Everyone likes the summer, but for many runners, this is their least favorite season. According to some studies (1), the average temperature for best running performances is 55 ºF (~12.5 ºC). When the temperature rises above 80 ºF (~26.5 ºC) the runner's performances are degraded by 10-15%.
But it is not only a matter of performance, extreme heat can be very dangerous.
Our body adapts to the heat and activates strategies to reduce it, but the risks of Heat Cramps, Heat Exhaustion, and finally Heat Stroke are real!
Source: CDC (3)
Now, being careful does not mean you cannot train. Here is some advice to run when it's hot.
Acclimatization: if the heatwave is sudden or if you traveled to a hotter location, allow your body to adapt. Cardiovascular adaptation starts within the first 3-5 days of training in the heat.
Hydration: drink during the day, start with a full tank then hydrate early and often. Do not leave for a run without water. Fuel with electrolyte tablets, sports drinks, and salty food.
Proper clothing pre-cooling, per-cooling: wear transpiring light-colored clothes; take a cold shower before heading out for a run; carry a cooling towel or a bottle of frozen water to cool you down during your run.
Adjust pace or training volume: run slower keeping the same volume (recommended for marathon training) or reduce the volume maintaining the pace
Common sense: run in the cool hours, possibly in the morning, chose a trail in the shade; avoid too much alcohol the night before your run, slow down, walk if necessary, stop if necessary!
Source: Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies (2)
Sources and bibliography:
(1) "Hot times", E.C. Frederick, Running, 9, p.52, 1983
(2) Périard, Julien Eijsvogels, Thijs, Daanen, Hein 2021/04/08 Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies