# Aerodynamic Stability and the Growth of Triangular Snow Crystals

## TL;DR

This paper explains the frequent occurrence of triangular snow crystals through an aerodynamic model showing how airflow-induced growth instabilities transform hexagonal plates into triangular forms.

## Contribution

It introduces a novel aerodynamic explanation for the formation of triangular snow crystals, linking airflow effects to crystal morphology changes.

## Key findings

- Triangular snow crystals are more common than random perturbations suggest.
- Airflow around a crystal can induce growth instabilities leading to triangular shapes.
- The model explains a long-standing puzzle in snow crystal morphology.

## Abstract

We describe laboratory-grown snow crystals that exhibit a triangular, plate-like morphology, and we show that the occurrence of these crystals is much more frequent than one would expect from random growth perturbations of the more-typical hexagonal forms. We then describe an aerodynamic model that explains the formation of these crystals. A single growth perturbation on one facet of a hexagonal plate leads to air flow around the crystal that promotes the growth of alternating facets. Aerodynamic effects thus produce a weak growth instability that can cause hexagonal plates to develop into triangular plates. This mechanism solves a very old puzzle, as observers have been documenting the unexplained appearance of triangular snow crystals in nature for nearly two centuries.