Air temperature plus dew point tells you how much the weather is going to cost you today. Enter both, plus the pace you would run in perfect conditions, and get your adjusted pace.
Not the humidity percentage. Your weather app lists dew point separately.
Dew point is the number that decides whether your sweat can actually evaporate, which is why it matters more than the temperature alone.
| Pace | Per mile | What it means |
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My 14-day heat acclimation protocol, and how I rewrite a workout when the dew point spikes. One email when a new training note drops. No spam, unsubscribe anytime.
Your muscles turn most of the energy they burn into heat. The harder you run, the more heat you make, and the only real way your body dumps it is by evaporating sweat off your skin. Once your core temperature climbs past roughly 102°F, your body starts protecting itself: it sends blood to the skin to cool you instead of to your legs to run. Performance falls off a cliff, and if you keep pushing, you are no longer training, you are risking heat exhaustion.
That is why the number that matters is not the temperature on its own. It is the temperature plus the dew point. Dew point is the absolute measure of moisture in the air. When it is high, your sweat has nowhere to evaporate to, so it drips off you and takes almost no heat with it. Relative humidity does not tell you this, because it is only a percentage relative to the current temperature.
The heat index is closer to useful, since it does combine temperature and moisture, but it was built to describe how hot the air feels to someone standing still. It says nothing about a body producing its own heat at six miles an hour. For adjusting a running pace, temperature plus dew point is the number I trust.
Add the two together in Fahrenheit and you get a combined stress score. The score maps to a percentage you should slow down. This combined temp and dew point method comes from Coach Hadley's Maximum Performance Running, and it has held up across every summer I have coached through.
Find your dew point down the left, your air temperature across the top. Cells are blank where the dew point would be higher than the air temperature, which cannot happen.
| Dew ↓ / Temp → | 50° | 55° | 60° | 65° | 70° | 75° | 80° | 85° | 90° | 95° | 100° |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 45° | 95 | 100 | 105 | 110 | 115 | 120 | 125 | 130 | 135 | 140 | 145 |
| 50° | 100 | 105 | 110 | 115 | 120 | 125 | 130 | 135 | 140 | 145 | 150 |
| 55° | — | 110 | 115 | 120 | 125 | 130 | 135 | 140 | 145 | 150 | 155 |
| 60° | — | — | 120 | 125 | 130 | 135 | 140 | 145 | 150 | 155 | 160 |
| 65° | — | — | — | 130 | 135 | 140 | 145 | 150 | 155 | 160 | 165 |
| 70° | — | — | — | — | 140 | 145 | 150 | 155 | 160 | 165 | 170 |
| 75° | — | — | — | — | — | 150 | 155 | 160 | 165 | 170 | 175 |
| 80° | — | — | — | — | — | — | 160 | 165 | 170 | 175 | 180 |
Each band is a range rather than one number for good reason. Some runners handle heat better than others, and acclimation moves the whole scale: the first 70° day in spring feels like a heat wave, while the same 70° day in September feels cool. Fitness and wind matter too. Start in the middle of the range, and let your effort make the final call.
Your normal pace runs across the top. Find your slowdown percentage down the left to read the adjusted pace.
| Slowdown | 4:00 | 4:30 | 5:00 | 5:30 | 6:00 | 6:30 | 7:00 | 7:30 | 8:00 | 8:30 | 9:00 | 9:30 | 10:00 | 10:30 | 11:00 | 11:30 | 12:00 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0.5% | 4:01 | 4:31 | 5:01 | 5:32 | 6:02 | 6:32 | 7:02 | 7:32 | 8:02 | 8:33 | 9:03 | 9:33 | 10:03 | 10:33 | 11:03 | 11:33 | 12:04 |
| 1% | 4:02 | 4:33 | 5:03 | 5:33 | 6:04 | 6:34 | 7:04 | 7:34 | 8:05 | 8:35 | 9:05 | 9:36 | 10:06 | 10:36 | 11:07 | 11:37 | 12:07 |
| 2% | 4:05 | 4:35 | 5:06 | 5:37 | 6:07 | 6:38 | 7:08 | 7:39 | 8:10 | 8:40 | 9:11 | 9:41 | 10:12 | 10:43 | 11:13 | 11:44 | 12:14 |
| 3% | 4:07 | 4:38 | 5:09 | 5:40 | 6:11 | 6:42 | 7:13 | 7:44 | 8:14 | 8:45 | 9:16 | 9:47 | 10:18 | 10:49 | 11:20 | 11:51 | 12:22 |
| 4% | 4:10 | 4:41 | 5:12 | 5:43 | 6:14 | 6:46 | 7:17 | 7:48 | 8:19 | 8:50 | 9:22 | 9:53 | 10:24 | 10:55 | 11:26 | 11:58 | 12:29 |
| 4.5% | 4:11 | 4:42 | 5:14 | 5:45 | 6:16 | 6:48 | 7:19 | 7:50 | 8:22 | 8:53 | 9:24 | 9:56 | 10:27 | 10:58 | 11:30 | 12:01 | 12:32 |
| 5% | 4:12 | 4:44 | 5:15 | 5:46 | 6:18 | 6:50 | 7:21 | 7:52 | 8:24 | 8:56 | 9:27 | 9:58 | 10:30 | 11:02 | 11:33 | 12:04 | 12:36 |
| 6% | 4:14 | 4:46 | 5:18 | 5:50 | 6:22 | 6:53 | 7:25 | 7:57 | 8:29 | 9:01 | 9:32 | 10:04 | 10:36 | 11:08 | 11:40 | 12:11 | 12:43 |
| 7% | 4:17 | 4:49 | 5:21 | 5:53 | 6:25 | 6:57 | 7:29 | 8:02 | 8:34 | 9:06 | 9:38 | 10:10 | 10:42 | 11:14 | 11:46 | 12:18 | 12:50 |
| 8% | 4:19 | 4:52 | 5:24 | 5:56 | 6:29 | 7:01 | 7:34 | 8:06 | 8:38 | 9:11 | 9:43 | 10:16 | 10:48 | 11:20 | 11:53 | 12:25 | 12:58 |
| 9% | 4:22 | 4:54 | 5:27 | 6:00 | 6:32 | 7:05 | 7:38 | 8:11 | 8:43 | 9:16 | 9:49 | 10:21 | 10:54 | 11:27 | 11:59 | 12:32 | 13:05 |
| 10% | 4:24 | 4:57 | 5:30 | 6:03 | 6:36 | 7:09 | 7:42 | 8:15 | 8:48 | 9:21 | 9:54 | 10:27 | 11:00 | 11:33 | 12:06 | 12:39 | 13:12 |
A normal July morning in North Carolina:
A score of 150 sits right at the top of the 141 to 150 band, so the penalty is 3% to 4.5%. If the plan called for 8:30 pace, the honest equivalent is 8:45 to 8:53. One more degree of dew point pushes you into the next band, where the same run becomes 8:53 to 9:01, so on a day like this it is fair to take the slow end and not feel bad about it. Same effort, same training benefit, slower watch.
The number is a starting point, not a verdict. On a high-score day I would rather an athlete run the workout by effort and let the pace land where it lands. The mistake I see most often is holding winter paces through a humid August, which quietly turns every easy run into a threshold run and every threshold run into a race. Adjust early, keep the effort honest, and you get to the fall with your fitness intact.
For the physiology behind all of this, plus the 14-day acclimation protocol that turns a hot summer into a fall personal best, read why heat is the poor man's altitude. If you want the paces this chart adjusts from, start with the VDOT calculator.
A chart adjusts your pace. It does not notice when your heart rate has been drifting for two weeks or decide which workout to move. I read your training, your recovery and your week, then tune the plan by hand. It starts with a free 20-minute call, no pressure.
Book a free intro callAdd the air temperature to the dew point, both in Fahrenheit. That combined number tells you how much to slow down. At 100 or below there is no adjustment. From 101 to 110 the penalty is up to 0.5%. It climbs from there, reaching 8% to 10% between 171 and 180. Above 180, hard running is not recommended. Multiply your normal pace by the penalty to get your adjusted pace: an 8:30 pace at a 5% penalty becomes 8:56.
Dew point measures the actual amount of moisture in the air. Relative humidity is only a percentage relative to the current temperature, so 90% humidity at 50 degrees and 90% humidity at 85 degrees are completely different running conditions. You cool yourself through the evaporation of sweat, not through sweating itself. When the dew point climbs above 65°F, sweat stops evaporating efficiently and drips off you instead, so your body loses its main way to shed heat.
There is no single temperature, because the dew point does as much damage as the heat. Use the combined score. Once air temperature plus dew point passes 180, hard running is not recommended: move the workout to a treadmill, shift it to early morning, or run easy by effort only. Between 161 and 180 you can still train, but the intensity has to come down with the pace.
Yes, and that is the adjustment most runners skip. In heat your heart rate drifts upward at the same pace because blood is being sent to the skin to cool you. Holding your normal easy pace on a humid morning quietly turns an aerobic run into a threshold effort. Slow down, keep the effort easy, and let the acclimation happen over 10 to 14 days.
Roughly 10 to 14 days of consistent heat exposure. Your plasma volume expands, your sweat rate rises, your sweat gets less salty, and your heart rate at a given pace comes back down. You will still be slower than in cool weather, but the gap narrows. That expanded plasma volume is why a hot summer of honest training tends to show up as a fall personal best. The full protocol is here.