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A 1.2 m3 continuous slow-expansion cloud chamber was used to simulate natural, liquid cloud formation on soluble cloud condensation nuclei (CCN). Droplet freezing was observed during continued simulated adiabatic ascent and cooling to −40°C. Sharply increasing ice nucleation rates were observed between −34° and −39°C, independent of the chemical composition of three CCN used. From the experimental data, nucleation rates are estimated assuming a homogeneous-freezing mechanism. It is concluded that homogeneous-freezing was observed. The results are compared to other laboratory and field studies. These results compare most closely with values calculated from data taken in real clouds and should be relevant to ice formation in cirrus clouds.
DeMott et al. (Tue,) studied this question.