When the Twin Cities Marathon canceled its race just hours before the 2023 start — only the second cancellation in the event’s 40-year history — it wasn’t a judgment call made on the fly. It followed a specific, science-based flag system that most casual race fans have never heard of. Here’s how race organizers actually decide when heat makes an event too dangerous to run, and what’s changing as extreme heat becomes more common.
Table of Contents
- The Flag System Races Actually Use
- Why Wet Bulb Globe Temperature, Not Air Temperature
- Real Race Cancellations and What Triggered Them
- How Races Are Adapting Instead of Canceling
- What to Do If You’re Registered for a Race During a Heat Wave
- Frequently Asked Questions
The Flag System Races Actually Use
Most major U.S. road races use a color-coded flag system, developed originally for road racing safety, to communicate real-time heat risk to runners and to guide organizer decisions:
| Flag | Risk Level | Typical Guidance |
|---|---|---|
| Green | Low | Conditions are safe for all runners |
| Yellow | Moderate | Runners should moderate pace, especially less-conditioned participants |
| Red | High | Considered dangerous for most runners; races often shorten distances or add extra medical support |
| Black | Extreme and dangerous | Race is typically canceled, postponed, or converted to non-competitive status |
When the Twin Cities Marathon canceled, organizers specifically cited “EAS Black Flag weather conditions — Extreme and Dangerous Conditions” in their announcement, showing this isn’t an informal judgment call but a defined operational trigger.
Why Wet Bulb Globe Temperature, Not Air Temperature
Serious race organizations don’t base cancellation decisions on air temperature alone — they use Wet Bulb Globe Temperature (WBGT), a measurement that combines temperature, humidity, wind, and direct sun exposure into a single number representing genuine heat stress on a working human body. This matters because two days with identical air temperature can carry very different real risk depending on humidity and sun exposure.
One real example: a Canadian marathon was called off roughly 75 minutes after it began specifically because WBGT readings exceeded the organization’s 30°C (86°F) safety threshold — even though the morning had started at a seemingly reasonable 25°C (77°F) air temperature. This illustrates why WBGT-based decisions can trigger mid-race cancellations that surprise runners checking only a standard weather app.
Real Race Cancellations and What Triggered Them
- Twin Cities Marathon (2023): Canceled hours before the start after forecasts showed record-setting heat approaching Black Flag conditions, with overnight lows already in the high 60s.
- Run Norwich (UK): Postponed under an official Met Office amber heat warning, with organizers citing risk to runners, volunteers, and the capacity of on-site medical support.
- Manitoba Marathon: Called off mid-race after WBGT readings crossed the 30°C threshold, despite a reasonable-seeming start temperature.
- 2026 Los Angeles Marathon: Rather than canceling outright, organizers offered a one-time “mile 18 option,” letting struggling runners exit the course roughly 8 miles early while still receiving a finisher’s medal — an example of adapting the event instead of scrapping it entirely.
- Hamburg Half Marathon (2026): Postponed as forecast heat approached 40°C (104°F), with organizers calling it “the most difficult decision” in the event’s history.
How Races Are Adapting Instead of Canceling
As extreme heat events become more frequent, race organizers are increasingly building flexibility into events rather than relying on binary cancel/proceed decisions:
- Earlier start times or shifting events to run partially or entirely before sunrise
- “Exit early” options, like the LA Marathon’s mile 18 policy, that still credit participants with a finish
- Air Quality Index thresholds layered alongside heat — an AQI above 150 can independently trigger distance reductions or postponement, since heat waves often coincide with poor air quality
- Moving annual events to cooler seasons entirely in regions where the traditional race date has become unreliably hot
- Enhanced medical and hydration stations triggered automatically at Yellow or Red flag conditions, before reaching cancellation-level risk
Climate researchers studying marathon conditions broadly have found that opportunities to race under genuinely optimal conditions are shrinking as average temperatures rise, a trend that’s reshaping event calendars across the sport, not just individual race-day decisions.
What to Do If You’re Registered for a Race During a Heat Wave
- Check the race’s official heat policy in advance, not just the day-of forecast — most major races publish their flag system and cancellation criteria publicly
- Monitor both temperature and humidity, not temperature alone, since humidity substantially raises real risk. Check your local Heat Index Calculator reading in the days leading up to race day
- Have a backup plan for hydration and pacing if the event proceeds under Yellow or Red flag conditions
- Don’t assume a race will definitely happen just because it hasn’t been officially canceled yet — many cancellations, like Twin Cities’, come the morning of the event
- Know the symptoms of heat exhaustion versus heatstroke before race day, since recognizing early warning signs matters more than any official flag color once you’re actually running
Frequently Asked Questions
What temperature cancels a marathon?
There’s no single universal number — most major races use Wet Bulb Globe Temperature (which combines heat, humidity, wind, and sun exposure) rather than air temperature alone, with a Black Flag designation around 30°C (86°F) WBGT commonly triggering cancellation or major event changes.
Can a race be canceled mid-event?
Yes. Since conditions can worsen as the day progresses, some races (like the Manitoba Marathon) have been called off after already starting, once real-time WBGT readings crossed the organization’s safety threshold.
Why do organizers use Wet Bulb Globe Temperature instead of just the forecast high?
Because WBGT captures the combined physiological stress of heat, humidity, wind, and direct sun exposure on a working body, giving a far more accurate risk picture than air temperature alone, especially for sustained physical exertion.
Do runners get refunds if a race is canceled for heat?
It varies significantly by race and organizer policy. Some offer credit toward a future event, others don’t guarantee refunds since weather-related cancellations are typically outside the organizer’s control. Check the specific event’s registration terms.
Are heat-related race cancellations becoming more common?
Yes. Multiple major races across the U.S. and Europe have canceled, postponed, or significantly altered events for extreme heat in recent years, and researchers studying the trend expect optimal racing conditions to continue narrowing as average temperatures rise.
Bottom Line
Race organizers don’t cancel events casually — they follow specific, science-based flag systems built on Wet Bulb Globe Temperature, not just the forecast high. If you’re registered for an endurance event during a heat wave, check the official heat policy in advance and monitor real conditions, not just the daily forecast, using our Heat Index Calculator.