One thing I find annoying is right before a relatively warm race when the obnoxious race emcee says, “It’s a perfect day outside for a race!” You can clearly tell from such a statement that the guy never runs, ever.
Because while 60-70°F (15-20°C) feels amazing if you’re out for an easy walk, or laying out in the sun… that temperature enters the somewhat-warm zone for distance runners in a race, who are moving a lot faster than a walk and producing a lot of body heat throughout their run. Add in substantial humidity, which interferes with the evaporation of sweat, and now it begins to feel really hot.
Jonathan Savage created a ‘perceived heat index for runners’ calculator to accurately show what a runner’s “heat index” is for a given pace at their height and weight, depending on the temperature and humidity.
Below is a chart from the calculator. All temperatures are shown in both Fahrenheit and Celsius, and assumes a person of my size (5’10”, 160 lbs as of this writing) running a 9:05 per mile pace.
As you can see, 65°F (18°C) at even a mild 40% humidity can feel like 101°F (38°C) for a runner my size (5’10”, 160 lbs) running at a 9:05/mile pace.
To be fair, that’s mild compared to how most of summer feels for me and others, running in 80-95°F (25-35°C) afternoons with 60-75% humidity. Even running at slower paces, my heat indices during summer reach the 130-145°F (55-60°C) range.
And, of course, you can train to handle this, and training in hot conditions has all sorts of fundamental long term benefits. But once you get to race day, all the heat is doing is making your race tougher. And the threshold where it begins to impair your performance isn’t particularly high.
In any case, I’m not big on clapbacks or “educating” people, but the next time someone attempts to point out that 60+°F weather (15+°C) is perfect running weather, you may want to throw some knowledge at them about how running changes everything.
P.S. As for the ideal temperature for running, your best conditions are 40-60°F, or 5-15°C.