The short answer
Yes, humidity affects running pace, but mainly on warm days, and the humidity percentage is the wrong number to watch. Read the dew point. At 25°C (77°F), raising it from 10°C to 20°C (50°F to 68°F) takes the expected marathon slowdown from 1.8% to 3.3%, by John Davis's model. In cool air, humidity barely matters.
Key takeaways
- Humidity slows you because sweat evaporates more slowly in moist air.
- Relative humidity swings with temperature; the dew point measures the moisture itself.
- Big studies find air temperature matters most; humidity adds to it on warm days.
- At 5°C (41°F), 90% humidity costs almost nothing. At 25°C (77°F), it costs about 4%.
On this page
Words this guide uses
- Relative humidity
- How close the air is to saturation at its current temperature, as a percentage.
- Dew point
- The temperature at which the air would be saturated; it measures the amount of moisture, whatever the temperature.
- Specific humidity
- The mass of water vapour in each kilogram of air, another direct measure of moisture.
- Pace-timed forecast
- The weather for each stretch of your route at the time you will actually run it.
Does humidity affect running pace?
Yes, humidity affects running pace on warm days, because moist air slows the evaporation of sweat that keeps you cool. On cool days it barely matters. The trick is to measure humidity as the dew point, not as a percentage.
It is one half of running in heat and humidity: heat makes the load, humidity decides how fast you can shed it. To see the dew point and heat stress along your own route at your pace, try the free tool Map your run.
Why is relative humidity a poor guide to pace?
Because relative humidity depends on the air temperature. The same percentage means very different amounts of moisture on a cold morning and a hot afternoon.
The US National Weather Service puts it simply: 30°F (−1°C) at 100% humidity feels much drier than 80°F (27°C) at 50%. Here is what that means for a marathon pace, by John Davis's model.
| Air, °C (°F) | Relative humidity (%) | Dew point, °C (°F) | Davis slowdown (%) |
|---|---|---|---|
| 5 (41) | 40 | −7.5 (18.6) | 0.2 |
| 5 (41) | 90 | 3.5 (38.3) | 0.2 |
| 25 (77) | 40 | 10.4 (50.8) | 1.8 |
| 25 (77) | 90 | 23.2 (73.8) | 3.9 |
At 5°C (41°F), going from 40% to 90% humidity changes nothing. At 25°C (77°F), it more than doubles the cost. Relative humidity is also highest at dawn, when the air is coolest, which makes early runs look more humid than they are. Our dew point running chart has every value in °C and °F.
What does the research say about humidity and marathon times?
The large studies agree that air temperature drives most of the slowdown. Humidity adds to it, and it shows up best through measures of actual moisture or heat stress.
- Vihma (2010) analysed the Stockholm Marathon from 1980 to 2008. Air temperature had the strongest link with finishing times. Relative humidity's link came only through its link with temperature. But the share of non-finishers rose with air temperature and specific humidity, a direct measure of moisture.
- El Helou and colleagues (2012) studied 1,791,972 finishes at six big marathons from 2001 to 2010. Air temperature had a major effect; humidity and dew point had no significant effect on their own.
- Ely and colleagues (2007) and Mantzios and colleagues (2022) measured heat stress as WBGT, which includes humidity. Both found performance falling as WBGT rose. Mantzios found WBGT predicted performance better than air temperature alone.
There is one more caveat. The 2015 consensus statement led by Sébastien Racinais says WBGT may underestimate heat stress when sweat can't evaporate. So on muggy, still days, treat the numbers as a floor.
How much does a higher dew point slow you?
At 25°C (77°F), each extra 5°C (9°F) of dew point costs roughly another 0.7–0.8% of marathon pace by Davis's model. Hadley's coaching chart gives slightly bigger steps.
Here is a 5:00 per km (8:03 per mile) runner on a sunny 25°C (77°F) day.
| Dew point, °C (°F) | Davis slowdown (%) | Davis pace per km | Davis pace per mile | Hadley chart (%) | WBGT in sun, °C (°F) |
|---|---|---|---|---|---|
| 10 (50) | 1.8 | 5:05 | 8:11 | 1–2 | 22.5 (72.5), Moderate |
| 15 (59) | 2.5 | 5:07 | 8:15 | 2–3 | 24.2 (75.6), High |
| 20 (68) | 3.3 | 5:10 | 8:19 | 3–4.5 | 26.3 (79.3), High |
Ten degrees of dew point move this runner from Moderate heat stress into the upper half of High. Over a marathon, the extra 1.5% is about three minutes for a 3:30 finisher. For what counts as acceptable, see what is a good dew point for running.
What happened on humid race days?
Humid days can carry more heat stress than warmer, drier ones. The Boston Marathon in the USA gives a clean comparison, from RunFast's analysis of ERA5 reanalysis (reanalysis, not a thermometer at the start line).
| Boston Marathon, four-hour finish | Start air, °C (°F) | Warmest air, °C (°F) | Peak dew point, °C (°F) | Peak WBGT, °C (°F) |
|---|---|---|---|---|
| 18 April 2016 | 19.3 (66.7) | 21.1 (70.0) | 3.1 (37.6) | 18.8 (65.8), Moderate |
| 11 October 2021 | 15.7 (60.3) | 20.7 (69.3) | 15.7 (60.3) | 21.6 (70.9), Moderate |
In 2016 the race started warmer, but the air was very dry. In October 2021, it started cooler with a dew point 12.6°C (22.7°F) higher. Peak heat stress came out almost 3°C (5°F) higher. Sun and wind differed too, so this isn't humidity alone. You can replay both days on the Boston race page.
How should you pace a humid run?
Slow down from the start, by the amount the dew point and temperature call for at the time you'll be running. Then run the effort, not the watch.
- Check the dew point for your run's hour, not the morning reading or the day's high.
- Pick your adjustment from a calculator or Hadley's chart. Our heat-adjusted pace calculators compared shows how they differ.
- Apply it to the late kilometres if the day is warming.
- Halve it for intervals, as Hadley suggests, because you cool in the recoveries.
- Run easy days by feel. Pace targets matter most in workouts and races.
On race day, official flags and medical advice override any pace plan. This guide is not medical advice.
Questions runners ask
Does humidity slow you down in cold weather?
Hardly at all. In cool air there is little heat to shed, and the air can't hold much moisture anyway. John Davis's marathon model gives the same 0.2% at 5°C (41°F) whether the humidity is 40% or 90%, and that small cost comes from the cold, not the moisture. Damp cold can feel raw, but it won't slow your pace the way humid heat does.
Is humidity or temperature more important for running pace?
Temperature, in the big studies. Vihma (2010) found air temperature had the strongest link with Stockholm Marathon times. El Helou and colleagues (2012) found no significant effect of humidity or dew point once temperature was known. Mantzios and colleagues (2022) rated air temperature the most important factor, at 40%. Humidity matters most when it is already warm.
Do heat-adjusted pace calculators use relative humidity or dew point?
It varies, as of September 2026. Running Writings accepts relative humidity, dew point or heat index, and the first two give identical results. The RunnersConnect calculator asks for the dew point. McMillan's heat calculator and RunDida's ask for relative humidity. Runna's Adapt for Heat works from the feels-like temperature. Any of them is fine if you enter the conditions for the hour you'll run.
Why do I feel soaked on a humid run?
Because your sweat is not evaporating. Sweat cools you only as it turns to vapour. When the dew point is high, the air takes up moisture slowly, so sweat collects on your skin and kit and drips off. Dripping sweat removes little heat, so you keep sweating, lose fluid and still warm up. That is why a muggy run feels harder than its temperature.
Can a breeze or a fan offset the humidity?
Partly. Moving air helps sweat evaporate even in humid conditions. The 2015 consensus statement led by Sébastien Racinais warns that WBGT may underestimate heat stress in high humidity or still air. It suggests fans and ice vests as practical cooling in hot, humid places. Outdoors, a breezy waterfront or open road can feel easier than a sheltered, still park at the same dew point.
See the heat at your pace.
Drop in your route (GPX, FIT or TCX), pick the day and your pace, and see the feels like, dew point and heat stress for every kilometre, timed to when you’ll be there. Free, in your browser, no sign-up.
Sources and method
- International Journal of Biometeorology (2010). Effects of weather on the performance of marathon runners (Vihma)
- PLOS ONE (2012). Impact of environmental parameters on marathon running performance (El Helou et al.)
- Medicine & Science in Sports & Exercise (ACSM) (2022). Effects of weather parameters on endurance running performance: discipline-specific analysis of 1258 races (Mantzios et al.)
- Medicine & Science in Sports & Exercise (ACSM) (2007). Impact of weather on marathon-running performance (Ely, Cheuvront, Roberts and Montain)
- British Journal of Sports Medicine (2015). Consensus recommendations on training and competing in the heat (Racinais et al.)
- US National Weather Service, La Crosse (2026). Why dew point vs humidity
- Running Writings (John Davis) (2026). Heat and humidity pace calculator for runners
- Maximum Performance Running (Mark Hadley) (2013). Temperature + dew point for pace adjustments
- 9to5Mac (2026). Runna now automatically adapts training paces based on heat and humidity
- Copernicus Climate Change Service (ECMWF) (2026). ERA5 hourly data on single levels from 1940 to present
Slowdowns are from John Davis's heat-and-humidity model in the Running Writings calculator (its published JavaScript, effort mode), with relative humidity from the dew point by the Magnus formula; the model was fit to marathon results, so treat the figures as long-race estimates. Paces are 5:00 per km (8:03 per mile) slowed by those percentages and rounded to the second. Hadley's percentages come from his published chart (air °F plus dew point °F, rounded). WBGT values are from RunFast's engine, which follows Liljegren et al. (2008), with wind 10 km/h (6 mph) at 10 m, sea level and strong sun (600 W/m² at a sun height of 45°). Boston figures are for a four-hour finish, from RunFast's analysis of ERA5 reanalysis (Copernicus Climate Change Service), hourly along the course: reanalysis, not a thermometer at the start line. Imperial values are our conversions.
Wes Thomson
Wes Thomson builds RunFast, which times the heat, humidity and dew point to a runner's own pace along the route, and its sister Windline, live wind forecasts for cyclists. Both run on the same forecast engine. He writes these guides with one focus: what the weather will do to your run, kilometre by kilometre, and how to plan around it.