Mountains add an element of unknown into grand tours. The sheer volume of vertical meters riders must climb over the course of three weeks pushes the GC contenders to the very edge, if not beyond what their bodies are capable of. The weather in the high mountains can also play a big hand in shaping a race's outcome.

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The mountains stages of the 2019 Tour de France were as unpredictable and as dramatic as any grand tour in recent memory. From unscripted tactics to flash floods and hail storms, the 2019 Tour had it all.

These are the must-watch mountain stages of the 2019 Tour de France.

Stage 6 – Mulhouse to La Planches des Belles Filles

La Planches des Belles Filles is well on its way to becoming an iconic mountain in the Tour. Stage six was the first summit finish of 2019 and was stolen by the breakaway duo of Dylan Teunes and Giulio Ciccone. Ciccone’s second place ride took the jersey off Alaphilippes back, but that would prove to be short lived.

The final kilometers featured gravel surfaces and steep slopes on which Alaphilippe and Geraint Thomas battled in a gritty finale. 

La Planches des Belles Filles was such a success that the Tour organizers decided to bring it back as the grounds for the individual time trial in the 2020 Tour de France.

Stage 8 – Mâcon to Saint-Étienne

Stage eight was a mid mountain stage with seven categorized climbs, but none greater than a category two. 

It was a relatively safe stage for the general classification contenders but two Frenchmen seized an opportunity in the technical and undulating terrain.

Julian Alaphilippe put on display his tenacity and guile, slipping away from the peloton with Thibaut Pinot and gained 20 seconds at the line. 

The entire field was outfoxed by Thomas de Gendt, who stole the show in a way only he knows how to do, while Alaphilippe's heroics allowed him to reclaim the jersey from Giulio Ciccone.

Stage 14 – Tarbes to Col du Tourmalet

The Col du Tourmalet is one of the most iconic climbs in cycling. It has been climbed close to 90 times in the Tour de France, but has only featured as the final climb three times.

In 2019 Thibaut Pinot etched his name into cycling history as he raised his hands on the summit. 

Viewers can feel the energy on the mountain as the Frenchman climbs through the massive crowds to the finish. 

Stage 14 is everything we love about the Tour de France.

Stage 18 – Embrun to Valloire

Stage 18 was not a summit finish, but it finished another iconic climb of the Tour de France. Riders crested the 23 kilometer long Col du Galibier, then plunged down the other side, where they finished 20 kilometers later in Valloire.

All the pressure fell on Team Ineos during stage 18. It was the first of the final three mountain days in the Tour and taking the yellow jersey away from Alaphilippe was proving harder than anyone had bargained for.

Stage 18 featured a redemptive ride by Nairo Quintana and desperately fast climbing by Ineos.

Stage 19 – Saint-Jean-de-Maurienne to Tignes

Stage 19 was intended to be a short and explosive 127 kilometer dash to Tignes where the peloton would finish atop the category one Monteé de Tignes. 

En route, Team Ineos traded blows against Julian Alaphilippe up the penultimate climb of the Col de l’Iseran, which allowed Egan Bernal to escape solo over the top. Alaphilippe cracked and tried to limit his losses up the climb, hoping to chase back to a group on the descent, but as riders dropped of l’Iseran they were stopped by the race officials.

Unbeknownst to the riders, a hail storm had swept through the valley, flooding the course. The road conditions ahead were hopeless and the organizers were forced to call off the race.

Time was taken at the top of the final climb. Riders donned jackets and were shepherd into team cars as they began to process what this meant for their race.

Foul weather would play its part again in stage 20, resulting in another shortened race. The climb up Col de l’Iseran ultimately decided the general classification in the 2019 Tour de France and for better or for worse was one of the most dramatic and consequential races of the decade.