Instrumentation for the detection of ice nuclei

The more common instruments used to count ice nuclei are described, and the relationship of their results to ice nucleation mechanisms is considered. All of the chambers considered (mixing, diffusion, and cloud-settling chambers and the Colorado State University isothermal cloud chamber) can activate nuclei by deposition, but the time available in the mixing and diffusion chambers is too short for any appreciable contribution by contact freezing. Deposition nucleation is probably inhibited in chambers with high transient supersaturations due to the occurrence of nuclei condensation. In the NCAR acoustical counter, which is essentially a continuous-mixing chamber, it is likely that most of the particles form ice crystals by condensation followed by freezing, since the residence time is too short for any appreciable nucleation by contact nuclei. In the case of the filter technique, modes of nucleation are probably limited to deposition and condensation on mixed nuclei followed by freezing. The applicability of the drop spectrometer is limited to the study of immersion freezing.