Assessing the size distribution of droplets in a cloud chamber from light extinction data during a transient regime

• We describe a method to measure the size spectrum of droplets in a cloud chamber.
• The variable light extinction during the transient regime of droplet settling is used.
• The method is rapid and enough sensitive to resolve fine features of the spectrum.
• The error sources and their quantitative evaluations are analyzed.

The paper describes a method of assessing the size distribution of fog droplets in a cloud chamber, based on measuring the time variation of the transmission of a light beam during the gravitational settling of droplets. Using a model of light extinction by floating spherical particles, the size distribution of droplets is retrieved, along with characteristic structural parameters of the fog (total droplet concentration, liquid water content and effective radius). Moreover, the time variation of the effective radius can be readily extracted from the model. The errors of the method are also estimated and fall within acceptable limits. The method proves sensitive enough to resolve various modes in the droplet distribution and to point out changes in the distribution due to diverse types of aerosol present in the chamber or to the thermal condition of the fog. It is speculated that the method can be further simplified to reach an in-situ version for real-time field measurements.