An experimental study was made of a membrane filter technique for the detection of ice nuclei in which air of controlled humidity flows across the cooled membrane as opposed to the usual static thermal diffusion procedure. This is necessary to overcome water vapor losses to various sinks on the sample substrate; the procedure also makes it possible to observe the nucleation process directly by microscope. However, for very small nuclei such as AgI, even this flow system is not able to activate the nuclei. This is overcome by letting a puff of supersaturated air pass over the deposited aerosol, which should be on a water-repellent substrate, to promote vapor flow to the ice and condensation nuclei, rather than onto the substrate. At temperatures warmer than −12°C, the puff procedure is not necessary since the water vapor pressure is sufficient to keep up with the vapor sinks. Following the above procedures, consistent results are obtained for AgI, kaolin, soil particles and pollution aerosols in both laboratory and field applications in the −5 to −20°C range.

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