Marathon race-day weather: how to read the forecast
Short answer: look at the hourly forecast for your start time and your expected finish time, not the daily high. Check the dew point, the sun and the wind on the course. Dress for the middle of the race, wear a throwaway layer at the start, and if it's humid or warm, start slower than your goal pace.
Check two hours: your start and your finish #
A 7 AM start and an 11 AM finish can be very different races. The temperature usually climbs all morning and the sun gets stronger. Find the hourly forecast for both, and plan your pace for the warmer one.
Dew point tells you more than temperature #
Humid air slows sweat evaporating, so you cool less. Dew point measures the water in the air directly:
- Under about 50°F: good racing air.
- 55–65°F: you'll feel it in the second half.
- Over 65°F: change your goal. How heat and humidity slow your pace.
Sun and shade #
Clear skies after sunrise add heat the thermometer misses. A dark, cloudy start feels cooler than the same temperature in full sun at mile 20. Wear a cap and sunglasses if the finish will be sunny.
Wind on the course #
Look at the wind direction as well as the speed, then at the course map. A headwind on the long exposed stretches costs more than the number suggests. Tuck in behind a group there, and don't chase your goal splits into it.
Rain and storms #
- Light rain in cool air is fine for racing. Anti-chafe balm and well-fitted socks save your skin and feet.
- Cold rain near 40°F is the risky one: wear gloves and a layer you can keep on.
- Thunder means the organizer may hold or stop the race. Follow their instructions. Running in rain, wind and storms.
Dress for mile three, not the start line #
- You'll feel 15–20°F warmer once you're running. At race effort it can be more.
- Wear an old sweatshirt or a trash bag over your kit in the corral and drop it before the start.
- Cheap gloves and arm warmers come off without stopping.
- Test anything new on a long run first. What to wear at each temperature.
The race heat flag #
Many races fly a flag by WBGT, a heat-stress reading that combines temperature, humidity, sun and wind. Yellow means slow down and drink. Red means slow down a lot. Black usually means the race is cancelled. WBGT and race heat flags, explained.
When to change your goal #
Most PR plans assume a cool day. If the dew point is in the 60s, the flag is Yellow or worse, or a strong wind sits on the back half, start slower than planned and run the first half by effort. A negative split in bad weather beats a blow-up at mile 20.
General guidance, not medical advice. On race day, follow the organizer's flag and instructions. For a number at your pace, try the running weather calculator with the forecast for your start and finish hours. Know the heat illness warning signs.
Questions #
What is the best weather for a marathon?
Cool, dry and calm: roughly 40s to low 50s°F at the start, a dew point under about 50°F, cloud or low sun, and light wind. Most runners slow down as it gets warmer and more humid, and slower runners lose more.
When is the marathon forecast reliable?
Watch the trend from about a week out, but plan on the hourly forecast from two or three days before. Check it again the night before and the morning of the race.
What should I wear for a cold marathon start?
Dress for the middle of the race, not the start line. Wear an old layer or a trash bag over your race kit and drop it before the gun, plus cheap gloves you can toss or tuck in a pocket.
Should I change my marathon goal for hot weather?
Yes, if the dew point is in the 60s or higher, or the race heat flag is Yellow or worse. Start slower than planned and run by effort for the first half. Speed up later only if you feel good.
Sources #
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