Running in heat and humidity: how much it slows you, with numbers

How much does running in heat and humidity slow you down?

Running in heat and humidity costs most runners 1–⁠5% of their pace, and more in extreme conditions. The cost rises steadily with heat stress, not at one cliff edge, and slower runners pay more.

The table shows one runner on a four-hour marathon, an even 5:41 per km (9:09 per mile), in six kinds of weather. The slowdown comes from John Davis's model in the free Running Writings calculator. Coach Mark Hadley's chart is alongside for comparison.

These are averages for steady conditions. Real race days warm up as you run, which is why a single number undersells the heat for anyone out past mid-morning. To see your own run kilometre by kilometre, drop your route into the free tool, Map your run.

The research agrees on the direction. Ely and colleagues (2007) studied seven North American marathons over up to 36 years. The top three men ran 1.7% slower than the course record when WBGT was 5–⁠10°C (41–⁠50°F), and 4.5% slower at 20–⁠25°C (68–⁠77°F). The 25th, 50th, 100th and 300th finishers slowed more.

Why does humidity make the heat so much worse?

Because you cool yourself by evaporating sweat, and humid air takes it more slowly. When the air already holds a lot of moisture, sweat drips off instead of carrying heat away.

The honest measure of that moisture is the dew point, not relative humidity. The US National Weather Service makes the point plainly: 30°F (−1°C) at 100% humidity feels far drier than 80°F (27°C) at 50%. Relative humidity says how full the air is at its current temperature, so it swings through the day even when the moisture doesn't.

Hold the air at 25°C (77°F) in sunshine and raise only the dew point. WBGT climbs from 22.5°C (72.5°F) at a 10°C (50°F) dew point to 26.3°C (79.3°F) at 20°C (68°F). That is the jump from Moderate to the upper half of High. Our guide to whether humidity affects running pace explains why studies often find temperature the bigger factor, and why dew point still matters.

What dew point is too high for a hard run?

Below a 10°C (50°F) dew point, humidity barely changes your pace. Above 18°C (64°F), most runners feel it on any hard effort, and above 21°C (70°F) it is oppressive.

The full chart, with every degree in both units, is in our dew point running chart. For what counts as good on a race morning, and how it depends on the temperature, see what is a good dew point for running.

How do you adjust your pace for heat and humidity?

Slow down before the heat makes you, by a set percentage for the conditions at the time you'll run. Two free methods dominate.

• Hadley's temperature plus dew point. Add the air temperature in °F to the dew point in °F. A sum of 100 or less needs no change; 131–⁠140 needs 2–⁠3%; above 180, hard running is not recommended. It is a coaching rule of thumb from 2013, not a peer-reviewed model.
• Davis's model. John Davis fitted a statistical model to 3,891 marathon runners across 754 races, from the dataset of Mantzios and colleagues (2022). His calculator takes temperature with humidity, dew point or heat index.

At 25°C (77°F) with a 15°C (59°F) dew point, Hadley's sum is 136, so 2–⁠3% slower. Davis gives 2.5%. A 5:00 per km (8:03 per mile) target becomes about 5:07 per km (8:15 per mile).

We compare the free tools side by side, with their strengths and limits, in heat-adjusted pace calculators compared.

What is WBGT, and why do race medical teams use it?

WBGT, the wet bulb globe temperature, is one heat-stress number that combines air temperature, humidity, sunshine and wind. Race medical teams use it because it tracks the risk of heat illness better than temperature alone.

The American College of Sports Medicine's 2007 position stand grades road-race risk by WBGT. Low is below 18°C (65°F), moderate 18–⁠23°C (65–⁠73°F), high 23–⁠28°C (73–⁠82°F) and extreme above 28°C (82°F). The Bank of America Chicago Marathon's Event Alert System uses four levels with the same names, from green to black, though its page gives no WBGT thresholds. RunFast computes WBGT with the model of Liljegren and colleagues (2008).

WBGT has one blind spot. The 2015 consensus led by Sébastien Racinais warns it may underestimate heat stress when sweat can't evaporate, in high humidity or still air. Our guide to WBGT for runners goes deeper, and marathon heat flags explains what each colour means for you on the day.

How hot is too hot to run?

There is no single cut-off, but the risk climbs fast once WBGT passes about 21–⁠23°C (70–⁠73°F) for a long race. Above 28°C (82°F), the ACSM's extreme band, its guidance is to cancel the race or advise runners to withdraw.

William Roberts studied finish-line medical records for a 2010 paper. Marathons in northern latitudes that started above a WBGT of 21°C (70°F) often ended in mass casualties or mid-race cancellation. For the Twin Cities Marathon in Minnesota, he calculated a "do not start" WBGT of 20.5°C (68.9°F).

Those numbers are for unacclimatised fields at spring and autumn races. Official race flags and medical advice override anything in this guide. Our guide on how hot is too hot to run sets out the limits in °C and °F, with their sources.

What is the ideal temperature for a marathon?

Cool: roughly 4–⁠10°C (39–⁠50°F) air, or a WBGT of about 7.5–⁠15°C (45.5–⁠59°F). Every degree above that costs time.

El Helou and colleagues (2012) analysed 1,791,972 finishes at Paris, London, Berlin, Boston, Chicago and New York from 2001 to 2010. The optimum air temperature ran from 3.8°C to 9.9°C (38.8°F to 49.8°F), depending on the group of runners. Above it, speed fell and more runners dropped out.

Mantzios and colleagues (2022) looked at 1,258 elite endurance races. Peak performance was most likely at a WBGT of 7.5–⁠15°C (45.5–⁠59°F), or 10–⁠17.5°C (50–⁠63.5°F) air. For every degree of WBGT outside that window, performance fell by 0.3–⁠0.4%. More in our guide to the ideal marathon temperature.

Why does a race get hotter the slower you run?

Because most races start in the cool of the morning and the day warms behind you. A three-hour runner is finished before the warmest hours; a five-hour runner is still out in them.

RunFast's analysis of ERA5 reanalysis shows it at the 2021 Berlin Marathon (reanalysis, not a thermometer at the start line). The race started at 16.5°C (61.7°F). A three-hour runner met air of up to 22.1°C (71.8°F) and peak WBGT of 21.6°C (70.9°F), Moderate. A five-hour runner met 24.1°C (75.4°F) and 22.9°C (73.2°F), reaching High from about km 40 (mile 25).

At Comrades in South Africa in 2015, an up run, the heat stress reached High from about km 42 (mile 26) for a nine-hour finisher. That is why a pace-timed forecast, the weather for each stretch at the time you'll run it, beats a city forecast. You can open any race day from 2015 to 2024 on the Berlin race page or the Comrades race page.

Over a whole day at ultra pace the effect grows: see running an ultramarathon in the heat and the desert stages of the Marathon des Sables.

How do wind and hills change the heat you feel?

Wind cools you by moving air across your skin, and hills slow you so less air moves. A slow, sheltered climb in the sun is where heat stress peaks.

A headwind cools but costs effort. A tailwind at your running speed can leave you in almost still air, which feels much hotter. Climbs add metabolic heat while your speed drops. Our guide to running into the wind and up hills puts numbers on both, from the classic studies of Pugh (1971) and Minetti (2002).

How long does it take to get used to running in the heat?

About one to two weeks of regular training in the heat. Most of the change arrives in the first week.

The 2015 consensus statement led by Racinais recommends repeated exercise-heat exposures over 1–⁠2 weeks. Heart rate, skin and core temperature fall, and sweat rate and work capacity rise. Acclimatising in dry heat also helps in humid heat, and the reverse. The benefits fade more slowly than they arrive, and most last 2–⁠4 weeks.

For a runner in London training for a warm Comrades, or a Bostonian heading to a humid autumn race, that means planning the heat block into the last weeks of training.

How do you plan a hot race day?

Check the heat at your pace along the route, set a slower opening pace for the hottest stretch, and decide in advance when you'll back off. Then let the race flags and medical teams have the last word.

1. Check the forecast for your stretch of road at your time. Look at the dew point and heat stress where you'll be in the final third, not at the start.
2. Set your pace from the hottest part. Apply the adjustment to the kilometres in the sun after mid-morning, not to the cool first hour only.
3. Start conservatively. A cool start tempts you to bank time; it usually costs more later.
4. Drink and cool to a plan. Use water on your head and neck, and the race's sponges and showers.
5. Respect the flags. At high or extreme heat stress, slow down further or stop.

The ACSM's 2021 consensus statement is clear that exertional heatstroke is a medical emergency, where stopping and rapid cooling save lives. If you feel confused or dizzy, stop. This guide is not medical advice.