Overcast and 45 degrees might not be the sort of weather most folks dream about, but it doesn’t get much better for an outdoor run. In fact, when researchers from the University of Tulsa examined the 25 fastest performances at different distances, along with the temperature that day, perfect running temps for hovered between 73.4 and 49.4 degrees. A comprehensive study out of France analyzing the collective finish times at six major marathons over 10 years also found that 44.2 degrees is ideal for women churning out the best performances.
45° F is ~7° C.
The claim is similar to why swimming pools tend not to heat the pools with lap lanes. I can’t swim in heated pools for very long before I hop out to sit on the side, feeling the heat radiant off me. Similarly, when I ran in hot temps, I’d sweat for a while after the cool down. Sometimes, over a half an hour. It was gross really, that much water/sweat running off me.