To understand the differences between wet bulb globe temperature (WBGT) and heat index we need to first understand each measurement and how it is calculated. I will go over both WBGT and heat index first to familiarize ourselves with them.
Wet Bulb Globe Temperature (WBGT)
Wet bulb globe temperature is a measure of the amount of heat stress that is placed on an individual by their environment. WBGT is a calculated number that takes the wet bulb temperature, dry bulb temperature (air temperature), and a globe temperature into account. Together these three temperatures take into account air temperature, humidity, sun energy, and wind. WBGT has been developed and refined for over half a century. In this time several guidelines have been developed around WBGT that are proven to decrease heat related illness. For a more in depth discussion of WBGT check out our previous blog: Wet Bulb Globe Temperature (WBGT) What It Is and Why We Care.
Heat Index (HI)
Unlike WBGT that takes into account air temperature, relative humidity, energy from the sun, and wind, the heat index only takes into account air temperature and relative humidity. Heat index was developed in 1979 and makes some assumptions that can make it inaccurate. For example the HI assumes the wind speed is 5.8 mph (9.3 km/h). Also the heat index is calculated in the shade. Unlike the WBGT, heat index does not provide any clues to the amount of heat stress added to the body due to energy from the sun. Because HI can be calculated quickly by knowing just the air temperature and relative humidity it has become a common number, but because of assumptions made in its formula and the lack of information about stress from the sun it can not be relied upon to be a good indicator of heat stress on a human body. Below is a nice summary produced by the national weather service of the different parameters involved in the heat index and the WBGT.
Differences Between Wet Bulb Globe Temperature and Heat Index
When you compare the WBGT and heat index values you will notice that the heat index is higher than the WBGT. In fact the heat index is almost always higher than the air temperature because of how it is calculated. On the other hand WBGT is usually lower than the air temperature. In fact the difference between the two numbers can easily be 20 or more degrees different. You can not correlate the heat index value to the wet bulb globe temperature.
Which Value Should You Use
In almost all use cases the wet bulb globe temperature is far superior than the heat index. The reason WBGT is used by the military, athletic associations, and OSHA is because it more accurately represents the heat stress on humans. The reason heat index is currently the more well know value is because before 2021 it was expensive and cumbersome to obtain an accurate WBGT value. The Zelus WBGT app has harnessed new technology to make the WBGT value free and accessible to everyone. In the coming years WBGT will be the number everyone is talking about when it comes to heat stress.
Until next time, stay cool! 😎