Stage 9: Vielha Val d’Aran to Andorre Arcalis

An exciting mountain stage finished in Andorra amidst a hailstorm. Today, we ask why such thunderstorms are so common in the Pyrenees.


Wet air from the Mediterranean is funneled up the river valleys of Eastern Spain, depicted nicely in this map from Pineda et al. (2010). As it goes further up the valleys, the air is pushed up in elevation until it reaches Andorra, the country with the highest average elevation in Europe.

The air cools down as it rises until it becomes too cool to hold all of that water it brought up from the Mediterranean, and the water falls out as precipitation. Combine this orographic effect with the large daily fluctuation in temperature that you see in the summer, and you have a large, wet and unstable air mass which results in a convective thunderstorm which drops hail as we saw today.

Thunderstorms are very common in Andorra during the summer, and flash floods from such thunderstorms are one of the major natural hazards Andorra faces (Trapero 1980).

  • Callado, A., and R. Pascual. “Diagnosis and modelling of a summer convective storm over Mediterranean Pyrenees.” Advances in Geosciences 2 (2005): 273-277.
  • Pineda, N., P. Esteban, L. Trapero, X. Soler, and C. Beck. “Circulation types related to lightning activity over Catalonia and the Principality of Andorra.”Physics and Chemistry of the Earth, Parts A/B/C 35, no. 9 (2010): 469-476.
  • Trapero, L., et al. “Mesoscale numerical analysis of the historical November 1982 heavy precipitation event over Andorra (Eastern Pyrenees).” Natural Hazards and Earth System Sciences 13.11 (2013): 2969-2990.



2 thoughts on “Stage 9: Vielha Val d’Aran to Andorre Arcalis

  1. Pingback: Stage 10: Escaldes-Engordany to Revel | Les Eaux du Tour

  2. Pingback: Stage 19: Albertville to Saint-Gervais Mont Blanc | Les Eaux du Tour

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