Supplementary data for J. Kleinheins et al. "Thermal imaging of freezing drizzle droplets: pressure release events as a source of secondary ice particles", Journal of the Atmospheric Sciences 2021

During freezing from the outside in, the volume increase of liquid water upon solidification leads to a pressure rise inside the droplet. The pressure is released in various ways, e.g. by cracking or by complete fragmentation of the ice shell.

These processes may be the source of secondary ice particles that are emitted during droplet freezing. In this study, the surface temperature of freezing drizzle-sized water droplets was measured with a high-resolution infrared thermography system, while recording the changes in shape and structure of the droplet by a high-speed video camera. The droplets were levitated in an electrodynamic trap under controlled conditions with respect to temperature, humidity and airflow velocity.