What is Wet Bulb Globe Temperature?

Most people are familiar with the Heat Index or the “feels like” temperature but there is another measurement that is probably a more suitable guide for the outdoorsy type. 

Heat index is calculated using the relative humidity and the actual air temperature in the shade. Therefore, a downside to that is that the heat index value is representing what it feels like in the shade as opposed to being directly in sunlight. The Wet Bulb Globe Temperature, or WBGT for short, gathers data from the ambient air temperature in the sun, relative humidity, wind speed, solar radiation and geographic location. 

There are a few instruments that are used to calculate the final WBGT value. A dry bulb thermometer measures the ambient temperature, the black globe measures the amount of sunlight, and the wet bulb thermometer will get the temperature in the air after evaporation takes place. All of these values are added together, after being multiplied by their respective weighted value, to get the final WBGT value. 

The WBGT scale can vary by geographic location, but you can get a good idea how it works from the National Weather Service’s scale below.

80-85 F - Working or exercising in direct sunlight will stress your body after 45 minutes. Take at least 15 minutes of breaks each hour if working or exercising in direct sunlight.

85-88 F - Working or exercising in direct sunlight will stress your body after 30 minutes. Take at least 30 minutes of breaks each hour if working or exercising in direct sunlight.

88-90 F - Working or exercising in direct sunlight will stress your body after 20 minutes. Take at least 40 minutes of breaks each hour if working or exercising in direct sunlight.

>90 F - Working or exercising in direct sunlight will stress your body after 15 minutes. Take at least 45 minutes of breaks each hour if working or exercising in direct sunlight.

WBGT can be most important for people that need to spend plenty of time outdoors and can help with activity modifications which is why athletic programs and the military have been using it for years. Factors to consider with a high WBGT would be to limit the length of time in the sun, adjust work/rest schedules, and increase water supply.

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