How it works
index = temperature°F + dew point°F → pace slowdown band
Step 1 — pull the forecast for the whole race window. Temperature usually climbs through a race morning, so for a 7 a.m. marathon look at the hour you expect to finish as well as the start. Step 2 — run both hours through the calculator. A runner targeting 3:40 needs 8:23 per mile. At the start, 68°F with a 62°F dew point sums to 130: a 1–2% slowdown, or 8:28–8:33 per mile. By the expected finish, 79°F with a 64°F dew point sums to 143: 3–4.5%, or 8:38–8:46 per mile. Held for 26.2 miles, that later band turns 3:40 into roughly 3:47–3:50. Step 3 — decide the goal before the gun. Our rule of thumb (not part of Hadley’s chart): if the finish-hour band costs only a minute or two, keep the goal and start at goal pace, or at the fast end of the early band if it already feels warm; if it costs more, race the adjusted time or switch to an effort or place goal. A second rule of ours, also not Hadley’s: once the sum passes 160 — say 88°F with a 75°F dew point, a 6–8% slowdown that turns 7:30 pace into 7:57–8:06 — a goal set in cool training weather is rarely realistic. Step 4 — if you reset, run the slowdown as a ramp. Start near the adjusted pace for the start hour and let the pace drift toward the finish-hour band as the day warms, rather than banking time while it is cool. Step 5 — metric works the same way. A half marathoner holding 5:10 per km in cool weather who faces 26°C with a 20°C dew point (147 in Fahrenheit terms) gets 3–4.5%: 5:19–5:24 per km, which turns a 1:49:00 goal into about 1:52–1:54.
Sources
- Hadley temperature + dew-point method Mark Hadley, Maximum Performance Running — heat & humidity pace-adjustment chart: temperature (°F) + dew point (°F) → percentage slowdown band.
- Why dew point, not just temperature Dew point quantifies absolute humidity; high humidity limits evaporative sweat cooling, raising heat strain independent of air temperature.
- Heat and endurance performance Endurance running pace declines as heat stress rises, with larger effects over longer races and for less heat-acclimated runners.
FAQ
What should I race for if my time goal is gone?
Pick a goal you can still control. An adjusted finish from the slower end of the band is the obvious one; an effort goal — first half at a pace that feels easy, strongest over the last third — is another; and a place or age-group goal works because everyone races in the same heat. Some marathoners turn a brutal day into a long training run and target a cooler race a few weeks later. What rarely works is racing the cool-weather goal and hoping.
Which forecast hour should I enter?
Enter both ends of the race. For a 5K or 10K the start hour is close enough, but for a half or full marathon the finish hour can be several degrees warmer while the dew point barely moves. Use an hourly forecast for the course itself rather than a nearby city average, and check it again the evening before and on race morning.
How much should I drink during a hot race?
Enough to stay ahead of thirst without overdrinking. Sweat losses vary widely between runners, so the most useful number is your own: weigh yourself before and after a warm training run and use the sweat-rate calculator to see what you lose per hour. On race day, drink small amounts regularly at aid stations, add sodium for longer races, and avoid finishing heavier than you started — drinking far more than you sweat can cause dangerously low blood sodium.
Is a hot 5K affected as much as a hot marathon?
Less, in practice. The chart gives the same band for every distance, but over 20-odd minutes your core temperature has less time to climb, so a 5K runner can usually race nearer the fast end of the band. In a marathon the heat load builds for hours: plan for the slow end and expect the second half to be the hard part.
What should change in my race-morning routine?
Shorten the warm-up and keep it in the shade, arrive well hydrated without forcing extra fluid, and consider pre-cooling with cold drinks, ice or a wet towel on the neck before the start. Wear light, loose clothing with a cap or visor, and use sunscreen. At aid stations, pour water over your head and neck as well as drinking; it does little for core temperature but helps you feel cooler and hold a steady effort.
How do I change my pace band for the heat?
Rebuild it around the adjusted finish time rather than the cool-weather goal. Take the slower band for the warmest hour you will race, convert it to a finish time, and print splits for that time with the printable pace band tool. Racing with the old band invites you to chase splits the weather has already taken off the table.
The pace bands come from a coaching chart applied to the forecast you enter; your own tolerance depends on fitness, acclimation, health, sleep and what you have eaten and drunk. Heat illness can be life-threatening — slow down, find shade and get medical help if you feel dizzy, confused or chilled or stop sweating, and treat this page as race-planning information, not medical advice.