"races in the 1970s for their cooler weather. Since 2000, more than half of yearly top-200 times have been run in April or October (the exception is Dubai, where the coolest month for a marathon is January). But since elite marathoners generate far more heat than regular runners, they might benefit from even colder conditions—a recent review by the French National Institute of Sport of nearly 2 million marathon finishers found that the optimal temperature for male pros was below 40°F. That raises an intriguing possibility: Focusing major marathons in April and October has whittled times to just under 2:03—perhaps to reach 2:00, those races should be held in March or November."