FOG DROP SIZE DISTRIBUTIONS-MEASUREMENT METHODS AND EVALUATION

In connection with evaluating fog modification efforts, several published methods of measuring the size distribution of fog droplets were reviewed. An evaluation cast doubt on the validity of the gelatin, Formvar, and polyvinyl alcohol, and oil collection media for recording droplets less than 4 micrometers diameter. Efficiency of several collection methods appeared to be poor for less than 4 micrometers diameter droplets when lack of drop size distribution correlation with visibility measurement was considered. During a fog dispersal test, pyrotechnically produced hygroscopic reagents were observed to cause an increase in the number of concentrations of the small sized droplets of a natural fog. During nonfog conditions, several types of pyrotechnic flares containing different hygroscopic reagents were tested for usefulness as fog modification agents. Measurement of drop and particle size distributions downwind from the ignition point of these pyrotechnics revealed that the effect of small droplet sizes in decreasing visibility in air of high humidity would negate the improvement made by removal of large size drops.