Guide · Heat & Humidity
Dew point and running: how humidity really slows you down
You ran the same loop, at the same effort, and your pace was 20 seconds slower per kilometre. The temperature looked fine — but the dew point was 22°C. For runners, dew point is the most honest number on a weather forecast. Here's why, and how to adjust your training when it climbs.
Why dew point beats humidity
Relative humidity is a ratio. 90% humidity at 15°C means the air is nearly saturated — but cool air can't hold much moisture, so there isn't actually that much water in it. 60% humidity at 32°C, by contrast, describes a sky full of water vapour. Same number, very different runs.
Dew point is the temperature at which the air would have to cool before water condenses. It's an absolute measure of moisture, so it doesn't lie. As dew point rises, the gap between your skin temperature and the air's saturation point shrinks, which means sweat evaporates more slowly — and sweat evaporation is the only thing keeping your core temperature in check while you run.
Air temperature
How hot it feels in the shade.
Relative humidity
Ratio — useless without temperature context.
Dew point
Absolute moisture. What actually limits cooling.
The runner's dew point scale
Use this scale before every workout. It's calibrated for trained runners; new runners should expect impacts one band earlier.
< 13°C (55°F)
Neutral
No noticeable effect on pace or effort.
13–16°C (55–60°F)
Slight
Faster sweat rate; barely felt on easy runs, mild on tempos.
16–18°C (60–65°F)
Noticeable
Workouts feel ~3–5% harder. HR drift starts earlier.
18–21°C (65–70°F)
Hard
Race pace drops 4–6%. Tempo runs feel like threshold.
21–24°C (70–75°F)
Brutal
Long runs degrade fast. Plan for 6–9% slower pace at the same effort.
> 24°C (75°F)
Dangerous
Effort-based only. Hydration and electrolytes are non-negotiable.
Pace adjustment: the 1%-per-degree rule
For every 1°C of dew point above 13°C, add roughly 1% to your easy and tempo pace at equal effort. It's a ceiling, not a discount — you don't get to run easier on cool days, but you do get permission to run slower on hot ones.
Dew point 13°C
Easy pace 5:00 /km becomes 5:00 /km
No change
Dew point 17°C
Easy pace 5:00 /km becomes 5:12 /km
+4% · +12s/km
Dew point 21°C
Easy pace 5:00 /km becomes 5:24 /km
+8% · +24s/km
Dew point 25°C
Easy pace 5:00 /km becomes 5:36 /km
+12% · +36s/km
The rule scales linearly enough up to ~25°C dew point. Beyond that, it under-predicts: dehydration and cardiac drift compound, and you're better off prescribing effort and HR than pace.
Four real cities, four very different runs
London, autumn dawn
Temp 10°C
RH 85%
Dew 8°C
Neutral — pace honestly
New York, summer evening
Temp 26°C
RH 65%
Dew 19°C
Hard — add 4–6% to pace
Singapore, midday
Temp 31°C
RH 82%
Dew 27°C
Dangerous — easy effort only
Phoenix, dry summer
Temp 38°C
RH 20%
Dew 12°C
Hot but low dew — pace holds better than HR
Phoenix illustrates the most common misread: people see 38°C and panic, then realise their pace is fine — it's the heart rate that's elevated, because the body is dumping more cardiac output toward skin cooling. Singapore is the opposite — pace looks normal until it suddenly doesn't, because there is no evaporative relief at all.
Three workouts, three adjustments
Easy run
Run by HR ceiling, not pace. Cap at 75% max HR. Add 10–30 seconds per kilometre and stop apologising for it.
Tempo / threshold
Drop target pace by the dew-point penalty before you start. Better to nail RPE 7/10 at a slower pace than to blow up trying to defend a winter PB.
Intervals
Keep the reps at goal pace, extend recovery by 30–60s, and cut the set short if HR can't recover. Quality over volume.
Heat-acclimation in 10–14 days
The good news: your body adapts. Plasma volume expands within a week, sweat starts earlier and saltier, and core temperature stabilises sooner. Two weeks of consistent runs in the heat — at sensible intensities — restores 70–90% of your cool-weather pace at the same effort.
- · Days 1–4: easy effort only. Walk breaks are fine.
- · Days 5–9: reintroduce strides and short tempo.
- · Days 10–14: normal sessions, with adjusted pace targets.
When to bail
Stop running and find shade if you stop sweating, feel chills in the heat, get a pounding headache, or can't think straight. These are early heat-illness signs, not toughness signals. Walk in, drink electrolytes, cool the neck and wrists.
Try the calculator
Get an adjusted pace for your actual conditions
adfez adjusts every run for heat, humidity, and dew point automatically — and shows the better-weather estimate so you can judge effort properly. Connect Strava once; the rest is automatic.