About racecast.io 🦄
The mission of racecast.io is to provide race forecasts that are a pleasure to use. We use data science to improve our endurance racing models and provide the most accurate representation of how the weather is going to affect your next race.
racecast.io began in response the Boston Marathon's famous wild weather. We believed that there had to be a better solution than burning through your refresh button hoping that the chance of rain is going to drop by 1%. Because of that we are proud to say that we are “Built in Boston“ - but in reality we are a fully remote team with day jobs. racecast.io is a passion project.
What is the RunScore?
RunScore rates race-day weather from 0 to 10 by how much it's likely to slow the typical marathoner down. It comes from the same pace model behind our race-day pace adjustments, which prices three things: heat (measured as WBGT, so humidity and sun count along with temperature), wind along the course, and rain. Their combined cost, as a share of finish time, becomes the score, from Great down to Brutal.
The numbers inside that model are calibrated on real race results: about 6.3 million finish times from 1,686 editions of 92 marathons since 2000, with each race compared against its own history so the course cancels out. Until September 2026, RunScore came from a model trained on more than 350,000 logged runs and runners' ratings of how the weather felt. That was a good guide to comfort and a poor one to race-day pace. The full method, and where the score is still off, is in Why We Rebuilt RunScore.
RunScore measures how much the weather slows a typical finisher, not how dangerous a day is. A cold, soaking, windy race can be risky for some runners while barely slowing the median, so read the forecast alongside the score.
Weather Sources
We rely on these weather providers:
- Visual Crossing: race forecasts and race-day weather history
- Open-Meteo: place search and historical weather used in our research. Weather data by Open-Meteo.com, licensed under CC BY 4.0.
- National Weather Service: US weather alerts and research
Contact Us
If you want to help out with racecast.io, have a problem, don't see your race, or just want to chat, do not hesitate to reach out contact@racecast.io.