How much do heat and humidity slow your running pace?
Short answer: on a muggy summer morning, expect to lose somewhere between 15 and 60 seconds per mile at the same effort, and more in full sun or real heat. Dew point matters as much as temperature, sun adds more on top, and slower runners lose more than fast ones. A cool, dry morning works the other way: you run a few seconds per mile faster for the same effort.
Your body sheds the heat of running mostly by sweating, and sweat cools you only when it evaporates. Anything that slows evaporation, or adds heat, leaves less of your effort for speed.
What drives the cost #
- Dew point. The amount of water already in the air. Humid air takes up sweat slowly, so it drips instead of cooling you. It's the biggest single factor most runners underrate.
- Air temperature. The warmer the air, the less heat your skin can lose to it. On its own, warm dry air costs far less than the same temperature when it's humid.
- Sun. Direct sun is a heat load the thermometer misses. The same hour costs more at midday in full sun than under cloud, in shade or at dawn.
- Wind. In the heat, a breeze helps you cool; still, heavy air makes it worse. In the cold, wind turns a mild day raw. Running into a headwind also costs effort directly.
- Your pace. Slower runners lose more, in seconds and as a share of their pace. They're out longer and make less cooling airflow.
- Your heat tolerance. Some runners handle heat better, and everyone gets better after 10–14 days of training in it.
- Rain, snow and altitude add their own costs: wet or icy footing, cold rain, and thinner air at elevation.
Dew point: the number to check #
Relative humidity depends on the temperature, so 90% at dawn can feel fine and 50% in the afternoon miserable. Dew point measures the water in the air directly, so it tells you more about a run:
- Under about 55°F: comfortable. Heat costs little unless the air itself is hot.
- 55–65°F: noticeable. A warm morning starts to cost real time.
- 65–70°F: sticky. Easy runs feel hard; slow down by effort.
- Over 70°F: oppressive. Expect a big slowdown, and take the heat illness warning signs seriously.
What it adds up to #
Rough ranges, in shade, for easy paces from about 8:00 to 12:00 per mile; the faster runner is at the low end:
- Cool and dry (about 50°F, dew point 35°F): a few seconds per mile faster.
- Mild (about 65°F, dew point 55°F): close to no change.
- Warm and humid (about 75°F, dew point 65°F): roughly 15–40 seconds per mile slower.
- Muggy (about 80°F, dew point 70°F): roughly 25–60 seconds per mile slower.
- Hot and steamy (about 85°F, dew point 75°F): roughly 30–85 seconds per mile slower, and more as it gets hotter.
Full sun adds more on top of each of these. These are planning ranges, not a prediction for your run: ArcPace works out the number for your pace, your heat tolerance and each hour's actual forecast.
Why slower runners lose more #
Most heat charts come from competitive race results. But in marathon data (Ely and colleagues, 2007), the leaders lost under 1% for each 5°C of heat stress while runners further back lost over 3%. Slower runners spend longer in the heat and make less cooling airflow, so a fixed adjustment undercounts the cost for them.
Estimates for planning easy runs, not medical advice. In real heat, slow down by feel, drink, and stop if you feel unwell. For race-day risk, see WBGT and heat flags; for how to stay cool, how to run in the heat.
Questions #
How much slower should I run in heat and humidity?
On a muggy summer morning, roughly 15 to 60 seconds per mile at the same effort, more in full sun or real heat. Dew point matters as much as temperature, and slower runners lose more than fast ones.
Is dew point or humidity better for running?
Dew point. Relative humidity depends on the temperature, so 90% at dawn can be comfortable and 50% in the afternoon miserable. Dew point measures the water in the air directly: under about 55°F feels fine, 55–65°F is noticeable, and over 70°F is oppressive.
How much does 80 degrees slow running?
It depends on the dew point, the sun and your pace. At 80°F with a muggy 70°F dew point, in shade, easy runners typically lose roughly 25 to 60 seconds per mile; with dry air, far less.
Does running in the sun make a difference?
Yes. Direct sun adds a heat load the air temperature doesn't show, so the same hour costs more at midday in full sun than under cloud or at dawn.
Do you run faster in cool weather?
A little. A cool, dry morning lets you run a few seconds per mile faster for the same effort than a mild day.
Related guides
ArcPace scores every hour of your forecast 0–100, with the pace cost at your easy pace and what to wear. Free for iPhone. Get it on the App Store