Concentrations of ice crystals of the order of 100 l−1 were found in some of the cumulus clouds sampled on four different days over or near the Australian mainland. These concentrations were 102 to 104 times higher than those of the ice nuclei present. The clouds had summit temperatures no colder than −13°C.
Penetrations of shallow stratocumulus clouds with similar top temperatures showed that ice particle enhancement was much less important in these clouds, a factor of 10 being the upper limit.
This difference in glaciation behaviour is considered in the light of differences in measured drop spectra, the stratocumuli having much narrower drop size distributions than the cumuli.
Of all the suggested mechanisms by which the number of ice crystals could be enhanced, the evidence presented here favours the production of secondary ice crystals when ice particles grow by riming.