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That coincides with three mountain stages in the Alps that test top riders at the best of times but will probe the outer limits of endurance in a heatwave.
Cycling fans worldwide and especially in France are transfixed by the 21-day Tour this year with two Frenchman in the victory frame ahead of Sunday's grandstand finish along the Champs-Elysees in Paris.
Julian Alaphilippe holds the leader's yellow jersey and his compatriot Thibaut Pinot sits firmly in the running to wrestle the title from defending champion Thomas with six stages remaining.
Thomas has already complained of overheating on a stage last week. But now he insists that he has no fear of temperatures soaring beyond anything experienced on the Tour so far.
"The heat won't make any difference, it's the same for everyone in the race," said the Welsh 2018 champion.
He was speaking ahead of Tuesday's stage, a loop around Nimes in France's southern region, where the pavements were already baking by morning.
But with the heat cranking up towards 40 degrees as the race crosses the baking Rhone Valley on Wednesday, riders will arrive for the first of three brutal Alpine slogs on Thursday for some dreadful reckoning in the mountains.
"This is the third week of racing and these high temperatures are sure to make a difference," Alaphilippe's boss Davide Bramati said.
The last stage to be run under high temperatures was last Thursday at Pau where Thomas, who had been expecting to win, was stunned to find himself trailing home 14 seconds slower than the former soldier Alaphilippe.
"I was overheating," said Thomas. "Over the final eight kilometres I just couldn't find the kick."
Former rider Denis Roux, who works with Tour partners Krys and who finished in the top ten on the 1988 edition, told AFP that riders react differently to heat with some fading while others prosper.
"Personally I loved the heat. Some like it hot, some don't," said Roux.
"Some guys just don't feel well, especially if there is no wind, which makes it much worse, then you start losing salt and that gets you," said Roux, who took part in six Tours de France.
Another former rider at the starting line Tuesday said the heatwave would have an effect but would unsettle some riders more than others.
"Of course it makes a difference," said 83-year-old Tour legend Raymond Poulidor.
"In this heat some riders just can't hack it," said the man famous for eight Tour podiums, but none of them on the top step.
Team doctors on the Tour de France are fully aware that the heat can make a rider hit the wall.
"Cyclists are usually psychologically tough," said Jacky Maillot, the team doctor at the FDJ team for whom French title hopeful Pinot rides.
"A rider can lose up to a litre of sweat per hour. And when that happens you cannot fully rehydrate, only by half a litre per hour," he said.
"Different riders react in different ways, but when the heat goes over 39 degrees, performances drop for all the riders," he added.
"So teams need to prepare for riders to have two water bottles (500ml each) with them at all times and hopefully drink three of them every hour," said Maillot on a stage that is likely to take five hours, meaning 15 bidons of water for each rider.
