Temperature
Warmer air adds to the heat around you.
What is WBGT?
Wet Bulb Globe Temperature is a measure of environmental heat stress. It brings together air temperature, humidity, sunlight, and wind.
Because the temperature alone doesn’t tell the whole story.
Understand the numberHumidity
Humid air makes it harder for sweat to evaporate.
Sunlight
Sun and hot surroundings add radiant heat.
Wind
Moving air helps sweat evaporate.
A different kind of degree. WBGT uses °F or °C, but has its own heat-safety thresholds. It is not air temperature, a “feels like” reading, or your body temperature.
WHY USE A DIFFERENT NUMBER?
More comprehensive heat assessment
Heat index adds humidity to temperature. WBGT also responds to sunlight, radiant heat, and the wind where you are.
| Accounts for | Air temp. | Heat index | WBGT |
|---|---|---|---|
| Air temperature | ✓ | ✓ | ✓ |
| Humidity | — | ✓ | ✓ |
| Sun & radiant heat | — | — | ✓ |
| Actual wind | — | — | ✓ |
Heat index assumes shade and light wind. National Weather Service ↗
01 / INSIDE THE NUMBER
Four factors.
Three sensor readings.
Humidity and wind don’t get separate percentages. They affect how the wet bulb and black globe warm up or cool down.
With sunlight, WBGT combines these readings:
70%
Natural wet bulb
A wet wick mimics sweat evaporating from skin. Humidity, wind, and radiation affect its cooling.
20%
Black globe
Radiant heat from sun and surroundings, also affected by wind.
10%
Air temperature
The surrounding air temperature, measured with a shielded thermometer.
WBGT
Environmental
heat stress
The percentages weight temperatures—not humidity, sunlight, or personal risk.
See the formula and a simple example
With solar load:
WBGT = 0.7 × natural wet bulb + 0.2 × globe + 0.1 × air temperature
For example, in °C:
0.7 × 25 + 0.2 × 45 + 0.1 × 32 = 29.7°C WBGT (85.5°F)
Without solar load:
WBGT = 0.7 × natural wet bulb + 0.3 × globe temperature
All readings must use the same unit. NWS calculation reference ↗
Explore the weather.
Change conditions to see impact on WBGT and heat stress
59°F113°F
Drier · 10%Humid · 100%
No solar loadStrong sun
Still air20 mph
Air temperature90°F
Strong sun · moderate humidity · a light breeze
Estimated heat index
°F
Air + humidity only
Sun and wind change WBGT. Heat index uses air temperature and humidity, assuming shade and light wind.
Ideal shade: 78.5°F WBGT.
About 8.3° lower with the same air, humidity, and wind.
An illustration, not a local forecast. The model holds sun angle and surface conditions fixed. Use your applicable WBGT policy for activity decisions.
How this estimate works
This tool adapts the Liljegren et al. (2008) heat-balance model to estimate the natural wet-bulb and globe temperatures. It uses the outdoor WBGT formula when there is solar load, and the no-solar formula at zero. It does not use a simple heat-index conversion.
Fixed assumptions: sea-level pressure (1,013.25 hPa), sun 60° above the horizon, 80% direct solar radiation when sunlight is present, a 150 mm black globe, and a surface at air temperature with 20% reflectivity and 0.98 emissivity. Wind is at sensor height (about 2 m), with a model minimum of 0.13 m/s.
Sun level: 0–100% represents 0–1,000 W/m² of incident solar radiation; it is not a cloud-cover percentage. Setting the slider to 0% removes all solar load for an ideal-shade comparison. Real shade can retain diffuse sunlight and radiant heat from surroundings.
Heat index: calculated with the National Weather Service method, including the Rothfusz regression, low- and high-humidity adjustments, and the simpler averaged estimate for cooler conditions. Only air temperature and relative humidity enter this calculation. Extreme combinations can exceed the formula’s validated range.
Flag colors: use the Klimo Insights example guidelines below, based on WBGT rounded to one decimal in °F. Changing display units keeps the same flag.
- Natural wet bulb
- 77.3°F
- Black globe
- 118.8°F
- Solar radiation
- 800 W/m²
Current formula: 70% natural wet bulb + 20% globe + 10% air temperature.
This product includes software produced by UChicago Argonne, LLC under Contract No. DE-AC02-06CH11357 with the Department of Energy. Software license and attribution. This educational adaptation is separate from Klimo’s forecast system.
03 / PUT IT TO USE
How to use WBGT.
Higher WBGT means greater environmental heat stress. Your activity and heat-safety policy determine what comes next.
- 01
Check the place and time.
Use forecasts to plan ahead. Check conditions where the activity will happen.
- 02
Use the right thresholds.
Follow your sport or workplace policy. Effort, clothing, equipment, and acclimatization matter.
- 03
Adjust as conditions change.
Apply your policy’s guidance for breaks, cooling, equipment, and modifying or stopping activity.
WBGT guidelines, in practice.
An example from the Klimo WBGT app. The recommended response changes with activity intensity.
Klimo Insights guidelinesEXAMPLE GUIDANCE| WBGT & flag | Activity Level | ||
|---|---|---|---|
| Low intensity | Medium intensity | High intensity | |
| Clear FlagBelow 80°FBelow ≈26.67°C | Normal activities. Maintain hydration and watch for signs of heat stress. |
||
| Green Flag80–84.9°F≈26.67–29.39°C | Regular activities. |
10-minute break each hour. Hydrate. |
10-minute break each hour. Hydrate. |
| Yellow Flag85–87.9°F≈29.44–31.06°C | 10-minute break each hour. Increase hydration. |
20-minute break each hour. Reduce overly strenuous activities. |
30-minute break each hour. Ideally reduce any activity to 45 minutes. |
| Red Flag88–89.9°F≈31.11–32.17°C | 15-minute break each hour. Increase hydration. |
20-minute break each hour. Reduce strenuous activities. |
30-minute break each hour. Ideally reduce any activity to 45 minutes. |
| Black Flag90°F or higher≈32.22°C or higher | 20-minute break each hour. Stay in the shade. |
20 minutes of activity. Then 40 minutes of rest. Ideally seek shade or an indoor environment. |
Suspend all outdoor activity. Move indoors or reschedule. |
WBGT values, not air temperature. °C values are approximate conversions of the app’s °F bands.
Use the policy that applies to your activity. These guidelines are an example; workload, acclimatization, clothing, and health can change the appropriate response. How to use WBGT thresholds ↗
Explore WBGT policies by stateCompare high-school practice and game requirements on an interactive map.Common Questions
Is WBGT the same as “feels like”?
No. “Feels like” often refers to heat index, which combines temperature and humidity. WBGT is a separate environmental heat stress index that also responds to radiant heat and wind. Its number is not directly comparable to heat index or air temperature.
What WBGT is too hot?
There is no single cutoff for everyone. Limits depend on exertion, clothing, equipment, acclimatization, and the policy for your workplace, sport, or event. Use WBGT-specific thresholds in the correct unit—°F or °C. See the Klimo example table above.
Understanding WBGT thresholds ↗Can WBGT be high on a cloudy day?
Yes. Clouds reduce direct sunlight, but warm, humid air and little wind can still limit evaporative cooling. The sky alone cannot tell you how much heat stress the environment presents.
Do I need a WBGT meter?
A meter measures conditions where you place it. Weather-based estimates and forecasts help with planning, but may miss local shade, blocked wind, or nearby heat sources. Use an onsite meter when your policy requires one.
Does WBGT tell me my body temperature?
No. WBGT describes the environment. It does not measure your core temperature or personal condition. People can respond differently to the same WBGT; take signs of heat illness seriously regardless of the number.
04 / THE EVIDENCE
Research behind
better heat decisions.
A few useful studies on heat policies, athletic performance, and event planning. Each finding has a specific context.
Georgia high school football
35–100% lower illness rates
Georgia’s combined WBGT, acclimatization, and practice rules were followed by lower illness rates.
Study context
Rates of heat syncope and heat exhaustion fell across WBGT categories after a combined policy introduced WBGT-based activity changes, acclimatization, and practice limits.
This before-and-after study covered 341,348 athlete-exposures. It evaluated the policy package; it cannot isolate the effect of WBGT alone. The 100% reduction reflects no observed cases in one category after the change.
Cooper et al. Heat Policy Revision for Georgia High School Football Practices Based on Data-Driven Research. Journal of Athletic Training, 55(7), 673–681.
Heat policies across states
55% lower heat illness rate
Mandatory preseason rules were associated with lower illness rates across 2.7 million athlete-exposures.
Study context
Mandatory preseason heat-acclimatization guidelines were associated with a lower exertional heat illness rate in high school football, across about 2.7 million athlete-exposures.
The adjusted rate ratio was 0.45 (95% confidence interval: 0.23–0.87). This observational analysis evaluated broader preseason rules, not WBGT alone; association does not establish causation.
Kerr et al. The Association between Mandated Preseason Heat Acclimatization Guidelines and Exertional Heat Illness during Preseason High School American Football Practices. Environmental Health Perspectives, 127(4), 047003.
Marathons and other endurance races
1,258 races analyzed
Performance declined about 0.3–0.4% per 1°C outside optimal WBGT conditions.
Study context
WBGT predicted performance better than air temperature alone. Across the events studied, performance declined roughly 0.3–0.4% per 1°C outside optimal WBGT conditions.
The analysis included running and race walking, with different optimal conditions and responses by event. The overall finding is not a fixed penalty for every marathon or athlete. A 1°C change equals a 1.8°F change.
Mantzios et al. Effects of Weather Parameters on Endurance Running Performance: Discipline-specific Analysis of 1258 Races. Medicine & Science in Sports & Exercise, 54(1), 153–161. Published online in 2021.
Planning match times around heat
14 of 16 host locations
Modeled WBGT above 28°C supports moving high-risk matches out of afternoon hours.
Study context
A historical-weather modeling study found outdoor WBGT could exceed 28°C in 14 host locations, supporting a shift away from afternoon matches at higher-risk venues.
Researchers used 2003–2022 weather data to assess June–July heat risk for the 2026 host locations. These are modeled regional conditions, not measured 2026 match conditions, indoor stadium conditions, or a universal cancellation threshold.
Mullan et al. Extreme heat risk and the potential implications for the scheduling of football matches at the 2026 FIFA World Cup. International Journal of Biometeorology, 69, 753–763.
PUT IT INTO PRACTICE
Check WBGT for your location.
Explore local conditions and forecasts with Klimo.