The molecules in ice crystals normally form a hexagonal lattice, which is why all snowflakes are six-sided (pictured). But on page 218, Lupi et al. report that the tiniest ice crystals prefer a different arrangement — a finding that has implications for climate models (L. Lupi et al. Nature 551, 218–222; 2017).
The authors used computational simulations to investigate how nanoscale ice crystals form from water. They found that a molecular arrangement consisting of randomly ordered layers of hexagonal and cubic arrays is the most thermodynamically stable arrangement in crystallites of up to 100,000 molecules.
The findings disagree with the classical theory of crystallite formation, which is used to predict the rates at which ice crystallites form in the atmosphere — a variable that influences cloud formation. The authors conclude that the classical theory must be corrected to improve climate and weather forecasts.