We are intrigued by the observation of persistent ice deposition from a thin single-layer mixed-phase cloud reported by Westbrook and Illingworth (2013). Their analysis suggests a strongly time-dependent nature of freezing in a natural cloud, other than what we are used to see in the analysis of aerosol particles in drop-freeze assays in our laboratory. Since 2015 we try to study the phenomenon in shallow, supercooled radiation fog, occasionally forming during winter in the Swiss Jura mountains.
