You checked the forecast. Twenty-six degrees, partly cloudy, light wind. It sounds manageable — a warm day, not a hot one. You've trained in worse.
Except the forecast doesn't tell you what 26°C feels like at hour six. It doesn't tell you what happens when humidity stops your sweat from cooling you. And it doesn't tell you that by the time you feel it on the run, the thermal debt has been accumulating since the first hour of the bike. Weather doesn't hit your race the way it hits your morning ride. It compounds — and a plan that doesn't account for conditions that seem perfectly reasonable is a plan that will come apart in the second half of the marathon.
The research places the threshold for cycling performance at around 22°C. For running, it's lower — closer to 20°C. Above those lines, sustainable output drops by roughly 0.5% per degree for unacclimatized athletes. At 28°C, that's a 3% reduction in what you can sustain. At 32°C, it's 5%.
But here's why athletes get caught out: the bike hides most of it. At race speeds above 35km/h, airflow over your body dissipates heat effectively — enough to reduce the practical penalty by a third or more on flat, fast sections. You feel warm, but you don't feel slower. The power target still feels achievable. The numbers still look right on the head unit.
The protection has holes. On climbs at 15km/h, there's almost no convective cooling. You're producing maximum heat at minimum airflow — the full temperature penalty applies. And above about 33°C, the equation flips entirely: the air is hotter than your skin, so airflow adds heat instead of removing it.
Then you get off the bike and start running, and every bit of protection disappears. Running pace doesn't generate enough airflow to matter. The threshold is lower. And you're carrying whatever thermal load the bike left you with. This is why athletes who felt fine on the bike can fall apart within the first few kilometres of the run.
Most race-season conditions sit squarely in the range where this matters. Aix-en-Provence has seen 35°C on the run. Nice has hit 33°C on the unshaded Promenade des Anglais. But you don't need extremes. A race day at 25 or 26 degrees is already above both thresholds — and the run will collect what the bike concealed.
Temperature gets all the attention. Humidity is what actually breaks races.
Higher humidity means the air is more saturated — less able to absorb your sweat, which is your body's primary cooling mechanism. Most athletes understand that much. What they don't account for is that the cost isn't proportional. Going from 40% to 60% humidity at the same temperature is noticeable but manageable. Going from 60% to 80% is dramatically worse — not twice as bad, but three or four times as costly in performance terms. The curve steepens because once the air is too saturated to evaporate sweat efficiently, your body has no fallback. Core temperature rises faster, and sustainable output drops sharply.
This is why thirty degrees at 40% humidity and thirty degrees at 70% humidity are two completely different races. At the lower humidity, your cooling system is functioning and the heat penalty is manageable. At 70%, military research found roughly double the aerobic capacity reduction compared to dry conditions at the same temperature.
It also explains why some moderate-temperature race days produce unexpectedly bad results. A 24°C morning with high humidity can be more destructive than a 30°C day with dry air — because the body's cooling system is compromised at a temperature most athletes wouldn't think to adjust for. If your plan was built for the temperature but not the humidity, it was built for the wrong conditions.
Heat doesn't settle into a steady state during long racing. It accumulates.
In cooler conditions, core temperature rises and then plateaus — the body finds equilibrium. In heat, that plateau never comes. Core temperature keeps climbing, and the performance cost climbs with it. In laboratory time trials, cyclists who rode at the same power in both cool and hot conditions showed no difference in the opening phase. The penalty emerged in the second hour and grew from there — by the back half, the hot group was producing measurably less power despite starting at the same output.
It's not just cardiovascular. Your metabolic efficiency — how much of your effort becomes forward motion — degrades in heat. One study measured it dropping from 20.5% to 19.6%. That's a 4.4% reduction that has nothing to do with heart rate or hydration. You're working the same. Less of it is moving you forward. More of it is becoming heat, which makes the problem worse.
This is what makes the back half of a hot race disproportionately hard. The temperature at hour five is the same as at hour one. Your body's ability to cope with it isn't. Duration and heat are multipliers, not additive — and a plan that applies the same adjustment from start to finish is underestimating the cost of every hour after the second.
The bike has built-in air conditioning. At speeds above 35km/h, airflow dissipates heat fast enough to meaningfully reduce the temperature penalty. It's why you can feel fine on the bike at 28°C — the wind is doing work your body can't do alone.
The run has nothing. No meaningful airflow at running pace. A lower temperature threshold — around 20°C versus 22°C for the bike. And a starting point of whatever core temperature the bike left behind, which after four or five hours of racing in warm conditions is already elevated.
This is why athletes blow up on the run on warm days they thought they'd managed. They did manage — on the bike. The airflow masked the accumulating thermal load. Every climb where the speed dropped and the cooling disappeared. Every hour where core temperature crept higher. It was all being absorbed by the bike's convective cooling advantage. At T2 that advantage vanishes, and the debt lands.
It's also why exposed climbs on the bike are more dangerous than they look in warm weather. Low speed, high power output, direct sun, no airflow. That's the thermal vulnerability point of any bike course — and on a warm day, it's where the damage that shows up on the run is actually done.
The numbers are dramatic. Unacclimatized athletes in hot conditions lose roughly 16% of their sustainable power capacity. After six days of heat exposure, that drops to about 8%. After fourteen days, it's close to zero. No other single intervention — not nutrition, not cooling vests, not pre-cooling — comes close to the impact of proper heat acclimatization.
But the adaptation decays quickly. Without continued heat exposure, you lose roughly 2.5% of the benefit per day. A two-week protocol completed three weeks before race day has already given back most of its gains.
Most age-groupers can't do a full fourteen-day protocol timed perfectly to their race. Work, family, geography — the logistics rarely align. Which makes weather-adjusted pacing targets even more critical. If you can fully acclimatize, the heat penalty is nearly eliminated and your cool-weather targets may still hold. If you can't — and most athletes can't — the plan has to account for the gap between your fitness and what the conditions will allow you to express.
Weather isn't a footnote on a race plan. It's a variable that changes every target across every discipline.
It starts in the water. Water temperature determines whether you're racing in a wetsuit or without one — a difference that directly affects buoyancy, drag, and the effort required to hold pace. Go high enough and wetsuits are banned. Go too high or too low and the swim distance may be shortened entirely due to safety risks. Your swim time projection changes before you've even started the bike.
A generic heat adjustment for the bike — "knock 5% off your power if it's hot" — is better than nothing. But it doesn't account for the difference between a sheltered valley and an exposed climb at the same temperature. It doesn't adjust for humidity. It doesn't model how the penalty compounds across hours of racing. And it applies the same correction at the start as at the finish, when the cost is lowest and highest respectively.
Keiro integrates the forecast into the full simulation. Temperature, humidity, and wind — applied to the actual course, segment by segment, accounting for how conditions interact with terrain, speed, and exposure. Water temperature feeds into the swim model. As the forecast sharpens toward race day, the plan updates with it. Bike power targets reflect what the conditions will actually allow. Run projections reflect the thermal load the bike will generate. The plan isn't adjusted for weather after the fact — it's built with weather as a core input from the start.
Weather is the one thing you can't control on race day. But you can plan for it. A warm day doesn't have to mean a blown-up run — it means different targets. A humid morning doesn't have to mean a surprise at kilometre fifteen — it means the plan already accounted for what the air can and can't do for you.
The athletes who race well in tough conditions aren't tougher. They're better prepared — with targets that reflect the day they're actually racing, not the cool morning they tested on three weeks ago.
Find your race and start planning.