Tour de France 2015: Mountains and heat pose threat to Chris Froome's grip on race

Riders face demanding climbs over the coming days

Froome is all smiles after retaining the yellow jersey - and his lead of over three minutes - on Wednesday (Reuters)

After Wednesday’s fireworks, three more spectacular stages in the Alps kick off on Thursday with an ascent of one of the hardest climbs in the region, the Col du Glandon.

Although the ascent is officially “only” 21 kilometres (13.1 miles) long, riders will actually be climbing for 47km, with a vertical gain in that distance of 1,600m – the equivalent of climbing Ben Nevis.

Immediately after a fast descent, the next challenge is the short but seemingly interminable sharp series of hairpin bends, the Lacets de Montvernier. Described by Chris Froome as one of the most photogenic ascents in France, the Lacets (there are 18 of them) could ensure only a small group of favourites will still be together for the finish at the town of Saint-Jean-de-Maurienne, 10km further on.

Friday’s stage, to La Toussuire, the ski station above Saint-Jean-de-Maurienne, is 138km long and therefore renders long-distance attacks more feasible. With an 18km ascent to the summit finish, stage 19 will be certain to create some more aggressive attacking. The descent from the climb preceding it, the Col du Mollard, is notoriously dangerous, and La Toussuire itself, if not the hardest climb, will, at this point in the race when riders are near exhaustion, be certain to do damage.

The final mountain stage of the race, Saturday’s stage 20 to Alpe d’Huez, is the most prestigious and arguably the most dangerous for Froome. The Sky rider’s rivals will, at this point, have little to lose and everything to gain by attacking, while it has to be remembered that in 2013 the Briton came closer than he would have liked to cracking in the face of Nairo Quintana’s attacks. Froome finally lost a minute but won the Tour.

The other big factor in these last Alpine stages is the weather. This has been one of the hottest Tours on record and, barring the odd, and very intense, thunderstorm, the high temperatures are set to continue to the weekend. Beyond the terrain, factors like the heat and sheer length and intensity of the race could play a role in how the Tour’s last major challenges unfold.