The short answer
WBGT (wet bulb globe temperature) is the heat-stress number for running that race medical teams use. It blends air temperature, humidity, sunshine and wind into one reading. Big-city marathons grade it in four levels: Low below 18.3 °C (65 °F), Moderate to 22.8 °C (73 °F), High to 27.8 °C (82 °F), and Extreme above.
Key takeaways
- WBGT weights humidity at 70%, sunshine at 20% and air temperature at only 10%.
- Sun and still air alone can lift WBGT by 8 °C (14 °F) at the same air temperature.
- The ACSM recommends postponing or cancelling a race when WBGT tops 28 °C (82 °F).
- WBGT is the place's heat; your pace and acclimatisation decide how it hits you.
On this page
- What is WBGT, and why do runners use it?
- How is WBGT different from temperature, heat index and feels like?
- What do sun, wind and humidity do to WBGT?
- Which WBGT levels do marathons use?
- Why does WBGT change along a race course?
- How should you use a WBGT forecast for your run?
- Questions runners ask
- Sources and method
Words this guide uses
- WBGT (wet bulb globe temperature)
- A heat-stress index made of 70% natural wet-bulb temperature, 20% black-globe temperature and 10% air temperature.
- Natural wet bulb
- A thermometer in a wet wick, open to sun and wind. It reads how well sweat can evaporate.
- Black globe
- A thermometer inside a matt black ball. It reads the heat of sunshine and hot surfaces.
- Pace-timed forecast
- The weather for each stretch of your route at the time you'll run it.
What is WBGT, and why do runners use it?
WBGT, the wet bulb globe temperature, is the heat-stress number behind most race-day heat flags. For running, it beats the thermometer because it also counts humidity, sunshine and wind, the things that decide whether your sweat can cool you.
It's one piece of running in heat and humidity. The formula is fixed: 70% natural wet bulb, 20% black globe and 10% air temperature, as the American College of Sports Medicine (ACSM) set out in 1996. The wet wick reads how well sweat evaporates; the globe reads the sun.
The US National Weather Service puts it simply: the heat index assumes you're in the shade, while WBGT is measured in direct sunshine. To see it kilometre by kilometre for your own route, Map your run works it out at the time you'll reach each stretch.
How is WBGT different from temperature, heat index and feels like?
Air temperature is only a tenth of WBGT. Heat index and feels like describe how hot the air seems to a person; WBGT describes how hard the environment makes it to shed heat.
| Measure | What goes in | Sun counted? | Who uses it |
|---|---|---|---|
| Air temperature, °C (°F) | Air in the shade | No | Every forecast |
| Heat index, °C (°F) | Air and humidity, in the shade | No | US forecasts |
| Feels like, °C (°F) | Air, humidity, wind, sometimes sun | Depends on the app | Weather apps |
| WBGT, °C (°F) | Humidity, sun, wind, air | Yes | Race medics, militaries, sports bodies |
Feels like matters too, but at your pace: see feels like temperature for runners.
What do sun, wind and humidity do to WBGT?
They can move it more than the air temperature does. We held the air at 25 °C (77 °F) and changed one thing at a time, using the Liljegren et al. (2008) WBGT model.
| Conditions at 25 °C (77 °F), dew point 14 °C (57 °F) | WBGT, °C (°F) | Level |
|---|---|---|
| No sun (night or thick cloud), light breeze | 19.7 (67.5) | Moderate |
| Low sun, 20° above the horizon | 22.7 (72.9) | Moderate |
| Morning sun, 30° up | 23.5 (74.3) | High |
| Midday sun, 60° up | 24.8 (76.6) | High |
| Midday sun, near-still air, 3 km/h (2 mph) | 27.9 (82.2) | Extreme |
| Midday sun, breezy, 25 km/h (16 mph) | 23.1 (73.6) | High |
The sun and a dead calm add 8.2 °C (14.7 °F) to the same thermometer reading. A breeze takes some back. Humidity works the same way: in that midday sun, a dew point of 6 °C (43 °F) gives 22.4 °C (72.3 °F), Moderate. A dew point of 23 °C (74 °F) gives 28.7 °C (83.7 °F), Extreme. Our dew point running chart shows how to read that humidity.
Which WBGT levels do marathons use?
Big-city marathons grade WBGT in four bands, the ones the ACSM laid out in 1996. The Chicago Marathon's Event Alert System gives them colours and actions, though its page doesn't publish any numbers. We pair them here by name only.
| Level and colour | WBGT, °C (°F) | ACSM 1996 risk | Chicago action |
|---|---|---|---|
| Low, green | below 18.3 (65) | Low, but not zero | Enjoy the event, be alert |
| Moderate, yellow | 18.3–22.8 (65–73) | Heat will rise during a morning race | Slow down, be prepared |
| High, red | 22.8–27.8 (73–82) | Any runner may suffer heat illness | Slow down, consider stopping |
| Extreme, black | above 27.8 (82) | Very high: postpone, reschedule or cancel | Participation stopped |
Some events set stricter limits from their own records. Roberts (2010) found that marathons in northern latitudes with a start WBGT above 21 °C (70 °F) often ended in a mass-casualty incident or a cancellation mid-race. He put the "do not start" level for the Twin Cities Marathon at 20.5 °C (69 °F). Our guide to marathon heat flags covers each colour in detail.
Why does WBGT change along a race course?
Because the sun climbs while you run. A 9:00 or 10:00 start puts slower runners into the late-morning sun, so one race can show two or three levels.
Our ERA5 reanalysis of past race days shows this. It's reanalysis, not a thermometer at the start line, but the pattern is clear. At the Berlin Marathon in Germany on 26 September 2021, the air was 16.5 °C (62 °F) at the start. A 5-hour runner reached Moderate from km 7 and High from km 40, with WBGT up to 22.9 °C (73 °F). At the Boston Marathon in the US on 15 April 2024, a 4-hour runner hit Moderate from km 4 (mile 2.5), peaking at 19.7 °C (67 °F).
At South Africa's Comrades Marathon in 2015, an up run, an 11-hour finisher reached High from km 33 (mile 20.5) and peaked at 27.1 °C (81 °F). A single start-line reading would have missed all of it.
How should you use a WBGT forecast for your run?
Use it to plan your pace, then let official flags and medical staff overrule it on the day. This isn't medical advice.
- Look at the peak. The highest WBGT on your route matters more than the start.
- Slow down by level. Moderate means ease off; High means a slower goal and more care.
- Read the dew point alongside it. A dew point above 18 °C (64 °F) means sweat stops working well. Our guide to a good dew point for running has the bands.
- Know the warning signs. The ACSM's 2007 position stand defines exertional heatstroke as a core temperature above 40 °C (104 °F) with signs such as confusion. Stop and get help.
- Train for it. Racinais et al. (2015) recommend repeated exercise in the heat over one to two weeks before a hot race.
- Move the start. A cooler start time is your biggest lever; see the best time to run in hot weather.
Questions runners ask
Is WBGT the same as wet-bulb temperature?
No. Wet-bulb temperature is one ingredient, usually measured in the shade. WBGT uses a natural wet bulb out in the open, then adds a black globe for sunshine and a dry thermometer for air. The wet bulb carries 70% of the weight, so humidity still dominates.
Can WBGT be higher than the air temperature?
Yes, though it's unusual. In our worked example at 25 °C (77 °F) with a dew point of 14 °C (57 °F), midday sun and near-still air push WBGT to 27.9 °C (82.2 °F), in the Extreme level. Most of the time WBGT reads below the thermometer, which can make it look reassuringly low.
Does WBGT take my running pace into account?
No. WBGT describes the place: the reading a medical tent's meter would show. Your own heat load also depends on how hard you run, your size, your clothing and your acclimatisation. Racinais et al. (2015) call heat acclimatisation over one to two weeks the most important step.
Why do some races measure WBGT on the course?
Because heat stress changes as the sun climbs and shade comes and goes. The ACSM's 1996 position stand asked organisers to measure the heat at the race site, not at a distant weather station, and to post the risk at the start and along the course.
Is a WBGT of 20 °C (68 °F) safe for a long run?
It's Moderate, not safe by default. The ACSM's 1996 guidance says this level reminds runners that heat and humidity will rise during a morning race. Roberts (2010) found Twin Cities Marathon problems climbed quickly above a start WBGT of 13 °C (55 °F). Slow down, drink to thirst, and follow any medical advice you've been given.
See the heat at your pace.
Drop in your route (GPX, FIT or TCX), pick the day and your pace, and see the feels like, dew point and heat stress for every kilometre, timed to when you’ll be there. Free, in your browser, no sign-up.
Sources and method
- American College of Sports Medicine (Medicine & Science in Sports & Exercise) (1996). ACSM position stand: heat and cold illnesses during distance running
- US National Weather Service, La Crosse (2026). Wet Bulb Globe Temperature (WBGT)
- Journal of Occupational and Environmental Hygiene (2008). Modeling the wet bulb globe temperature using standard meteorological measurements (Liljegren et al.)
- Bank of America Chicago Marathon (2026). Rules and safety (Event Alert System)
- American College of Sports Medicine (Medicine & Science in Sports & Exercise) (2007). ACSM position stand: exertional heat illness during training and competition (Armstrong et al.)
- Medicine & Science in Sports & Exercise (2010). Determining a 'do not start' temperature for a marathon on the basis of adverse outcomes (Roberts)
- British Journal of Sports Medicine (2015). Consensus recommendations on training and competing in the heat (Racinais et al.)
- Copernicus Climate Change Service (ECMWF) (2026). ERA5 hourly data on single levels from 1940 to present
The worked examples use the WBGT model of Liljegren et al. (2008), as coded in the RunFast engine, at sea level. Fixed case: air 25 °C (77 °F), relative humidity 50% (dew point 13.8 °C, 57 °F), wind at 10 m of 10 km/h (6 mph) unless stated. Sunshine is clear-sky irradiance for the sun's height (Haurwitz): 318 W/m² at 20°, 490 W/m² at 30° and 890 W/m² at 60°. The humidity rows hold the midday case and change relative humidity to 30%, 70% and 90%. Race examples come from RunFast's ERA5 reanalysis of race days (Copernicus Climate Change Service), at a 5-hour Berlin finish, a 4-hour Boston finish and an 11-hour Comrades finish, along each course at the time a runner on that schedule reaches each kilometre. Levels follow the ACSM (1996) bands that the engine uses. °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.