Athletic trainers and coaches
Practice modification, break schedules and equipment decisions set against published thresholds rather than judgment calls.
Wet bulb globe temperature, almost always shortened to WBGT, is a heat stress index that folds air temperature, humidity, wind speed and solar radiation into a single number. It is the measurement written into heat safety rules for athletics, the military and occupational safety, rather than the temperature on a car dashboard.
The reason is simple. Ninety degrees in dry shade with a steady breeze and ninety degrees in still, humid air under a cloudless sky are not the same event for a human body, but a thermometer reports them identically. WBGT separates them. The first might read in the high seventies; the second can clear ninety and sit in black flag territory.
The term itself comes from the instruments. A wet bulb thermometer wrapped in a damp wick measures how much evaporative cooling the air will allow. A black globe thermometer, a copper sphere painted matte black, measures radiant heat from the sun and from hot surfaces. Add the ordinary dry bulb air temperature and you have all three words in the name.
The four things WBGT measures
Air temperature answers one question. WBGT answers four at once, which is why it tracks what a body actually feels on an open field or a hot slab of pavement.
Air temperature
The dry bulb reading, how hot the air itself is.
Humidity
Moisture that stops sweat evaporating, so cooling stalls.
Solar radiation
Radiant heat loading the body, turf and pavement.
Wind speed
Air movement that carries heat and sweat away.
Where WBGT came from
WBGT was developed by the U.S. Marine Corps and Navy in the 1950s at Parris Island, after a run of heat casualties in recruit training. The services needed something a duty officer could read off a device and act on without interpretation, and the flag system, white through black, came out of that work.
It spread outward from there. WBGT is now the basis of ISO 7243 for assessing heat stress in the workplace, the ACGIH threshold limit values that industrial hygienists screen against, the NATA position statement on exertional heat illness in athletics, and a growing list of state high school athletic association rules. Texas made it mandatory for outdoor practice and marching band through the UIL WBGT policy.
How the number is calculated
A reference WBGT reading comes from three thermometers read together on site: a dry bulb for air temperature, a natural wet bulb for evaporative cooling, and a black globe for radiant heat. Outdoors in sunlight they combine as WBGT = 0.7 × natural wet bulb + 0.2 × black globe + 0.1 × dry bulb. Indoors or in shade, where there is no solar load, the dry bulb term drops out and the globe term carries 0.3.
Most people do not have that instrument standing by. The WBGT app estimates the same quantity from standard weather data using the Liljegren et al. (2008) model, a peer-reviewed method developed at Argonne National Laboratory that solves the physical heat balance of the globe and wet bulb rather than curve-fitting an approximation. Weather inputs come from Open-Meteo.
The result is a reading grounded in the same physics as the instrument, available anywhere your phone has a signal.
What the three thermometers contribute
| Component | What it responds to | Weight outdoors |
|---|---|---|
| Natural wet bulb (Tnwb) | Humidity and wind acting on evaporative cooling, measured unaspirated so it reflects real conditions | 0.7 |
| Black globe (Tg) | Radiant heat from the sun, sky, turf and pavement | 0.2 |
| Dry bulb (Ta) | Air temperature alone | 0.1 |
Humidity dominates deliberately. Evaporation is the body's main cooling route during exertion, and when humid air shuts it down, nothing else compensates.
What a WBGT reading means
WBGT values live on a lower numeric scale than the heat index, which catches people out. A WBGT of 88°F is not a mild afternoon. It is a red flag condition in most guidelines, where unacclimatized people should stop strenuous work. Air temperature on that same afternoon might be 97°F and the heat index 108°F.
Two readings are never quite comparable across situations either. The same 85°F WBGT means something different for a cross country runner in shorts, a lineman in full pads, an infantry squad in body armor and a roofer over black membrane. That is why guidelines apply clothing and equipment adjustments, and why the app carries separate Athlete, Military, Outdoors, Aviation and OSHA modes instead of one generic scale.
WBGT flag conditions
A number only helps if it tells you what to do. These are the five flag categories the U.S. military established and athletics and industry adopted. Thresholds shift by guideline and by how acclimatized and how heavily dressed people are, so treat these as the common baseline rather than a universal law.
WhiteUnder 80°F
- Normal activity. Heat stress is low for most people.
- Water available and encouraged throughout.
Green80.0 to 84.9°F
- Normal activity for acclimatized people.
- Watch anyone unacclimatized, unfit, or in heavy gear.
Yellow85.0 to 87.9°F
- Reduce intense exertion and total time in the heat.
- Add rest breaks in shade; increase fluid intake.
Red88.0 to 89.9°F
- Cut hard training sharply. Unacclimatized people should stop strenuous work.
- Frequent breaks, minimal equipment, close supervision.
Black90.0°F and above
- Cancel or postpone outdoor exertion.
- Essential work only, with short cycles and active cooling on hand.
A cold-water immersion tub or equivalent rapid-cooling method should be ready and staffed from roughly 80°F WBGT upward, not improvised after someone goes down.
WBGT compared with the heat index
Both are heat numbers, but they answer different questions, and swapping one for the other is the most common mistake in heat planning.
| Factor | Heat index | WBGT |
|---|---|---|
| Inputs | Air temperature and relative humidity | Air temperature, humidity, wind speed and solar radiation |
| Assumed conditions | Shade, light wind | Direct sun, measured wind |
| Typical use | General public heat warnings | Athletic, military and occupational safety thresholds |
| Reported as | An apparent or “feels like” temperature | A heat stress index with flag categories |
| Reads higher? | Usually higher than air temperature | Usually lower in number, but triggers action sooner |
That last row trips people up. A WBGT of 88°F is not milder than a heat index of 105°F; they are different scales. 88°F WBGT is a red flag condition where hard training should largely stop.
Who relies on WBGT
Military and first responders
Heat categories have governed training and work limits for seventy years, with matching work-rest cycles and water intake tables.
Industrial safety and outdoor crews
Construction, roofing, landscaping and utility work, where heat illness prevention plans are built on a measured number.
Band and activity directors
Marching rehearsal on open asphalt is one of the hottest environments in any school day, with none of the shade a field offers.
Warning signs of heat illness
Heat stress shows up before it becomes an emergency. Anyone on site should speak up immediately on any of these:
Dizziness or lightheadedness
Nausea or vomiting
Chills or goosebumps in the heat
Severe or throbbing headache
Confusion or out-of-character behavior
Stumbling, cramping or collapse
When in doubt, cool first and call for help. Collapse or confusion in the heat should be treated as possible exertional heat stroke: begin cold-water immersion immediately and call 911. Cooling comes before transport.
Check WBGT in the WBGT app
The free WBGT app for iPhone gives you a real-time reading for wherever you are, or any city you search, with the flag level and the guidance that goes with it.
- Real-time WBGT for your location or any city
- Hourly and 14-day WBGT forecasts to plan ahead
- Color-coded flag levels with mode-specific guidance
- Humidity, wind, sun and cloud cover behind every reading
WBGT is estimated from weather data with the peer-reviewed Liljegren model. Where a policy requires on-site measurement, use the app for planning and as a cross-check alongside your instrument.
Common questions
What does WBGT stand for?
Wet bulb globe temperature. The name lists the three instruments in a reference measurement: a wet bulb thermometer for evaporative cooling, a black globe thermometer for radiant heat, and a dry bulb thermometer for air temperature.
Is wet bulb globe temperature the same as wet bulb temperature?
No, and the difference matters. Wet bulb temperature on its own measures only evaporative cooling. WBGT is a weighted composite of natural wet bulb, black globe and dry bulb readings, so it also accounts for radiant heat from the sun and for wind.
What is a dangerous WBGT?
There is no single universal cutoff. Most guidelines treat 88°F WBGT as the point where hard training should be cut sharply and 90°F as the point to cancel outdoor exertion, but the thresholds move with acclimatization, equipment and the standard you follow. Use the policy that governs your activity.
Is WBGT measured in Fahrenheit or Celsius?
Both. U.S. athletic and military guidance is usually written in Fahrenheit; ISO 7243 and ACGIH limits are in Celsius. The app displays either, and a chart in one unit should never be read against a reading in the other.
Can you calculate WBGT without a black globe thermometer?
You can estimate it. Models such as Liljegren et al. (2008) solve the physical heat balance of the globe and wet bulb from standard weather data and get close to a measured value. A WBGT calculator is good for planning; where a policy requires on-site measurement, measure on site.
Why is WBGT lower than the heat index?
They are different scales, not different accuracies. The heat index is expressed as an apparent temperature and runs above air temperature. WBGT is an index value that typically lands below air temperature in dry air. A lower WBGT number can still mean a more dangerous situation.
Related WBGT guides
Know the real heat number
Real-time WBGT, flag levels and forecasts. Free on iPhone.
