Humid, Not Just Hot: What a Sticky Race Day Does to Pacing and Cooling

Commentators at the 2026 IRONMAN World Championship in Kona described conditions as "really, really humid." The professional races were decided late in the run. Magnus Ditlev was reported walking, and in the women's race Solveig Løvseth went down twice in the final kilometre. Casper Van Riel was seen vomiting on the bike. Nobody can say from the outside what caused each athlete's problems, but humid heat is one of the toughest conditions in endurance sport.

Most triathletes plan for heat by looking at the forecast temperature. Fewer think about humidity separately, even though it changes how well your body can cool itself. TriForward's "Heat Acclimation for Race Day" and "The Watts the Heat Takes" cover heat in general. This article focuses on what humidity adds, and on what the evidence says about cooling during a race.

Why humidity matters: evaporation

When you exercise, most of the energy your muscles use ends up as heat. At rest in cool conditions, your body loses heat to the air easily. During hard exercise in the heat, you rely mostly on sweating.

But sweat cools you only when it evaporates from your skin. Evaporation takes heat with it. Sweat that drips off or soaks into your clothing without evaporating does very little.

How fast sweat evaporates depends on the difference between the water vapour pressure at your skin and in the surrounding air. In humid air, that difference is small, so less sweat evaporates. You sweat as much or more, but you cool less. The skin stays warmer, which narrows the gap between your core and your skin, and that makes it harder for blood flowing to the skin to shed heat.

Air movement helps evaporation, which is why the bike usually feels more bearable than the run on a humid day. At running speeds, there's much less airflow.

Relative and absolute humidity

Weather forecasts give relative humidity: how much water vapour the air holds compared with the most it could hold at that temperature. Warm air can hold much more water than cool air. So 60% relative humidity at 20 °C means much less moisture in the air than 60% at 32 °C.

For cooling, what matters is the absolute amount of water vapour in the air, often measured as vapour pressure. That's one reason most heat studies have struggled to separate the effects of temperature and humidity: if you keep relative humidity the same and raise the temperature, you also raise the absolute humidity.

The study that separated temperature from humidity

Jenkins and colleagues designed a study to pull these apart, published in Experimental Physiology in 2023. Fourteen trained adults (seven women and seven men) each completed four trials in a crossover design. Every trial was 45 minutes of cycling at a fixed moderate intensity (70% of maximal oxygen uptake), followed by a 20-km time trial. A fan provided about 4.5 m/s of airflow, roughly 16 km/h.

The four conditions were:

• Cool: 18 °C, with a vapour pressure of 1.96 kPa
• Moderate: 27 °C, same vapour pressure
• Hot: 36 °C, same vapour pressure
• Hot-humid: 36 °C, with double the vapour pressure, 3.92 kPa

Inference: at 36 °C, those vapour pressures correspond to roughly 33% and 66% relative humidity, based on standard tables. That's our arithmetic, not a figure the paper reports in its abstract.

What heat alone did

Raising the air temperature, with the moisture in the air held constant, increased skin temperature (by about 0.5 °C for each 1 °C of air temperature), heart rate (about 1 beat per minute per degree), sweat rate and how hot participants felt. Core temperature barely changed during the fixed-intensity cycling.

Time-trial performance was similar in the cool and moderate conditions, but 3.6–6% slower in the hot condition.

What humidity added

Doubling the humidity at 36 °C:

• Raised core temperature
• Made participants feel hotter and more uncomfortable
• Didn't change skin temperature or heart rate
• Slightly lowered blood pressure
• Made the time trial 3.4% slower (95% confidence interval ±2.2%)

The authors concluded that absolute humidity matters, alongside air temperature, when heat stress is severe.

The finding that heart rate didn't rise is important for pacing. Inference: if you rely only on heart rate on a humid day, you may not see the extra strain that humidity adds, at least during the first hour. Your perceived effort may be a better early warning.

A second view: humidity at 30 °C

An earlier study by Maughan, Otani and Watson, published in the European Journal of Applied Physiology in 2012, varied humidity at a fixed temperature. Eight men cycled at 70% of maximal oxygen uptake to exhaustion at 30 °C, with relative humidity of 24%, 40%, 60% and 80%.

Average time to exhaustion fell steadily as humidity rose: 68 minutes at 24%, 60 minutes at 40%, 54 minutes at 60% and 46 minutes at 80%. Core temperature at exhaustion was about 39 °C in every condition, and sweat rate was higher in the more humid trials. Skin temperature was higher at 80% humidity than at 24%.

So athletes stopped at a similar core temperature, but got there sooner when the air was humid. The study had only eight men, and a test to exhaustion is not the same as a race, but the direction matches the Jenkins study.

What this means for race pacing

Neither study tested anything as long as an Ironman or a 70.3, and both used cycling in the lab. But they agree on the direction: humidity makes the same temperature harder.

Inference: if a humid day costs a few per cent on top of heat alone, a 4:30 Ironman marathon could be around 9 minutes slower from humidity alone, before any effect from heat. That's a rough extrapolation from a 20-km time trial, and the real effect over a full race is unknown. It could be larger, because heat strain builds over hours.

Practical pacing adjustments:

• Expect to be slower, and decide that before the start. Setting a lower target in advance is easier than reacting late.
• Pace the bike conservatively. The bike is where you have airflow and the best chance to manage heat. Going hard there leaves you hotter at the start of the run. TriForward's "The Watts the Heat Takes" covers bike power in the heat.
• Use perceived effort and heart rate together. Humidity may raise core temperature without an obvious change in heart rate early on.
• Start the run slower than usual. TriForward's "Why the Ironman Marathon Falls Apart After 25 km" explains why the run fades late, and heat makes that worse.

Cooling: what the evidence shows

Cooling strategies can be split into precooling (before exercise) and percooling (during exercise).

Precooling and percooling both help

Bongers and colleagues pooled 28 studies in a meta-analysis published in the British Journal of Sports Medicine in 2015. All tested cooling strategies in men exercising in heat above 30 °C. Twenty studies tested precooling and eight tested percooling.

On average, cooling improved performance by 6.7%. Precooling improved it by 5.7% and percooling by 9.9%, a difference that wasn't statistically significant. Combining several precooling methods worked best before exercise, and an ice vest worked best during exercise. Interestingly, performance wasn't closely related to core temperature, which suggests that feeling cooler may itself matter.

Ice slurry versus cold-water immersion

Choo and colleagues compared two popular precooling methods in a 2018 meta-analysis of 22 studies in the European Journal of Sport Science. Cold-water immersion before exercise clearly improved endurance performance and lowered peak core temperature. Ice ingestion, including ice slurry, had no clear effect on performance. It was also associated with a greater rise in core temperature once exercise started.

One proposed explanation, not tested directly in the meta-analysis, is that very cold drinks cool the core and may reduce the body's sweating response, so the benefit is lost once exercise warms you up. Inference: ice slurry before a long race may make you feel better at the start, but there's little reason to expect it to change your result hours later.

Why external cooling matters more on humid days

When humidity limits evaporation, your sweat can't do its job. Water poured over your head and neck, cold sponges and ice against the skin cool you by direct contact, which doesn't depend on evaporation in the same way. Inference: on humid days, external cooling is likely to matter more than usual, because your own sweating is less effective. This is based on the physics of heat transfer rather than a trial comparing humid and dry race conditions.

Race-day cooling options

These are practical options for long-course racing. They're consistent with the evidence above, but haven't been tested in Ironman races:

• Before the start: stay in the shade, keep your warm-up short, and consider cold towels or a cold drink. Cold-water immersion is rarely practical at a race venue.
• On the bike: pour water over your head, neck and forearms at aid stations. Wear a light-coloured, well-vented helmet and tri-suit.
• On the run: put ice in your hat, under your tri-suit and in your hands. Use sponges at every aid station. Drink cold fluids to thirst.
• Start early. Cooling before you overheat tends to work better than trying to recover once you're already struggling.

A humid race-day scenario

Here's how this might look for an age-grouper at a humid 70.3, using illustrative numbers rather than data from any study. In cool conditions, they'd plan to ride at 200 W and run at around 5:15/km. The forecast is 29 °C with 75% relative humidity, and they've done ten days of heat sessions on the turbo in a warm room.

Before the start, they stay in the shade and skip most of the warm-up. On the bike, they set a ceiling of about 190 W and pour water over their head and neck at every aid station. They drink to thirst rather than to a schedule. At the start of the run, they hold back to around 5:30/km for the first 5 km, put ice in their hat and tri-suit at each aid station, and take a cold sponge every time one is offered. If they feel hotter and more laboured than usual at that pace, they slow further, even if their heart rate looks normal.

They may finish a few minutes slower than they would on a cool day. Inference: on a humid day, that's likely to be a better result than holding normal targets and fading badly in the second half.

Heat acclimatisation is still the foundation

The 2015 consensus recommendations led by Racinais advised heat acclimatisation through repeated exercise-heat exposures over one to two weeks, starting events well hydrated, using cooling strategies, and race organisers providing shade, cooling and rehydration. Acclimatisation is generally associated with higher sweat rates and a lower heart rate and core temperature at a given effort. In humid conditions, some of that extra sweat drips off uselessly, but the cardiovascular benefits still apply. TriForward's "Heat Acclimation for Race Day" covers how many days you need.

Inference: if your race is likely to be humid, acclimatising in humid conditions, or in a heat chamber or warm indoor room with limited airflow, is more specific than training in dry heat.

Common mistakes

• Checking only the temperature. A 30 °C day with high humidity can be harder than a 34 °C dry day.
• Pacing to your normal heart rate. Humidity may raise core temperature before heart rate shows it.
• Waiting too long to start cooling. Cooling works better as prevention than rescue.
• Drinking instead of pouring. Water on the skin cools you; drinking more than you need doesn't, and can cause hyponatraemia.
• Trusting a pre-race ice slurry to carry you. The evidence for it is weaker than for other methods.

What we don't know

• How much humidity costs in a full Ironman. The studies lasted from about one to two hours.
• How humidity affects running specifically. Both main humidity studies used cycling, where airflow is greater.
• How cooling works in women and older athletes. The cooling meta-analysis included only men, though the Jenkins humidity study had an equal sex split.
• The best combination of race-day cooling. Few studies test realistic, multi-method cooling during long races.
• How individual differences play out. Body size, sweat rate, fitness and acclimatisation status all change how someone copes with humid heat, and the studies here report group averages.

References

• Jenkins EJ, Campbell HA, Lee JKW, Mündel T, Cotter JD. Delineating the impacts of air temperature and humidity for endurance exercise. Experimental Physiology, 2023;108(2):207–220. doi:10.1113/EP090969
• Maughan RJ, Otani H, Watson P. Influence of relative humidity on prolonged exercise capacity in a warm environment. European Journal of Applied Physiology, 2012;112(6):2313–2321. doi:10.1007/s00421-011-2206-7
• Bongers CCWG, Thijssen DHJ, Veltmeijer MTW, Hopman MTE, Eijsvogels TMH. Precooling and percooling (cooling during exercise) both improve performance in the heat: a meta-analytical review. British Journal of Sports Medicine, 2015;49(6):377–384. doi:10.1136/bjsports-2013-092928
• Choo HC, Nosaka K, Peiffer JJ, Ihsan M, Abbiss CR. Ergogenic effects of precooling with cold water immersion and ice ingestion: a meta-analysis. European Journal of Sport Science, 2018;18(2):170–181. doi:10.1080/17461391.2017.1405077
• Racinais S, Alonso JM, Coutts AJ, Flouris AD, Girard O, González-Alonso J, Hausswirth C, Jay O, Lee JKW, Mitchell N, Nassis GP, Nybo L, Pluim BM, Roelands B, Sawka MN, Wingo JE, Périard JD. Consensus recommendations on training and competing in the heat. British Journal of Sports Medicine, 2015;49(18):1164–1173. doi:10.1136/bjsports-2015-094915
• TRI247. IRONMAN Kona 2026 as it happened as Knibb and Luis crowned champions (live race report), 10 October 2026. tri247.com