“Thirteen degrees at the start, no wind, clear skies”: What’s the ideal weather to break your marathon record?

Before thinking about perfectly tied shoelaces, a securely fastened bib, or one last bathroom stop, another player quietly enters the preparation: the sky. For years, marathoners have been monitoring the weather with the same intensity surfers use to watch the waves. Pre-race conversations revolve as much around temperature, wind, and humidity as they do around training sessions. Some check forecasts ten days in advance, while others follow strict protocols to handle heat, rain, or gusty winds. Sometimes, breaking a personal best depends more on a little atmospheric help than on perfectly timed training.

---

Sunday morning. You check your weather app before heading out for the marathon you’ve been talking about to your colleagues for weeks. You know that air, light, humidity, and wind influence performance just as much as a solid training plan. No matter how well you prepare—eating well, sleeping early, stashing gels meticulously—the weather can derail even the most carefully crafted strategy.

In popular imagination, breaking a marathon record is mostly about training, equipment, and mental freshness. Yet science shows that the ideal “window” often falls in a narrow range between 8 and 12°C (46–54°F). A “thermally stress-free” environment, as sports physiotherapist and athlete Jérôme Auger puts it, where “the body works at its optimum when it doesn’t have to fight heat or cold.” Cool temperatures, dry air, and soft sunlight—this is exactly what exercise physiologist Stéphane Bermon of World Athletics describes as “conditions in which the body dissipates heat efficiently,”the type of conditions under which world records often fall.

Julien Devanne, 2019 French marathon champion who ran 2:14:55 in Valencia, recalls it vividly. That day, “conditions were good, even if the heat rose quickly.” For runners targeting sub-3:30, he notes, “it’s still perfect, but beyond that, it starts to weigh.” With the wry humor typical of marathoners, he admits that when you do break your record, even in sweltering conditions—30°C (86°F) or more—“you feel like it was the perfect day.” Ideal weather, in other words, is as much a mental state as a set of numbers.

| Humidity: the invisible brake

Runners often focus on the thermometer, yet humidity can be far more decisive. When the air is saturated, sweat no longer evaporates, thermoregulation collapses, and the engine overheats. In Vietnam, Devanne experienced this firsthand during a 6 a.m. start: moderate heat but extreme humidity, resulting in “ten minutes lost on the half, just because of that.” A painful but clear lesson. Even a finely tuned athlete “can be caught out by humidity,” he says, to the point where the race “becomes survival.”

Bermon calls this humidity “a silent killer,” preventing sweat from doing its job. Too dry isn’t better: in arid air, sweating works so efficiently that dehydration sets in faster than expected. The balance is delicate—almost surgical.

| Wind: a double-edged ally

Enter another often underestimated factor: wind. Auger points out that a tailwind can feel like running downhill, a little “free boost” that quickly turns into a handicap when it shifts head-on. “On the track, it feels like it’s always in your face, even when it should be behind you,” Devanne jokes. On the marathon course, it can be brutal: “A headwind for one kilometer at mile 35 is tougher than a hill.” Wind not only increases resistance; it “disrupts mechanics and even the mind,” creating that eerie feeling of “running while sitting down,” as he describes it.

| A subtle balance to find

Too hot, you overheat. Too cold, you expend more energy. Too humid, you can’t sweat. Too dry, you dehydrate. Too windy, you struggle. Not enough wind, you overheat. Bermon sums it up clinically: “The cooler the air relative to your body, the more efficiently heat is transferred.” Beyond numbers, there are often-overlooked factors, like air quality. Auger reminds us that pollution “constricts airways, reduces breathing efficiency, and lowers performance,” often without us realizing it.

No wonder a top marathoner like Devanne describes his ideal almost like a vision: “Thirteen degrees at the start, no wind, clear skies…”—the alignment of the stars every runner dreams of experiencing. The meteorological bliss of a 42.195 km addict.

| Adapting your plan to the weather

Perfect weather exists but rarely presents itself. The key is to adjust. Devanne prepares “always for the worst-case scenario.” If he knows rain is coming, he doesn’t invent a ray of sun. “Either I run, or I don’t, but I go prepared.” Simple philosophy.

Weather shouldn’t dictate the mind, but the mind can turn tough conditions into opportunity. “Fundamentals must never be abandoned,” stresses Auger. The basics: sip regularly—three or four mouthfuls every 20–25 minutes—acclimate to heat in advance, adjust pace according to wind, know your course, plan hydration points. And avoid the trap of overhydration, which slows you down and upsets digestion.

Thermoregulation has become central. Previously, it was secondary. Today, “it’s essential,” says Devanne, to the point that a water bottle in hand can become “the best way to survive race season.” Adaptation yes, but not surrender. “Wind, heat, humidity… but not all at once.”

| Controlled vs. endured weather

Finally, there’s the ability to act rather than suffer. Mental strength shapes performance as much as the thermometer reading. Devanne notes: “Some thrive in the rain, others wilt in the sun,” proving each athlete responds differently. Turning adverse weather into advantage is almost an art: “The mind can make a terrible day perfect,” he says, convinced that perception matters as much as physical conditions.

At the elite level, nothing is left to chance. Clarifying your strategy before the start, anticipating surprises, knowing your target pace, and keeping margin for real-time adjustment is crucial. As climates shift, marathons will have to adapt: earlier starts, more northern courses, perhaps fewer records. The ideal weather window narrows, but the drive to seize it remains.

In the end, a simple truth prevails: weather influences but doesn’t dictate. Perfect days exist—morning at 13°C, windless, clear skies, a city waking just to cheer runners—and when they arrive, you’d better be sharp, confident, and ready to seize them. The rest belongs to the magic of the marathon: a delicate mix of meticulous preparation, unpredictable sensations, and an almost intimate rapport with the day’s weather.

➜ Discover the marathon calendar

---