The short answer
A black flag at a marathon means extreme heat stress: under the Chicago Marathon's Event Alert System, the event is cancelled or participation stopped. Green means low risk, yellow moderate, red high. The colours follow WBGT bands the American College of Sports Medicine set in 1996, with black above 28 °C (82 °F).
Key takeaways
- Green, yellow, red and black match Low, Moderate, High and Extreme heat stress.
- The ACSM recommends postponing or cancelling when WBGT exceeds 28 °C (82 °F).
- Under Chicago's system, red means slow down and consider stopping; black means participation stops.
- In our data, none of 35 race days at four marathons reached Extreme.
On this page
Words this guide uses
- Heat flag
- A colour posted at the start and along a race course to show the current risk of heat illness.
- Event Alert System
- The Chicago Marathon's four-level warning scale, from Low (green) to Extreme (black), with an action for each level.
- WBGT (wet bulb globe temperature)
- A heat-stress index combining humidity, sunshine, wind and air temperature. Race medics set heat flags by it.
- Pace-timed forecast
- The weather for each stretch of your route at the time you'll run it.
What does a black flag mean at a marathon?
A black flag at a marathon means extreme, dangerous heat. At the Chicago Marathon, black is the Extreme level of its Event Alert System: the event is cancelled, or participation is stopped and runners follow officials' instructions.
Black is the top of a four-colour scale, set by heat stress rather than the thermometer. It's the sharp end of running in heat and humidity, and the number behind it is WBGT, the wet bulb globe temperature. It blends humidity, sunshine, wind and air temperature into one reading.
The American College of Sports Medicine (ACSM) set the WBGT bands in its 1996 position stand on distance running. Above 28 °C (82 °F), it said, the risk of heat exhaustion or heatstroke is very high. It recommended postponing, rescheduling or cancelling the race.
Official race flags and medical advice always override this article. It isn't medical advice.
What do green, yellow, red and black flags mean?
Green is low risk, yellow moderate, red high and black extreme. The table pairs the Chicago Marathon's wording with the ACSM's 1996 WBGT bands.
| Flag | Level | WBGT, °C (°F), ACSM 1996 | Chicago's conditions | Chicago's action |
|---|---|---|---|---|
| Green | Low | below 18 (65) | Good | Enjoy the event, be alert |
| Yellow | Moderate | 18–23 (65–73) | Less than ideal | Slow down, be prepared for worse |
| Red | High | 23–28 (73–82) | Potentially dangerous | Slow down, follow officials, consider stopping |
| Black | Extreme | above 28 (82) | Extreme and dangerous | Participation stopped |
Chicago's page names the levels but publishes no WBGT numbers, so the pairing is by name. Low doesn't mean safe. The ACSM noted that heatstroke can still happen at Low; the level only says the risk is low. Our WBGT explainer for runners shows how sunshine and still air push the number up.
Which marathons use heat flags?
The Chicago Marathon publishes its four-level Event Alert System, with colours and actions. The ACSM's 1996 position stand also asked organisers to post the heat risk on large signs at the start and along the course.
Don't assume your race uses the same numbers. We found no published flag thresholds from the London Marathon or the Boston Marathon. Read your own race's safety page and pre-race emails. If you're running abroad, check the local rules, whether that's Berlin in Germany or Comrades in South Africa.
How often do big marathons reach each flag?
Rarely black, often yellow. We modelled every race day from 2015 to 2024 at four races and recorded the highest heat-stress level a mid-pack runner met. This is reanalysis, not a thermometer at the start line, and it isn't the flag each race flew.
| Race (finish time) | Race days | Low, green | Moderate, yellow | High, red | Extreme, black | Peak WBGT range, °C (°F) |
|---|---|---|---|---|---|---|
| Berlin (4:00) | 9 | 6 | 3 | 0 | 0 | 14.0–22.4 (57–72) |
| London (4:00) | 9 | 8 | 1 | 0 | 0 | 9.3–20.6 (49–69) |
| Boston (4:00) | 9 | 5 | 4 | 0 | 0 | 8.3–21.6 (47–71) |
| Comrades (11:00) | 8 | 0 | 5 | 3 | 0 | 20.1–27.1 (68–81) |
There was no race in 2020. Boston 2021 ran on 11 October and London on 3 October 2021 and 2 October 2022. Comrades skipped 2020 and 2021, and ran on 28 August in 2022.
Comrades stands out. Its course runs about 90 km (56 miles), so an 11-hour finisher is still running after 16:00. In 2015, 2016 and 2017, that runner reached High. Our marathon weather history has every year in full.
Has a marathon ever been stopped for heat?
Yes. In a 2010 study, William Roberts found that start WBGTs above 21 °C (70 °F) led to mass-casualty incidents or mid-race cancellations in several marathons.
His data came from the Twin Cities Marathon in Minnesota, USA. Problems there rose quickly above a start WBGT of 13 °C (55 °F). At a start WBGT of 22 °C (72 °F), the event had a mass-casualty incident, with 160 "unsuccessful" starters per 1,000 finishers. He set a "do not start" level of 20.5 °C (69 °F) for that race.
That's below the black band. It's why medical teams watch the forecast for the hours ahead, not just the start. His study looked at northern races where runners weren't used to the heat.
What should you do under each flag?
Change your race to match the colour, and decide before the gun. These steps follow the Chicago actions and ACSM guidance.
- Green: run your plan, but drink to thirst and watch for signs of trouble.
- Yellow: slow down from the start. Expect the heat to rise during a morning race.
- Red: set a slower goal, or run for the finish rather than a time. Stop if you feel unwell.
- Black: stop when told. The race is over, whatever your watch says.
The ACSM's 2021 consensus statement calls exertional heatstroke a true medical emergency. Fast recognition, stopping activity and rapid whole-body cooling are essential for survival. Confusion, stumbling or collapse need medical help at once. Our guide to how hot is too hot to run sets out the limits for training days too.
Organisers can also show supporters the heat where each runner is, live. See live tracking for running events.
Questions runners ask
Can you keep running under a red flag?
Usually, yes, but with care. Chicago's Event Alert System asks runners at High (red) to slow down, follow officials' instructions and consider stopping. The ACSM's 1996 guidance says anyone particularly sensitive to heat or humidity probably should not run at that level. If you feel dizzy, confused or stop sweating, stop and get medical help.
Who decides the heat flag on race day?
The race organisers and their medical team decide, using conditions measured at the race. The ACSM's 1996 position stand asked organisers to measure heat stress at the race site rather than relying on a distant weather station. Chicago says its levels are based mainly on the weather, as well as other conditions. Their call overrides any forecast or article, this one included.
Does a black flag always mean the race is cancelled?
Under Chicago's Event Alert System, Extreme (black) reads "event cancelled" and participation is stopped. The ACSM's 1996 guidance for WBGT above 28 °C (82 °F) was to postpone the race until conditions ease, reschedule it, or cancel it. Each race sets its own rules, so read your event's safety page before race week.
Can the flag colour change during a marathon?
Yes. Heat stress usually climbs through a morning race as the sun gets higher, and slower runners meet the hottest part. In our reanalysis of the 2021 Berlin Marathon, a 3-hour runner peaked at Moderate while a 5-hour runner reached High from km 40 (mile 25). Watch the flags at each aid station, not only at the start.
Is there a heat flag for cold weather?
Heat flags don't cover cold, but the risk is real. The ACSM's 1996 position stand warns that a WBGT below 10 °C (50 °F) can bring hypothermia in slow runners over long distances, especially when wet and windy. At the 2018 Boston Marathon, our reanalysis puts the coldest feels like at about −3 °C (27 °F) in the rain.
Your race, at your pace.
Pick Comrades, Berlin, London or Boston, set your finish time, and see the heat stress, feels like and dew point you’d have met at every landmark, on every race day from 2015 to 2024.
Sources and method
- Bank of America Chicago Marathon (2026). Rules and safety (Event Alert System)
- American College of Sports Medicine (Medicine & Science in Sports & Exercise) (1996). ACSM position stand: heat and cold illnesses during distance running
- Medicine & Science in Sports & Exercise (2010). Determining a 'do not start' temperature for a marathon on the basis of adverse outcomes (Roberts)
- American College of Sports Medicine (Current Sports Medicine Reports) (2021). ACSM expert consensus statement on exertional heat illness: recognition, management, and return to activity (Roberts et al.)
- US National Weather Service, La Crosse (2026). Wet Bulb Globe Temperature (WBGT)
- Copernicus Climate Change Service (ECMWF) (2026). ERA5 hourly data on single levels from 1940 to present
Flag meanings are quoted from the Chicago Marathon's Event Alert System page, which publishes no WBGT thresholds. WBGT bands come from the ACSM's 1996 position stand (18, 23 and 28 °C; 65, 73 and 82 °F), which RunFast's engine grades as 18.3, 22.8 and 27.8 °C. The race-day counts are RunFast's own: ERA5 hourly reanalysis (Copernicus Climate Change Service) along each course, timed to a runner finishing in 4 hours at Berlin, London and Boston and in 11 hours at Comrades, with WBGT from the Liljegren et al. (2008) model. Each count is the highest level that runner met, not the flag the race flew. Race days run from 2015 to 2024: no 2020 races; Boston 2021 ran on 11 October, London 2021 on 3 October and 2022 on 2 October; Comrades had no race in 2020 or 2021, and 2022 ran on 28 August. °F values are our conversions.
Wes Thomson
Wes Thomson builds RunFast, which times the heat, humidity and dew point to a runner's own pace along the route, and its sister Windline, live wind forecasts for cyclists. Both run on the same forecast engine. He writes these guides with one focus: what the weather will do to your run, kilometre by kilometre, and how to plan around it.