In late May 2021, 172 runners set out to tackle a 100km ultramarathon in north-west China. By the next day 21 of the runners had been killed by hypothermia after an unexpectedly intense storm brought freezing temperatures, strong winds and hail to an upland section of the course. Weather forecasts had predicted a cold front, but had not captured how extreme the conditions would be. There are no meteorological stations in the area and survivor reports are subjective, but now a new hyper-local weather model – using topographic data at…