Secondary Ice Processes

In many cases, mixed phase clouds contain orders of magnitude more ice than there are primary ice nucleating particles. This superabundance of ice is commonly explained by secondary ice processes, i.e. the generation of more than one ice particle in the process of freezing.
With the help of synchronized high –speed and thermal video imaging, Kleinheins et al. have been able to observe up to 15 subsequent pressure buildup and release processes during the freezing of an individual levitated cloud droplet.

Internal pressures up to 170 bars have been found. These are indicative of ice shell cracking and are believed to be a source of secondary ice particles.

Fig.: (a) Experimental etup for measuring the internal pressure in freezing cloud dropets. (b) Temperature- (red) and pressure (blue) trace during droplet freezing. The arrow marks a rapid pressure release that is associated with a crack in the ice shell (red circle in the insert).