How Weather Affects Marathon Pace
Temperature, humidity, and the math behind adjusting your race day goal.
Weather is the single biggest external variable on race day. A runner trained to break 3:00 in ideal conditions might run 3:06 to 3:12 in heat and humidity — not because they're less fit, but because the physics of thermoregulation work against you as conditions worsen.
Understanding how weather impacts pace — and adjusting your plan accordingly — is the difference between a smart race and a death march.
The Temp + Dewpoint Method
The most useful way to gauge race day conditions is to add the temperature and dewpoint together. This combined number captures both heat and humidity in a single metric that correlates well with marathon performance impact.
| Temp + Dewpoint | Conditions | Pace Impact | Example (7:00 base pace) |
|---|---|---|---|
| Under 100 | Ideal | No adjustment | 7:00/mi |
| 100–120 | Mild | +1 to 2% | 7:04–7:08/mi |
| 120–140 | Warm | +2 to 4.5% | 7:08–7:19/mi |
| 140–150 | Hot | +4.5 to 6% | 7:19–7:25/mi |
| 150–160 | Very Hot | +6 to 8% | 7:25–7:34/mi |
| 160+ | Extreme | +8 to 10% | 7:34–7:42/mi |
To use this: check the forecast for race morning. Take the expected temperature at start time and the dewpoint (not relative humidity — dewpoint is more accurate). Add them together and look up your adjustment.
Why Dewpoint Matters More Than Humidity
Relative humidity is misleading because it changes with temperature. A 50% humidity reading at 80°F is very different from 50% at 50°F. Dewpoint, on the other hand, is an absolute measure of moisture in the air. A dewpoint of 65°F means the same thing regardless of temperature — it's sticky and your sweat won't evaporate efficiently.
As a rough guide for dewpoint alone: under 55°F is comfortable, 55–65°F is noticeable, and above 65°F significantly impacts performance. Most weather apps show dewpoint — check it the morning of the race alongside temperature.
How Heat Slows You Down
When you run, about 75% of the energy your muscles produce is heat. Your body has to dump that heat to keep your core temperature from rising to dangerous levels. The primary cooling mechanism is sweat evaporation.
In hot or humid conditions, sweat doesn't evaporate as efficiently. Your body has to work harder to cool itself — diverting blood flow from your muscles to your skin, increasing heart rate, and ultimately forcing you to slow down to manage core temperature. This is not a willpower problem. It's thermodynamics.
Real example: At the 2024 Boston Marathon, race day conditions reached a temp + dewpoint sum of approximately 140–150. Many elite and sub-elite runners came in 5–10 minutes over their target times. Conditions-adjusted, these were still strong performances — but runners who didn't adjust their pacing strategy blew up catastrophically in the second half.
Adjusting Your Race Plan for Heat
Pacing
The most important adjustment: slow your target pace based on the table above. Start conservative and let your body find a sustainable rhythm. The worst thing you can do in hot conditions is go out at your cool-weather goal pace — you'll overheat by mile 15 and the back half will be brutal.
Hydration
Increase your fluid intake by 15–30% in warm to hot conditions. If your baseline target is 22 fl oz/hr in mild weather, bump it to 25–28 fl oz/hr in warm conditions and 28–32 in hot. You'll also want to take water at every aid station, even if it's just a few sips.
Sodium
More sweat means more sodium loss. In cool conditions you might need 400–500 mg/hr. In hot conditions, that jumps to 1,000–1,100+ mg/hr. If you're mixing your own bottles with electrolyte powder, increase the concentration for hot weather.
Gel timing
Your gel schedule stays the same — same intervals, same count. But make sure you're washing gels down with extra water in the heat. Concentrated carbs without adequate fluid can cause GI distress, which is worse when you're already overheated.
Cooling strategies
Pour water over your head and neck at aid stations. Wear a light-colored hat or visor. If the race offers ice or sponges, use them. These don't replace slowing down — but they help manage core temperature alongside a smart pacing adjustment.
When to Abandon Your Goal
If the temp + dewpoint sum is above 150, seriously consider adjusting your goal time rather than forcing it. You're looking at 6–10% slower than ideal conditions, which means a sub-3 attempt becomes a 3:06–3:12 effort at the same fitness level. Running that day as a "training race" with an adjusted goal is smarter than blowing up at mile 18 and limping home in 3:30.
There's no shame in adjusting for conditions. The fast marathons (Berlin, London, Chicago) are fast partly because they tend to have favorable weather. If your race day is hot, you're running a harder race — period.
Calculate Your Adjusted Pace
Our Weather Pace Tool takes your goal pace, race day temperature, and dewpoint, and gives you an adjusted target pace so you can race smart in any conditions.
Check Your Adjusted Pace →You can't control the weather. You can control how you respond to it. The runners who adjust their plans come home with smart races. The runners who don't come home with stories about the wall.