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.
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.