Wet Bulb Globe Temperature (WBGT): Everything You Need to Know

Wet bulb globe temperature. Say that three times fast!

It’s difficult to say and isn’t easy to calculate, but monitoring wet bulb globe temperature is a stress-free way to protect student athletes, employees, and overall operations from extreme heat. 

Wet bulb globe temperature has been a popular topic for schools, universities, and sports leagues around the world. Educational and athletic institutions look at wet bulb globe temperature as a comprehensive measure of all the weather-related factors that impact the bodies (and therefore, the performance) of athletes on any given day.

So, what is the wet bulb globe temperature and how can you use it? It’s time to become a WBGT expert so you can see why it is the best way to measure heat and manage heat stress for your organization and how you can go about doing that.


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What is Wet Bulb Globe Temperature (WBGT)? 🤔

According to the National Weather Service, wet bulb globe temperature (WBGT) is an apparent measurement used to estimate the most accurate level of heat stress in direct sunlight.

Temperature is just the reading on the thermometer. Heat index considers two factors: Temperature and humidity. But the WBGT takes five factors into account. These include:

Illustration of a school with a sports field and the five aspects factored into wet bulb globe temperature (WBGT) Temperature, sun angle, wind speed, humidity, and cloud cover

Temperature

Humidity

Wind speed

Sun angle

Cloud cover (solar radiation)

 

This helps produce the most accurate temperature readings. With WBGT, groups like military agencies, industrial hygienists, and sports programs can manage their workloads effectively and optimize performance in high-temperature environments.

 

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How is Wet Bulb Globe Temperature Calculated? 🧮

As we mentioned before, WBGT takes several environmental factors into account. Because of this there is a mathematical equation that includes all the pertinent factors when calculating WBGT. The equation is:

Wet bulb globe temperature (WBGT) calculation formula

Let’s break it down.

Natural wet bulb temperature, which indicates humidity

Tg part of the wet bulb globe temperature equationGlobe thermometer temperature, which indicates radiant heat

td in the wet bulb globe temperature equationDry bulb temperature, which indicates ambient air temperature

That’s it! It’s important to note that there is a different equation for calculating WBGT indoors or when solar radiation is negligible. You probably won’t need that equation. If you’re wondering what that equation is our friends at the Korey Stringer Institute have it on their website.

What About Wet Bulb Temperature?

While wet bulb globe temperature is a calculation that helps us discover how heat affects the human body, there is a different measurement called wet bulb temperature.

Wet bulb temperature is a measurement taken by a thermometer that has a bulb covered by a water-soaked cloth. When air passes over the cloth to evaporate the water, the evaporative cooling shows the wet bulb temperature on the thermometer.

But why do we care about this measurement? We often use this temperature to find the dew point and/or humidity as we compare it to the dry bulb air temperature.

If you’re a Sferic Maps user, you can access hyperlocal wet bulb globe temperatures and wet bulb temperatures. Make sure you know the difference and use wet bulb globe temperature if you’re trying to protect athletes or employees from the heat.

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The History of WBGT 📚

When did temperature and heat index stop being enough?

Some of the earliest users of WBGT were the U.S. Marines and U.S. Army. These branches of the military used WBGT to help drill sergeants and officers know how hard they could push recruits during outdoor activities while avoiding heat illness. They created activity level/rest time ratios based on WBGT to maximize training results. 

They developed a flag system for training areas. The flag colors let officers know the WBGT status. White flags are Category 1, Green are Category 2, Yellow are Category 3, Red are Category 4, and Black are Category 5. 

Wet bulb globe temperature (WBGT) flag graph
 

Over the years other groups began using WBGT and created their own guidelines. Some of the most popular reference groups today include:

 

Heat Acclimatization

Man sweating because of high wet bulb globe temperature

Each category includes water intake guides for both heat acclimated and unacclimated individuals.  

What is heat acclimatization? Let’s review for those of you who are not familiar or just need a refresher.

Heat acclimatization is the process of adjusting one’s body to high-temperature conditions.

This process lowers the possibility of heat illness or exertional heat stroke (EHS). This improves overall outdoor performance in athletes and laborers. If properly done, heat acclimatization can lower your heart rate, reduce body and skin temperature responses, and increase sweat production and blood flow. 

For athletics, begin heat acclimatization early in the season by slowly and steadily introducing heat stress to the body. This can mean lengthening workouts and exposure a little each day, adjusting your heart rate under stress, wearing limited gear and more. Other steps toward acclimatization in athletics include: 

Do not practice longer than 3 hours per day

No consecutive double practices

Increase workout intensity gradually over a few days

Increase sodium intake as sweat increases

Hydrate regularly throughout games and practices

Take frequent rests to avoid overheating

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What is the Purpose of Wet Bulb Globe Temperature? 🏃‍♀️

Football players practicing agility drills on a turf field as the sun rises

Wet bulb globe temperature may just seem like another way to read the thermometer or an annoying stipulation to keep you from conditioning student-athletes. However, the real purpose of using the wet bulb globe temperature is to make your team healthier and better prepared to perform in high heat situations.

The real purpose of using wet bulb globe temperature as a heat measurement is to keep people safe while performing strenuous outdoor activities at high temperature.

It is the most comprehensive and therefore most reliable measure of how hard or easy it is for athletes to perform in their specific outdoor environment. 

Athletic Directors, Athletic Trainers, Coaches, Recreation Managers and players themselves use wet bulb globe temperature to help with heat acclimatization during warmer months to prevent heat-related illnesses and injuries by adjusting their activities accordingly. 

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Suggested Actions and Impact Prevention 👒

So how are the WBGT guidelines broken down? According to the National Weather Service (NWS), There are four main categories of WBGT. Each category comes with its own effects and suggested actions for outdoor activity. These categories don’t start until the WBGT hits 80 degrees F.

Remember, when we refer to temperature here, we are referring to WBGT.

Wet bulb globe temperatures suggested actions and impact prevention chart. When the WBGT is above 90 degrees Fahrenheit the body is stressed after 15 minutes and needs at least 45 minutes of breaks every hour of working or exercising in direct sunlight. When the WBGT is 88-90 degrees Fahrenheit the body needs at least 40 minutes of breaks. When the WBGT is 85-88 degrees Fahrenheit the body needs at least 30 minutes of breaks. When the WBGT is 80-85 degrees Fahrenheit the body needs at least 15 minutes of break.

When it’s 80-85 degrees F (in WBGT), working or exercising in direct sunlight will stress an individual’s body after 45 minutes. To prevent heat illness and lessen the overall impact of heat stress, plan 15 minutes of breaks for each hour you’re out in direct sunlight.

When it’s 85-88 degrees F (in WBGT) your body will stress from working or exercising in direct sunlight after just 30 minutes. Since your body stresses quicker, you’ll need longer breaks. If the WBGT is in this category make sure you take at least 30-minute breaks.

When it’s 88-90 degrees F your body will stress from just 20 minutes of working in direct sunlight. Take at least 40 minutes of breaks. For coaches this may seem a little extreme, but you can use the shaded break times to go over strategy and other critical concepts.

If the WBGT is 90 degrees F or above working or exercising in direct sunlight will stress your body in just 15 minutes. That means breaks increase to 45 minutes for each hour working in direct sunlight. You may benefit from moving practice indoors at this point. Make sure you keep a close eye on both acclimated and unacclimated athletes.

Breaks

Take all breaks in the shade. If there is no natural shade in the area surrounding your strenuous outdoor activity, improvise with pop-up tents. Shade has a tremendous effect on lowering the heat stress levels. That’s because it’s often 10-15 degrees cooler in the shade. Even 10 degrees will make a huge difference to someone on the verge of heat illness.

Hydration

Make sure those exerting themselves in high temperature environments are drinking plenty of water. Bodies need water to help keep them cool. Remind athletes to drink water even when they aren’t thirsty. Also advise them to increase water intake with increased time outdoors. Sports drinks with sodium and electrolytes are also key in replacing nutrients we sweat out. Athletes and outdoor workers alike should always take in more water than they are losing or dehydration is unavoidable.

Sunscreen

One last suggested action when using WBGT guidelines is to reduce sun exposure. You can do this with sunscreen, proper clothing, and hats (when possible).

The best way to reduce sun exposure in athletes is by planning outdoor activities in the early morning or evening. Before 10 a.m. and after 4 p.m. are the best times for reduced sunlight exposure. Athletes should also wear a waterproof/sweat proof sunscreen. This sunscreen should have a SPF of at least 30. It should also have broad spectrum protection (against both UVA and UVB rays).

We’ve found that stick formula suncreens are good for faces because the sunscreen stays out of your eyes even if you sweat. Athletes should apply sunscreen at least 20 minutes before they head out into the sun. Ensure they reapply every two hours or after excessively sweating.

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Monitoring WBGT

To put the suggested actions and impact prevention plans into effect, you must measure WBGT at your facility. There are a few monitoring and alerting methods, devices, and services you can use to protect athletes at your facility.

MEASURE WBGT

Manually Calculating WBGT 

If you don’t have a physical WBGT device or an alerting service from a trusted real-time weather data provider, you can use a chart (like the one provided below) to estimate wet bulb globe temperature using a combination of the current temperature and relative humidity. As this is just an estimate, we do not recommend this as a foolproof WBGT monitoring method.

Wet bulb globe (WBGT) graph from temperature and relative humidty
This chart in particular is based on a formula that is valid for full sunshine and a light wind. It’s important to note that use of this chart will often lead to over-alerting since conditions rarely are “full sun” and “light wind.”

Handheld Monitoring Devices

That brings us to other methods of monitoring WBGT. To get a more accurate reading of this life-saving temperature measurement, you can use a handheld monitoring device. There are plenty of different models on the market today for you to choose from. However, it’s important to note that these devices can fluctuate in accuracy and may require calibration and maintenance. In addition, the measurement stays with one person rather than being available to a team. Another drawback to relying on handheld WBGT devices is human error, which is quite common. 

On-Site Weather Station

The final way to monitor WBGT at your facility is from a hyperlocal weather station network that measures it. Our weather stations measure WBGT as a current condition from a station level. This condition can then be monitored in our Sferic Maps weather map or Sferic Mobile application. You can trust professional weather networks because they calibrate weather data to ensure its factually correct. Automated WBGT category alerts via text or email ensure everyone is in the know as quickly as possible and prepared to activate their impact prevention protocol. 

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WBGT: The Bottom Line

animated yellow sun gift with smiley faceSo what does all this mean?

WBGT is the number one way to keep student-athletes (or anyone exerting themselves outdoors) safe from heat illness. That means it’s better than just monitoring the temperature or heat index.

Wet bulb globe provides an accurate measurement of environmental heat stress

It’s been around for over 60 years

You can use WBGT as a preventative measure to reduce heat illnesses by modifying work/rest ratios

WBGT modifications vary for acclimated and unacclimated individuals

WBGT is a more comprehensive heat stress measure than temperature or Heat Index

Automatic WBGT warnings based on real-time readings are the easiest and quickest form of monitoring and alerting method

When wet bulb globe temperature goes up against temperature or heat index, it always wins as the best tool to help you and your athletes have a successful and safe season.

Now What?

Now that you’re a regular wet bulb globe temperature expert it’s time to think about how you might be able to use this measurement in your life.

If you’re a coach, athletic director, or athletic trainer you can provide safer heat stress guidelines, faster heat illness response, and increase the wellness of all your student athletes.

It’s time to start thinking about monitoring and alerting on wet bulb globe temperature at your school.

Click the button below to learn how to get real-time alerts for WBGT and other severe weather threats at your school:

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