WBGT Explained: The Number That Decides If Your Race Gets Cancelled
Temperature alone doesn't predict heat risk. WBGT, the composite index race directors actually use, weighs humidity, sun, and air temp together, and here's how to use it.
In 2007, the Chicago Marathon hit 88°F and organizers stopped the race mid-course. Water ran out at aid stations. A runner died. It remains the only time a World Marathon Major has ever been halted for heat.
The number that would have flagged that morning as dangerous hours before the gun wasn't the temperature. It was WBGT, wet bulb globe temperature, the composite heat-stress index that race medical directors, the military, and sports governing bodies actually use to decide whether an event is safe to run. Almost no consumer weather app shows it. Almost every serious heat-illness study uses it as the standard.
Three thermometers, one number
Standard temperature comes from one sensor in the shade. WBGT comes from three, and the combination is the entire point.
Dry bulb is ordinary air temperature. Nothing fancy, just a thermometer.
Natural wet bulb is measured with a wet wick exposed to the air and whatever breeze exists. As water evaporates off the wick, it cools the sensor, the same mechanism that cools you when you sweat. A high wet bulb reading means the air is already close to saturated and evaporation is struggling, exactly the situation dew point is trying to describe from a forecast instead of a physical measurement.
Black globe is a matte black copper sphere with a thermometer inside. It absorbs solar radiation the way your body does, so it reads meaningfully hotter than air temperature on a sunny day and barely above air temperature under cloud cover or at night. This is the variable neither dry bulb nor dew point captures at all.
The official outdoor formula, from the National Weather Service's WBGT estimation methodology, weights these three readings unevenly:
WBGT = 0.7 × wet bulb + 0.2 × black globe + 0.1 × air temperature. The wet-bulb reading, which rises with both humidity and temperature, gets 70% of the vote. Sun exposure gets 20%. Plain air temperature, the number every weather app leads with, gets 10%.
That weighting is the whole story. It's why an overcast, muggy 68°F morning can carry more real heat stress than a dry, breezy 82°F afternoon, and why two race mornings with an identical thermometer reading can be completely different races depending on what the sun and the air moisture are doing.
Why WBGT beats dew point, and when dew point is still your best option
Dew point, covered in our dew point deep dive, is the best single number a normal forecast gives you, and it tracks the wet-bulb term that dominates the WBGT formula. What it doesn't capture is the other 20%: solar radiation.
Run the same dew point on a sunny, exposed course and a shaded, overcast one, and WBGT will diverge sharply between them even though dew point reads identical. That's the black globe term doing its job. A marathon that runs through open parkland at 10 AM in full sun can carry a meaningfully higher WBGT than the dew point alone would suggest, and a course that's tree-covered or runs at dawn can run cooler than the dew point implies.
This is why WBGT, not dew point and not air temperature, is what shows up in the actual research literature on marathon heat illness, and what race medical directors use for go/no-go decisions. Dew point is the best proxy available on a standard forecast. WBGT is the real thing.
The flag system race directors actually use
The American College of Sports Medicine and USA Track & Field both publish WBGT-based flag categories for road race risk. The exact cutoffs vary slightly by organization, but the widely used version looks like this:
| Flag | WBGT | What it means |
|---|---|---|
| White | Below 50°F (10°C) | Low heat risk. Cold itself may be the concern instead. |
| Green | 50–65°F (10–18°C) | Moderate risk. Normal racing conditions for most runners. |
| Yellow | 65–73°F (18–23°C) | High risk. Slower, less-acclimatized runners should adjust effort. |
| Red | 73–82°F (23–28°C) | Very high risk. Race organizers typically warn all participants and recommend pace caution. |
| Black | Above 82°F (28°C) | Extreme risk. The threshold most commonly cited for cancelling or postponing an event. |
Black flag doesn't mean every race stops automatically. ACSM has deliberately avoided a hard mandatory rule, leaving the call to race medical staff. But 82°F WBGT is the number that comes up again and again in the research and in race operations planning as the point where continuing gets genuinely dangerous, not just uncomfortable.
By our estimate from the hourly weather, Chicago 2007 started near 68°F WBGT, already a yellow flag, and passed 73°F into red by mid-morning as the air climbed toward its 88°F high. The race was stopped in red-flag conditions, not black.
What racecast actually computes
Being straight about this: WBGT isn't something you can read off a phone's weather app. It requires either a physical globe thermometer on site or a model estimate built from several inputs at once. Most race forecasts skip it entirely and just show you air temperature, because that's what the underlying weather API hands them by default.
We compute an estimated WBGT for every race with our own in-house formula, not the NWS's or Liljegren's globe-temperature model: wet bulb temperature from air temperature and humidity (the Stull 2011 psychrometric formula), a simplified black-globe term from solar radiation and wind cooling, and the standard 0.7/0.2/0.1 weighting to combine them. Below 50°F WBGT, our pace model applies no heat penalty at all, the same threshold the White/Green flag boundary uses. Above it, pace slows by roughly 0.35% per degree, a figure consistent with the range Ely et al. (2007) found across seven major marathons over 36 years, and calibrated against more than six million finish times across 27 years of the races we cover.
The point isn't that our number is fancier than a thermometer. It's that a single dry-bulb temperature reading, the number every generic weather app leads with, is missing 90% of what actually determines how hard a hot race is going to feel.
How to use this on race morning
Look for a WBGT estimate, not just a temperature. If a race's official communications mention a flag color or a heat advisory, that's WBGT-based guidance, even if they never say the word. Take it seriously regardless of what the plain temperature reads.
Green or below: race normally. Standard pacing, standard hydration plan. This is what your training predicts.
Yellow: build in a small, deliberate cushion. Slightly slower goal pace, slightly more attention to fluids and electrolytes from the start rather than reactively.
Red: shift from time goals to effort goals. This is the range where trying to hold a fixed pace regardless of how you feel starts to be genuinely risky, not just slower than you wanted.
Black: reconsider whether today is a PR day at all. Organizers may not cancel, but that doesn't mean racing all-out is the right call for you individually. Finishing safely and finishing fast can be in direct conflict on a black-flag morning.
No WBGT available anywhere? Fall back to dew point. It tracks the dominant term in the formula (wet bulb) even without a solar radiation input, and it's a real, meaningful upgrade over looking at temperature or "feels like" alone.
The flag color exists to answer one question before you're out there sweating through it: is today's heat stress mostly about temperature, mostly about humidity, or mostly about sun. WBGT is what actually answers that.
More guides
- Dew Point Is the Marathon Stat That Predicts Your Race Day — the accessible proxy for the humidity term in WBGT
- How to Run a Marathon in the Heat — race day strategies for warm conditions
- How Weather Affects Your Marathon Pace — the full breakdown of heat, wind, and humidity adjustments
- What to Wear on Race Day by Temperature — clothing guide from 20°F to 90°F
- Race Day Gear Guide — a full temperature-based gear plan
- Race Pacing Guide — adjusting effort and splits for tough conditions
- Find your race — live WBGT-informed forecasts and pace adjustments