Wet Bulb Globe Temperature (WBGT) is a comprehensive measure of heat stress in direct sunlight.
Unlike the heat index, which considers only temperature and humidity for shaded areas,

 

Why we use it:

CA State Law AB 1653 and CIF Bylaw 503 K.
 

WBGT takes into account:​

• Temperature: The basic measure of how hot it is.

• Humidity: The amount of moisture in the air, affects sweat evaporation and cooling.

• Wind Speed: Influences heat loss through convection.

• Sun Angle: The position of the sun impacts the intensity of solar radiation.

• Cloud Cover: Determines the amount of solar radiation reaching the ground.

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Why is it important for us?
WBGT is especially important for athletes because it helps us understand the real impact of the weather on our bodies. When we know the WBGT, we can make better decisions about:

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• Practice Intensity: Adjusting how hard we push during training.

• Rest Breaks: Scheduling more frequent breaks when necessary.

• Hydration: Ensuring you drink enough water to stay hydrated.

• Practice Length: Shortening practice if conditions are too intense.

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Playing your part

When you see the WBGT is high be prepared

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• Drink plenty of water before, during, and after practice.

• Wear light, breathable clothing.

• Know the importance of taking breaks and listening to your body.

• Understands that safety comes first, even if it means adjusting training plans.

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WBGT Components

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1. Dry-bulb temperature (Aka air temperature)

2. Natural wet-bulb temperature

   • ​Similar to an old-fashioned thermometer, wrapped in a wet cloth. 
     As water evaporates from that wet cloth, evaporation cools the thermometer, which is identical to how the human body cools itself with sweat. Ultimately, the natural-wet bulb temperature incorporates the effect of air temperature, humidity, and solar radiation into one individual temperature.

3. Black globe temperature

   • ​Temperature measured by a thermometer inside a black globe that is actually or simulated to be ~6 inches in diameter. Solar radiation from the sun heats the outside of the black globe, the wind blowing across the globe cools it, and the thermometer inside measures these changes combined. This allows the black globe thermometer to react in the same way as human skin, including the effects of solar radiation and wind speed on body temperature.

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The impact of hyper-localized conditions on WBGT

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Due to the factors/components of WBGT. Measurements will vary by location. Even just a change in surface (e.g. track surface, paved, grass, dirt), shade, or humidity can cause great differences. 

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Therefore it's important to take field WGBT measurements at the location of the outdoor activity when the forecast WBGT is predicted to be at or near National Athletic Trainers' Association (NATA) guidelines for practice modification or cancelation and the activity is taking place in a contained area with similar characteristics throughout. This will ensure the specifics of the location are taken into account when making decisions based on WBGT regarding practice or competition.

For example, the humidity of the air near a lake or water source is usually higher than that over a dry open field, thus WBGT will be higher in humid areas than dry ones. Thereby indicating higher heat stress.​

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WBGT History

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In the 1950s, the US military developed the Wet-Bulb Globe Temperature Index. 

This was in response to a large quantity of heat illness cases being observed during basic training.

The military's previously existing policy used heat index to evaluate heat risk for personnel in high-heat situations. To better represent risk, wind speed, sun angle, and cloud cover were incorporated, forming the Wet-Bulb Globe Temperature.  After implementing new policies based on the WBGT measures, there was a drastic reduction in heat illness during basic training.

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How come I don't see it in weather reports?

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Given that this is not a new metric you may wonder how you may have never heard about it before or seen it in weather reports. This is because weather stations do not have the sensors required to directly measure WBGT. As such it is not possible to get readings directly from the government/public-funded weather stations. The only way to get this measurement from weather stations is derived from the calculation of other measurements available using scientifically tested and derived formulas.

 

However, there are various calculation methods and no standard has been agreed upon.
There is a generalized consensus on which formulas and methodologies are the most accurate. Some are very calculation intensive and others are not, and the error bars vary compared to direct measurement by methodology.

In the dashboard on this website, the most accurate not majorly intensive calculation methodology has been deployed, since this is live data and at each page refresh many calculations need to be redone. There, however, is a more precise and calculation-intensive method available which was also tested. This other method takes a noticeable amount of time to compute and it only results in a very minor improvement in precision specifically in the late evening to sunrise hours. This delta is smaller than the delta between various other calculation methodologies and mainly impacts the hours when WBGT is not critical.

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