How freezing raindrops create extra ice in clouds

Mixed-phase clouds contain both water droplets and ice crystals at the same time. They are common and play an important role in the climate system. To make reliable predictions about Earth’s energy balance, precipitation patterns, and future warming, weather and climate models need to represent these clouds accurately.

Researchers often measure far more ice crystals in mixed-phase clouds than there are ice-nucleating particles (INPs) to trigger their formation. This gap suggests that extra ice must be forming through other pathways. These additional pathways are known as secondary ice production processes that generate new ice crystals without needing INPs.

In a recent study, Pfeifer et al., observed this process during a refreezing rain event in Hyytiälä, Finland, on 16 February 2024. A cloud radar identified a layer where melted snowflakes were refreezing. At the ground, a Video In Situ Snowfall Sensor captured thousands of close-up images of the frozen particles that reached the surface. By analyzing these images, the team found clear signs of droplet breakup. They estimated that each freezing drop produced between 1 and 6 new ice crystals. These results show that droplet breakup can be a powerful way for producing extra ice in clouds. They also provide new information that can help improve weather and climate models, making predictions more accurate. 

Figure: Pfeifer, N., Mom, B., Moisseev, D. et al. Efficient ice multiplication from freezing raindrop fragmentation. Commun Earth Environ 6, 942 (2025). https://doi.org/10.1038/s43247-025-02953-3