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The wet bulb globe temperature reading is based on simplified formulas and estimated variables. Only rely on data from on-location sensors to make safety decisions.
What is Wet Bulb Globe Temperature (WBGT)?
If you're making heat safety decisions for student-athletes, workers, or outdoor events, you've likely come across the term Wet Bulb Globe Temperature. But what exactly does it mean and how is WBGT calculated?
Wet Bulb Globe Temperature, or WBGT, is your body's true heat stress scorecard.
WBGT is a measure of how direct sunlight truly feels to the human body, factoring in multiple environmental elements to gauge heat stress. Unlike the heat index, which only considers temperature and humidity, calculating WBGT factors in the sun's intensity, wind, and cloud cover.
What is the WBGT Index?
The WBGT index is the gold standard for measuring the impact of heat stress on the human body in real-world conditions. It's the official metric used by everyone from the NCAA and state high school athletic associations to OSHA and the U.S. Military to make critical heat stress decisions.
WBGT readings, unlike regular temperature readings, combine the effects of:
- Air temperature and air movement
- Relative humidity
- Radiant heat or Sun angle
- Cloud cover (solar radiation)
WBGT is a composite value made up of:
- Dry bulb temperature (Tdb) – your standard air temperature
- Natural wet bulb temperature (Tnwb) – measures evaporative cooling (humidity + airflow)
- Globe temperature (Tg) – measures radiant heat from the sun and surroundings
How WBGT is Calculated: The Wrong Way vs. The Right Way
If you've ever Googled "how to calculate WBGT," you've likely found a simple formula that seems easy enough. But that common online calculator is a theoretical shortcut — and it's dangerously incomplete.
The Common Shortcut: The Stull Method of WBGT Estimation
Many free apps and online tools use an estimation known as the Stull Method. It's a formula designed to guess one component — the Wet Bulb Temperature (Tw) — using only two basic data points: air temperature and relative humidity.
Tw ≈ T × atan(0.151977 × √(RH + 8.313659)) + atan(T + RH) – atan(RH – 1.676331) + 0.00391838 × RH3/2 × atan(0.023101 × RH) – 4.686035
Where T = dry bulb air temperature in Celsius (°C) and RH = relative humidity in percent (%).
The problem? It completely ignores the two biggest factors that determine real-world heat stress on an athlete:
- Solar Radiation (the sun beating down)
- Wind Speed (a cooling breeze)
The Full WBGT Formula
The professional formula combines all three measured components:
WBGT = (0.7 × Tw) + (0.2 × Tg) + (0.1 × T)
- T = Temperature in Celsius
- Tg = Globe thermometer temperature — measured by a thermometer placed inside a special black globe to estimate solar radiation
- Tw = Wet-bulb temperature in Celsius
Here is the equation for calculating wet bulb temperature:
Tw = T × arctan[0.151977 × (RH% + 8.313659)^(1/2)] + arctan(T + RH%) – arctan(RH% – 1.676331) + 0.00391838 × (RH%)^(3/2) × arctan(0.023101 × RH%) – 4.686035
Here, "T" is the temperature in Celsius, and "RH" represents the relative humidity. With this final calculation, you can calculate WBGT completely.
WBGT Risk Levels
The WBGT index is used worldwide by sports organizations, military branches, and occupational health agencies to guide safe activity levels in hot conditions.
Low Risk
Conditions are safe for all activities. Maintain routine hydration habits.
Moderate Risk
Use caution with unacclimatized individuals. Increase fluid intake and monitor for symptoms.
High Risk
Limit activity duration and intensity. Mandatory rest breaks and shade access required.
Extreme Risk
Cancel or postpone strenuous outdoor activity. Serious heat illness risk for all participants.
Why do I need to monitor WBGT?
Wet bulb globe temperature helps maintain a proper workload in direct sunlight and deters heat strokes during excessive heat events. This is especially important because it differs from the heat index, which only accounts for temperature and humidity in shaded areas.
When schools and organizations follow WBGT guidelines, they can make better decisions about:
- When it's safe to be outside practicing
- Hydration and how often to take water breaks
- Whether to use pads during practices
- If they should cancel practice and physical activity
- If they should move practice or the event indoors
Why Use WBGT to Calculate Heat Stress?
- It's Apt for Outdoor Activity: Since WBGT is a measure of heat-related stress specifically, it's essential for construction activities, athletic events, and any time people perform outdoor work. Regulations are in place for WBGT monitoring, making accurate calculation essential.
- WBGT is Highly Valuable for Athletics: WBGT is highly valuable for high school and collegiate athletics, event venues, OSHA-regulated construction, city sports leagues, and military training facilities. During a high school football practice, knowing the WBGT ensures that players in direct sunlight won't put dangerous stress on their bodies.
- WBGT shows you when humidity decreases: A humidity decrease generally causes a rise in temperature — knowing the WBGT is also beneficial for winter sports and snowmaking operations.
Best Ways to Calculate WBGT
Whether you're an athletic trainer or organizing an outdoor event, knowing how to calculate wet bulb globe temperature can be a game-changer for heat safety. Not all WBGT methods are created equal.
- Option 1 – By Hand: Requires a black globe thermometer, psychrometer, and dry bulb thermometer. Scientifically sound but unscalable for real-time monitoring.
- Option 2 – Weather Apps: Use the Stull method and pull data from weather stations up to 25 miles away. No site-specific data, only hourly updates, often inaccurate.
- Option 3 – Handheld WBGT Devices: Portable and durable, but too manual, lacking automated logging, and small black bulb sensors cause inaccurate WBGT spikes.
- Option 4 – On-site automated monitoring: On-site sensors that automatically log WBGT every 5 minutes with policy-based alerts triggered instantly when thresholds are crossed. The most reliable approach.
Frequently Asked Questions
The wet bulb globe temperature (WBGT) can be measured with on-site weather stations, handheld devices known as WBGT monitors, or online WBGT and weather monitoring apps. These tools calculate the temperature, humidity, and sunlight to compute the WBGT index.
Wet bulb globe temperature readings can be taken using a WBGT monitoring device like a weather monitoring station or a handheld monitor. Readings must be taken by an athletic director, administrator, or medical personnel overseeing the event.
The WBGT should be monitored throughout the event and readings taken every 20 to 30 minutes, with athletic activities adjusted accordingly. Participation modifications must be communicated to the coaching staff before and during practice.
Regularly monitoring WBGT levels allows coaches and athletic trainers to adjust training schedules, implement hydration protocols, and make informed decisions to protect athletes from high temperature and heat-related risks. Tracking WBGT is crucial to protect athletes' health and safety during outdoor practices, games, and events.
WBGT should be measured regularly, especially during periods of high heat or physical activity. The frequency depends on the specific situation and governing body guidelines — most recommend readings every 20–30 minutes during outdoor athletic events.
While some apps provide estimates of WBGT, dedicated WBGT monitors and professional weather stations offer more accurate and reliable measurements. Free apps typically use the Stull method — which ignores solar radiation and wind speed — and pull data from weather stations up to 25 miles away.
No. WBGT is best used outdoors — outside any building, athletic center, or climate-controlled area. Heat index, on the other hand, is best used indoors within any building, athletic center, and casual shaded environments. WBGT accounts for solar radiation and wind; heat index does not.