Measure WBGT to Mitigate Heat Stress

By John Crosby, Meteorologist

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Heat Stress: What is it and how is it measured?

Heat stress is a leading cause of weather-related deaths in the United States - more than lightning, flooding, or tornadoes. It is a serious health condition that affects workers, athletes, and anyone active in high temperature environments.

Climate change exacerbates the problem, with increased air temperatures and humidity. Colin Raymond of NASA’s Jet Propulsion Laboratory reports that extreme levels of heat stress have more than doubled over the past 40 years and the trend is expected to continue.

In simple terms, heat stress is caused by the body being overheated. Whether called heat exhaustion, heat stroke, or heat cramps, typical symptoms include confusion, inability to concentrate, excess sweating, lack of sweating, dizziness, and collapse.

Challenge: How can we measure heat stress and take measures to avoid it?

We know that humidity has a major effect on cooling the body. If the humidity is high, sweat on the surface of the skin does not evaporate as quickly, slowing the cooling of the body. Wind speed impacts the body’s ability to evaporate sweat - the higher the wind, the more effective the evaporation and cooling.

The four main contributing factors to heat stress: high temperatures, high humidity, low winds, and direct sunlight can make for a deadly combination. But also at play are the type of clothing being worn, the metabolic rate (exertion level), and the acclimation level of the individual.

The National Weather Service (NWS) regularly reports the HI when the temperature is > 27C (80F) and many weather stations (including those from Columbia Weather Systems) report HI as a standard feature.

While the Heat Index is a readily available tool, it doesn’t give us the most information to address heat stress. Fortunately, there is a better measurement available, that factors in a more useful parameter, called Wet Bulb Globe Temperature.

Wet Bulb Globe Temperature (WBGT) is a greatly improved measurement of heat stress as it accounts for real world conditions and utilizes additional measured parameters. WBGT is a measure of heat stress in direct sunlight.

It was developed in the early 1950s after the US military noticed a high number of heat casualties during boot camp and training exercises. The WBGT index has become the gold standard for measuring heat stress and is now a standard with the military, OSHA, and many states who oversee K-12 athletic programs.

Traditionally, the WBGT index was calculated using shaded air temperature, black globe temperature, and wet bulb temperature as described below.

The formula clearly demonstrates that WBGT is much more dependent on the Wet Bulb Temperature (seven times) than the dry bulb air temperature. It should be noted that the Wet Bulb Globe Temperature is always lower than the ambient air temperature.

With the HI value always higher than air temperature and the WBGT value always lower than the air temperature, the two methods are not directly comparable.
The following table shows the differences:

The development of the WBGT has led to a series of charts that the user must refer to understand the reading. These charts are different for the different climatic regions of the country and the level of exertion. For example, the U.S. State of Georgia, one of the early adopters of WBGT, uses the data in the WBGT decision table for student athletes.

Additional sources of information on implementing WBGT include:

• – The International Organization for Standardization ISO 7243:2017 that contains exhaustive details on the WBGT method.
• – The Korey Stringer Institute of the University of Connecticut guidelines for each US state, along with other useful information.
• – The National Federation of State High School Associations article entitled “Wet Bulb Globe Temperature (WBGT) – Why Should Your School Be Using It?”

Solution: Monitor WBGT to Mitigate Heat Stress

Columbia Weather Systems first developed a monitoring system with WBGT for the U.S. Coast Guard in Florida. The extreme heat and humid environment in Florida make it an important location to monitor heat stress.

Two methods were discussed in this white paper to measure heat stress as summarized below:

Heat Index: Unlike WBGT, heat index is a measurement that only takes into account air temperature 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. While heat index remains the default measurement in some organizations, it’s not always the ideal measurement to use. For general public weather forecasts, the Heat Index might be more applicable because it gives a quick “feels-like” temperature for a general area.

WBGT: WBGT is more comprehensive than Heat Index because it takes into account many more factors (solar radiation, wind, cloud cover, etc) to give a better idea of heat stress in particular environments. For situations where precision and heat safety are needed, like in outdoor sports practice or outdoor events, WBGT is preferred.

Contact Columbia Weather Systems to learn how their professional grade weather systems can help mitigate the effects of heat stress in your organization. Click to view Environmental Heat Stress System page.

Call or email for a quote | 503-629-0887 | info@columbiaweather.com

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