Running in the heat

Heat is the single biggest weather factor working against a runner. As air temperature climbs past roughly 50–54°F, your body diverts more blood to the skin to shed heat, leaving less for your working muscles, so the same effort produces a slower pace and a higher heart rate. The hotter it gets, the more it slows you.

But the thermometer only tells half the story. Your body's main cooling system is sweat evaporation, and how well that works depends on the dew point, not the air temperature. When the dew point climbs above about 60°F, the air is already so saturated that sweat can't evaporate efficiently: heat builds up even on a day that doesn't look that hot. That's why a muggy 75°F morning can feel far harder than a dry 85°F afternoon. RunWeather folds heat, humidity, sun, and wind into one wet-bulb-globe-temperature (WBGT) heat-strain number, the same metric race directors use to flag risk.

The fixes are practical: run in the coolest window (usually pre-dawn or after sunset in summer), slow down to hold effort rather than pace, drink to thirst and add electrolytes on long sweaty runs, and give your body 10–14 days to acclimatize when a heatwave hits. Acclimatization genuinely raises your heat tolerance. Check the score before you head out so the conditions, not your watch, set the pace.