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On Monday, stage 3 of La Vuelta a España, the cycling race, was cancelled after a hailstorm forced riders to take cover and left hail covering the road. Let’s take a look at what happened! www.youtube.com/watch?v=ddn7... 1/14

The storm was in the eastern French Pyrenees. This region is no stranger to hail — the Pyrenees is a well-known hail hotspot. This map shows where we get the most frequent hail environments in Europe (www.nature.com/articles/s41... CC-BY-4.0). 2/14

The hailstorm hit when the riders were 15 km from the stage finish, cycling through Mont-Louis, at about 17:30 CEST (15:30 UTC). Radar data at that time show high reflectivities (over 50 dBZ) indicating the presence of large particles and lots of rain (zoom.earth/maps/radar/#...). 3/14

Some protruding clouds appear in satellite imagery at that time, likely developed by strong updrafts that can support hail growth (MTG GeoColour RGB with MSG IR10.8 in the background, view.eumetsat.int). 4/14

Hailstorms are small in scale, so it’s hard to forecast or model exactly where and when hail will happen. But we can consider whether the atmosphere is more or less likely to produce hail. 5/14

We often consider the atmospheric “ingredients” for hail. Two common ones are instability (propensity for updrafts to form) and wind shear (difference in wind by height). If you have lots of instability and wind shear together, hail is more likely and the environment is “hail-prone”. 6/14

So was there instability and wind shear in this case? Yes, at least in the vicinity, in a model forecast (adapted from charts.ecmwf.int, CC-BY-4.0). 7/14

The European Severe Storms Laboratory’s more sophisticated forecast also showed probabilities of up to 60% for large hail, in regions around where the hail occurred (www.stormforecast.eu). 8/14

It’s important to keep in mind that actual hail is usually highly localised within larger hail-prone regions, and hail-prone environments often do not produce any hail at all! That's because even the most hail-prone environment requires a storm to be triggered before you can get hail. 9/14

What about climate change? This is a hail-prone region so storms like this are not unusual in summer. We also can’t say any one storm was “caused” by climate change. But we can test whether storm environments are changing over time, and if storms are getting more or less frequent or severe. 10/14

Future hail projections remain highly uncertain. For hail frequency changes with global warming, our recent work showed a general poleward shift in hail conditions, but no significant projected changes over the Pyrenees (adapted from www.nature.com/articles/s41..., CC-BY-4.0). 12/14

And Zhang et al. (www.nature.com/articles/s41...) projected increasing hail damage potential in general, but a locally varying signal in southwest Europe with both increases and decreases across the Pyrenees region. 13/14