Snowflake dance analysis could improve rain forecasts

The research, observing the physical motion of falling ice crystals, will help scientists better estimate where and when these crystals will melt into raindrops, a crucial stage in the formation of many types of rain.

Published today (Thursday 10 October) in the journal Atmospheric Chemistry and Physics, the study involved scientists watching how fake snowflakes fell in a substance imitating the atmosphere.

Jennifer Stout, who led the research, said: "Watching snow gently falling can be mesmerising, so it has been a joy to uncover the ways in which different ice crystal shapes pirouette and zigzag on their downwards journey.

"Understanding the dance of a snowflake is not only beautiful but can help us understand the reflectivity of clouds. Each snow crystal in a cloud acts like a tiny mirror, reflecting and refracting the light that passes through it. By predicting the choreography of an entire cloud, we could better improve our understanding of the atmosphere and the processes which lead to rain and snow. This intricate coordination of snowflakes can also create a big visual impact, causing stunning phenomena such as sun dogs and ice halos."

3D printed snowflakes

The research team used 3D-printed "snowflakes" of various shapes and sizes, from simple hexagonal plates to complex multi-branched dendrites. These artificial crystals were dropped through a tank filled with a water-glycerine mixture, simulating atmospheric conditions. High-speed cameras captured their descent, allowing researchers to reconstruct their three-dimensional trajectories and orientations.

The study revealed four main types of ice crystal motion: stable (falling straight down), zigzag (swinging back and forth), transitional (a mix of zigzag and spin), and spiralling (rotating while falling). Surprisingly, complex shapes like dendrites remained stable in motion despite their tendency to create turbulence in their wake, while simpler shapes became unstable much earlier.

Improving rain forecasts

These discoveries have significant implications for weather forecasting. Weather radar, which plays a key role in observing oncoming rain, bounces signals off water and ice particles in the air. With better understanding of how different ice crystal shapes move and orient themselves, meteorologists can interpret these radar signals more accurately, and better estimate when ice becomes rain. This more detailed data can lead to improved predictions of when, where and how much rain will fall.

The study's findings may also potentially improve scientists' understanding of how clouds reflect sunlight and trap heat in the atmosphere, with the potential to improve climate models and longer-term weather predictions.