Covariation of droplet size distribution and air humidity in fog: A methodological approach

The dynamics of fog droplets depend on their size and chemical composition as well on the ambient conditions such as relative humidity (RH) and turbulence. We analyze the droplet size distribution (DSD) in clouds and fog and how it covaries with the air humidity. We developed a new setup to remove fog droplets from the foggy air in order to quantify RH in the droplet-free air. The setup consists of a newly developed fog droplet separator with an attached H₂O gas analyzer and precision thermometer. The fog droplet separator works similarly to an active fog collector, yet with a high collection efficiency of 98.4% for the liquid water content. The water vapor concentration and the air temperature are measured in the droplet-free air at a frequency of 1 Hz. Using this system and a FM100 Fog Monitor, we analyzed the droplet size distribution (DSD) and studied how it covaries with RH. We examined two long-lasting fog events in March and April 2017 on Mt. Lulin, Taiwan. We found that the DSD differed between data with high RH (>100.3%) and low RH (<99.7%). For one event, we were able to confirm the hypothesis of the Köhler theory, which states that droplets are smaller at a low RH than at a high RH. In the other event, the droplets were larger at low RH, which was caused either by entrainment of air masses of different compositions or by droplet growth from below the detection limit to a detectable droplet size, or by a decrease of droplet sizes beyond the critical diameter with decreasing RH. The results show that the developed setup is appropriate to analyze how the DSD covaries with RH in fog.