Water freezing is crucial to Earth's climate, and accurate weather and climate forecasts depend on reliable predictions of ice nucleation rates. Such predictions are typically based on classical nucleation theory, which assumes that the structure of the initially formed ice crystallites corresponds to that of thermodynamically stable hexagonal ice. Laura Lupi et al. now report simulations and energy calculations with a simple water model to show that, for emerging crystallites, ice with disordered stacking is more stable than hexagonal ice and results in ice nucleation rates more than three orders of magnitude higher than predicted by classical nucleation theory. This effect must be accounted for in cloud models and when interpreting ice nucleation rates measured in laboratory conditions and extrapolating them to temperatures important to clouds.
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