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WEATHER LAB | Is Wet Bulb Globe Temperature a better measure of heat stress?

Heat index is the most commonly used parameter for measuring the dangers of summer heat. But is it the most accurate?

DES MOINES, Iowa — When it comes to dangerous summer heat, there are a number of factors to consider beyond the temperature.

Meteorologists typically show the heat index to show what the combination of heat and relative humidity feels like to the body.

But did you know the heat index is only a measure of what it feels like in the shade when you only factor in the air temperature and humidity?

That begs the question: How useful is the heat index for athletes and workers exerting energy in the hot sun?

Well, there is another parameter that takes into account numerous weather variables called the Wet Bulb Globe Temperature (WBGT).

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Unlike heat index, the WBGT takes into account the temperature measured in direct sunlight.

WBGT also factors in humidity, wind speed, sun angle and cloud cover.

If the temperature is 90° and the dew point is 65°, the heat index will be 92° no matter the sky cover, wind speed or sun angle.

With the same temperature and dew point plus 5% cloud cover and a 3 mph wind, the WBGT would be 89. The WBGT would fall to 81 with 65% cloud cover and a 13 mph wind. 

As you can see below, the difference in 81 and 89 on the WBGT scale is significant in regards to potential impacts.

Credit: WOI

Heat index is typically a higher number than the WBGT simply because the two use completely different equations and scales.

OSHA and many countries use WBGT as a guide to manage workload in the sun.

Athletic trainers also commonly use this parameter to limit the threat of heat-related illnesses to athletes.

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