Measuring Droplet Size Distribution in Fog

Abstract

We evaluate a direct photographic method to measure droplet size distribution (DSD) in fog using an instrument, consisting of a digital camera and a stroboscope, designed for onboard use in a hot air balloon, which is safer than other aircrafts in fog. This method allows observation of fog droplets without mechanical suction or impaction. Drops of diameter 5-100 µ m could be imaged with a relative standard deviation of 20-21%. Extinction coefficients derived from the DSD show good agreement with those derived from the visibility. The feasibility of the instrument attached to a hot air balloon was validated in a field observation, demonstrating that the direct photographic method is feasible for determining fog DSD.