If you’ve ever slogged through a hot, humid marathon and wondered why it felt exponentially harder than your training runs, you’re not alone.
Weather conditions—especially heat and humidity—can have a profound effect on marathon performance, far more than many runners anticipate.
New insights from a broad analysis of nearly 4,000 marathon performances help us understand how temperature and humidity shape race day outcomes, offering important lessons for anyone planning to toe the start line.
The Hidden Impact of Heat
Unlike battling headwinds or conquering hills, where the added effort is clear and measurable, heat works in a more sneaky way. It acts as an additional source of fatigue, magnifying the perception of effort long before your body is technically in danger.
Studies, like one published in the Journal of Applied Physiology (Nybo, 2008), show that during time trials, runners slow down in the heat well before their core temperature spikes to dangerous levels. Essentially, your brain senses potential trouble ahead and eases the brakes early to protect you—even if it feels frustrating in the moment.
This “protective slowdown” helps explain why so many runners see their splits deteriorate on warm race days. It’s not just about staying hydrated or keeping cool externally—it’s a deep, built-in survival response that kicks in whether you like it or not.
The Sweet Spot for Marathon Conditions
Thanks to the large-scale analysis of 3,891 marathon performances across 754 races, we now have a clearer picture of ideal racing weather.
The data shows that the optimal temperature range for marathoning falls between 35–55°F (2–13°C). Within this window, performances are consistently strong, and humidity levels have little to no effect.
Things change dramatically once the temperature climbs above 65°F (18°C). At that point, humidity starts to compound the stress on your body, making conditions feel even hotter and more oppressive.
The study found that hot and humid conditions could slow runners by up to 8%, a staggering 15 minutes or more for someone aiming to run a three-hour marathon.
In practical terms, this means that a chilly start might not be comfortable, but it’s far preferable to a warm, sticky morning. A little too cold is better than a little too hot.
Adjusting Expectations on Race Day
Given how drastically heat and humidity can sap performance, smart runners adjust their race plans when the forecast calls for a scorcher.
One simple way to do this is by looking at the heat index — sometimes labeled “Feels Like” on weather apps. Unlike pure air temperature, the heat index factors in humidity and gives a more accurate sense of how oppressive the conditions really are.
Pacing adjustments based on heat index can help runners avoid mid-race blowups.
According to pacing tables developed from empirical data and guidelines like those from the Road Runners Club of America (RRCA, 2022), even a 10-degree increase in “feels like” temperature can mean adding 10–20 seconds per mile to your planned pace. It may feel counterintuitive to slow down intentionally, but adjusting early can save your race in the long run.
Lessons From Real-World Races
If you want a real-world example of why this matters, look no further than the 2007 Chicago Marathon. That year, race day temperatures soared to 88°F, and hundreds of runners suffered from heat-related illnesses. The race was eventually canceled mid-event. Part of the problem was that many runners were Midwesterners used to training in much cooler conditions, and their bodies simply weren’t prepared for the brutal heat.
Contrast that with the Honolulu Marathon, which takes place every December in reliably hot and humid weather. Runners who sign up for Honolulu often plan for the conditions, arriving early to acclimate or training in warmer environments beforehand. As a result, finish rates are typically much better, despite the challenging weather.
Heat acclimation isn’t just anecdotal, either. Research from Sports Medicine (Patterson et al., 2019) shows that spending 7–14 days training in hot conditions can dramatically improve heat tolerance, helping runners maintain pace and avoid overheating.
Prepare for the Weather, Not Just the Course
The bottom line is that weather can’t be an afterthought when you’re preparing for a marathon. Choosing a race with historically favorable conditions—or at least adjusting your strategy when race day forecast looks tough—can make a huge difference.
Remember:
- Aim for marathons where temperatures hover between 35–55°F (2–13°C).
- Be extra cautious if the temperature is above 65°F (18°C) and humid.
- Adjust your pacing based on the heat index to avoid catastrophic blowups.
- Consider acclimating to heat if your target race is known for warm conditions.
By treating the weather as seriously as your training, you can give yourself the best shot at crossing the finish line strong, no matter what the thermometer says.
