Ever wondered why a blazing hot day doesn’t feel the same on a breezy soccer field as it does on a crowded construction site? Or why some schools cancel outdoor activities while others power through? The answer lies in a clever tool called the Wet Bulb Globe Temperature (WBGT)—the ultimate measure of heat stress. By understanding WBGT categories and zones, you’ll not only make sense of those numbers but also learn how they help protect athletes, workers, and even weekend gardeners from heat-related dangers. Stick with us, and we’ll break it all down!

What are the Wet Bulb Globe Temperature (WBGT) Categories?

Simply put, WBGT categories are the language of heat safety, turning invisible dangers into clear, actionable decisions. WBGT Categories represent thresholds of heat stress risk, determined by specific WBGT readings. These categories provide a structured guide to understanding how dangerous the heat is for physical activity based on environmental conditions.

WBGT categories break down thresholds for heat stress levels and dictate when it’s safe to work or play outside and when it’s time to cool off or call it quits. WBGT temperatures may vary depending on the region and territory you belong to, owing to acclimatization. For instance, here are the WBGT categories for the state of California, as recommended by the California Interscholastic Federation (CIF):

What are WBGT Zones and why do they depend on your region?

WBGT Zones are the geographic or environmental areas where WBGT measurements are monitored, often considering microclimates, such as urban heat islands or shaded regions, to provide localized heat stress insights. They account for regional climate variations and the specific risks to individuals in different environments. Zones are location-specific, adapting WBGT values to account for unique environmental factors like direct sunlight or urban heat islands.

Different regions, or “zones”, of the US have their own heat thresholds and WBGT guidelines.

There are 3 different WBGT zones in the US, each with its own threshold:

• Class 1 zones: threshold of 84.2 degrees F
• Class 2 zones: threshold of 87.8 degrees F
• Class 3 zones: threshold of 90.14 degrees F

Northern states tend to have lower thresholds. Southern states tend to have higher thresholds. 

Depending on the zone your area is in dictates your WBGT threshold and the recommendations you should take when that threshold is reached like: 

• adding more water breaks
• cutting the length of practice
• limiting the amount of physical activity
• canceling the practice completely

Example:

A high school football practice in Bismarck, North Dakota – a Class 1 WBGT zone – should cancel football practice if WBGT reaches 84.2 degrees F.

However, a high school in Lubbock, Texas – a Class 2 WBGT zone – should cancel football practice if WBGT reaches 87.8 degrees F.

And to show how WBGT guidelines can change even within the same state because of zones, a high school in Dallas, Texas – a Class 3 WBGT zone – should cancel practice if WBGT reaches 90.14 degrees F.

It’s important for coaches and athletic directors to familiarize themselves with which WBGT zone they are in, so they know what guidelines to follow.

Keep in mind, some states like North Carolina are entirely one zone, while states like Pennsylvania make up all 3 different zones.

Low-Risk (Green Zone):

• Safe for most activities.
• Basic hydration and rest breaks are sufficient.

Moderate-Risk (Yellow Zone):

• Increased caution required.
• Athletes or workers may need longer rest periods and frequent hydration.
• This is a transition zone where mild heat-related symptoms could begin​

High-Risk (Red Zone):

• Strenuous activities should be limited or adjusted.
• Scheduled breaks in shaded or cooled areas become essential.
• Organizations often implement specific heat safety protocols to reduce heat illnesses​

Extreme-Risk (Black Zone):

• High likelihood of heat illness if precautions aren’t followed.
• Activities may be suspended or moved to cooler parts of the day.
• Critical interventions include immediate cooling stations and medical readiness​

Learn all you need to know about wet bulb globe temperature here.

What is Heat Acclimatization?

Heat acclimatization is the body’s ability to adapt to hot environments over time. Individuals in warmer climates acclimate faster, leading to different work-rest guidelines compared to cooler regions. This is why each state has its own unique set of region-specific WBGT activity guidelines.

The Science behind WBGT

Wet bulb globe temperature involves measuring three temperatures: dry bulb, wet bulb, and globe. 

• Dry Bulb Temperature: This is the temperature you think of when checking a wet bulb thermometer. It measures the air temperature in the shade, without the effects of relative humidity or radiant heat from the sun.

• Wet Bulb Temperature: This is a measure of the air temperature as felt when the skin is wet and exposed to moving air. It accounts for the cooling effect that happens when moisture (like sweat) evaporates. Wet bulb temperature is measured using a thermometer wrapped in a wet cloth, capturing the combined effects of temperature and humidity.

• Globe Temperature: Globe temperature captures the impact of radiant heat, like sunlight or heat from machinery, on our bodies. It’s measured using a blackened bulb or globe with a wet bulb thermometer inside, allowing it to absorb radiant heat and tell us how that heat might affect us.
  

Read more about wet bulb globe temperature measurements and black bulb sensors here.

Heat Index vs WBGT

Two terms come up often when talking about heat: heat index vs wet bulb globe temperature.

Heat Index: Unlike WBGT, heat index is a measurement that only takes into account air temperature in globe and humidity. Think of it as the “feels-like” temperature in a shaded area. Heat Index is measured in the shade rather than the sun, and doesn’t factor in cloud cover (solar radiation), wind, or sun angle.


WBGT: Like we’ve mentioned, WBGT (wet bulb globe temperature) is more comprehensive than Heat Index because it’s taking into account many more factors to give a better idea of heat stress in particular environments.

While heat index remains the default measurement in some organizations, it’s not always the ideal measurement to use.

• For general heat stress and public weather forecasts, the Heat Index might be more applicable because it gives a quick “feels-like” temperature.
  
• For situations where precision and heat safety are needed, like in outdoor sports practice or outdoor events, WBGT is preferred. 

Learn more about Heat Index vs. Wet Bulb Globe Temperature.

WBGT in sports and athletics

Outdoor sports are where WBGT is becoming the go-to measurement for heat stress. Between record-hot weeks and evolving heat stress regulations, using WBGT for heat stress monitoring in outdoor sports has never been more important.

Heat stress can severely impact the health and safety of athletes, especially depending on the region of the US they’re located in. Without monitoring WBGT, athletes are at risk of heat-related illnesses like heat exhaustion, heat cramps, and heat stroke. 

These conditions can lead to serious health complications and, in severe cases, can be fatal. 

Read more about WBGT in athletics here.

Comparison of WBGT across athletic surfaces: 

You’re a high school or university athletic trainer and need to measure WBGT for your entire athletic complex. But you have: 

• concrete and acrylic tennis courts
• artificial turf football fields
• grass soccer fields

Does one centrally-taken WBGT measurement accurately account for the different athletic surfaces in your complex? The short answer: yes. 

The University of Georgia conducted a 2019 study where they compared WBGT measurements from different athletic surfaces in their complex and found no statistically significant differences in measurements between each surface. 

Read more about this study.

Should I use Wet Bulb or Wet Bulb Globe Temperature?

For Outdoor Heat Stress Activities, Use WBGT

If your primary concern revolves around outdoor activities in hot weather, WBGT is the way to go. This measurement is becoming the standard for school athletics, collegiate sports, and various outdoor activities. 

For Cold-related Activities, Use Wet Bulb

Wet bulb temperature, on the other hand, is more appropriate for cold-weather activities. It’s the preferred metric when dealing with activities like snowmaking.

For a more in-depth guide to Wet Bulb vs WBGT, our guide has you covered. 

How to add WBGT to heat stress policies

With increasing global temperatures and extreme extreme heat waves, the need to have WBGT included in heat stress policies is growing. 

Integrating WBGT into your heat stress policy:

Implementing WBGT into existing heat policies is pretty straightforward:

• Find out what WBGT region your outdoor sports programs fall into
• Note the WBGT category, or zone, for that region
• Check that zone’s WBGT class
• Note the recommended guidelines and actions that WBGT class’s threshold triggers (e.g., mandatory water breaks or practice cancellations). 

Take a look at our example WBGT policy to help guide you.

Also consider implementing clear guidance on how WBGT will be measured and how often. Handheld WBGT devices exist, but a solution like Perry Weather’s WBGT monitoring is far more comprehensive and removes a lot of the guesswork that comes with monitoring and calculating wet bulb globe temperature.

Plus, Perry Weather makes sure monitoring and alerting is tied back to your policy and its thresholds.

If you don’t already have a heat stress policy…

This is a good opportunity to start crafting one. Start by considering your organization’s or use case’s needs and circumstances like where you’re located and who is involved in outdoor activities.

To learn more about crafting a comprehensive heat stress and WBGT policy, read our guide.

If you need help crafting a heat stress and WBGT policy for your needs, Perry Weather offers policy consultations backed by expert meteorologists to make sure your policy is accurate, tailored, and comprehensive. 

Policy reviews and updates:

Like any policy, your WBGT policy also requires regular reviews and updates. With temperatures rising, and local weather trends evolving, your WBGT policy could be impacted if, for example, your region was originally a Class 1 zone but becomes a Class 2 zone. So make sure you’re keeping up with policy reviews and any necessary updates. 

What’s the Best Way to Measure WBGT?

Precision is key when it comes to measuring wet bulb temperature, and not all measurement solutions are up to the task. 

Manual calculation: 

A manual WBGT calculator involves a formula, but you first need to calculate wet bulb temperature which itself also has a formula. So there are two complex wet bulb calculation formulas involved. This isn’t the best method, as it requires a lot of manual math, metrics, and is prone to human-error.

To see these formulas and learn how to calculate WBGT manually, read our guide. The National Weather Service also offers a WBGT forecasting tool, although this isn’t recommended when providing heat stress safety. 

On-site device: 

To truly get wet bulb globe temperature measurement right, having an on-site WBGT sensor like the one offered with Perry Weather’s weather station is the way to go. 

On-site sensors provide real-time, accurate data tailored to your exact surroundings, enabling better decision-making and accurate measurements when you need them.

Not sure about needing an on-site solution? Read our guide on why an on-site WBGT device is best. 

What tools and solutions exist for WBGT monitoring?

Handheld devices: 

These are portable but prone to human-error, especially if they require frequent calibration or if users don’t use them properly. For instance, if the device is being held in the shade, the WBGT reading will be inaccurate. And if the device isn’t calibrated properly, the reading will be inaccurate as well. 

Outdoor weather stations: 

Outdoor weather stations equipped with a WBGT monitoring device offer the most accurate readings and can provide real-time data directly to computers or smartphones. These weather stations remove much of the manual involvement, human error, and lack of clarity around WBGT calculations and decision-making. 

How Perry Weather helps you accurately measure and monitor WBGT

Perry Weather’s approach to WBGT measurement and heat stress monitoring lets you provide accurate, comprehensive heat safety while removing the guesswork of what to do when your wet bulb globe temperature meter threshold is reached. 

Accurate Wet Bulb Readings and Alerts:

Perry Weather’s outdoor weather station, equipped with a more capable 3.5” diameter black bulb, provides precise and timely WBGT readings. That data is then reflected in a user-friendly dashboard when using Perry Weather’s WBGT monitoring software. Depending on the reading and your policy, clear and instructive alerts are then delivered via SMS, PA warnings, and more. 

Learn more about Perry Weather

Perry Weather offers accurate and done-for-you WBGT measuring and monitoring, providing solutions and features like an outdoor weather station, customizable alerts, policy consultations, and 24/7 on-call meteorologist support.

We make sure you’re able to provide comprehensive heat safety to those you’re responsible for. If you’d like to better understand how Perry Weather can elevate your WBGT monitoring and ensure safety, contact us to learn more.