The temperature near the new National Stadium in central Tokyo, where the Olympic marathon will start and finish, hit 30 degrees Celsius at 6 a.m. on Friday, weather service provider Weathernews said.
With exactly one year to go until the 2020 Olympic women's marathon, which is scheduled to start at 6 a.m., the Weathernews team measured the temperature along the route, stopping their vehicle every 5 kilometers to take readings.
In addition to the sweltering heat, they also measured humidity levels of more than 70 percent as of 6 a.m.
"There were some thin clouds. Once you're exposed to direct sunlight the temperature will feel different," said Kazuo Asada, who runs the sports meteorology team at Weathernews and is also a member of the science committee at the Japan Association of Athletics Federations.
The statistical data they collected will be used to plan measures to safeguard runners against heat-related illnesses.
The women's marathon at the 2020 Games is scheduled to run on Aug. 2. The men's marathon, also due to start at 6 a.m., is scheduled for Aug. 9.
Originally, the start time was set for 7:30, which was then revised to 7 a.m. before it was moved again to counter the possible health risks to athletes, spectators and workers.