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The largest snow crystal ever measured was 10 millimetres across, discovered by Kenneth Libbrecht, who photographed the record-breaker in Ontario, Canada, in December 2003. Libbrecht is a professor at the California Institute of Technology where he specialises in the dynamics of ice crystal formation. In our interview, he explains the complex physics at play when atmospheric conditions are just right for snow to form and how he creates perfect snow crystals in his lab.