The Heat Index Can Be Misleading. Here’s Something Better.

There's a reason why many sports teams, public services, and even the military prefer Wet Bulb Global Temperature (WBGT) over the Heat Index.


CHARLOTTE, N.C. — As we enter the heart of summer, things are looking quite different compared to this time last year – or any year in recent memory, for that matter. Practices and games have been canceled for Pee Wee and professional sports leagues alike, once-bustling streets and highways are now empty and lifeless, and holiday travel has gone by the wayside. To make matters worse, we’re in the middle of one of the longer heatwaves we’ve seen since the turn of the millennium. How we measure that heat, however, has largely gone unchanged since Sir Thomas Allbut invented what we consider the modern thermometer in 1867. However, as anyone in the Carolinas could tell you, the temperature doesn’t tell you the whole story.

Heatindexchart 650

A simple heat index chart, as used by all National Weather Service branches.

Enter stage heat index. Unlike temperature, the heat index itself isn’t a measurement, but instead a combination of two measurements. Originally defined as “a combined measurement of temperature and humidity, arrived at by adding degrees of temperature to the percentage of relative humidity and dividing by two,” in 1957,  the heat index is also known as a “feels-like” temperature. While the exact definition and formula of the index have both changed significantly over the years (I’ve attached the current formula below), it is still trusted by professional and amateur weather watchers alike as an accurate depiction of what it feels like on a hot, humid, and hazy day.

Heat Index Formula

As you can see, the actual heat index formula is quite complicated. Aren’t calculators great?

Despite this level of trust given to the heat index, even it has its flaws. Obviously, the formula you see above is nearly impossible to calculate by hand, assuming you don’t have some sort of Ivy League mathematics degree. To make matters worse, the heat index only calculates “feels like” temperatures in the shade. While there are a lot of trees and forests here in the Carolinas, you’ll still find yourself out in the sun more often than in the shade when going outside on any given day. Even the National Weather Service (NWS), one of the heat index’s greatest proponents, says that the system is flawed and uses quite a few assumptions. On its own site, the NWS says that the heat index measures for a shady location with light wind, and that exposure to direct sunlight can increase the heat index value by up to 15º.

Another problem with the heat index is that it’s hard to convey relativity. What does 120º actually feel like? As you can see in the NWS heat index chart above, the heat index can actually work in the negative as well. For example, an 84º temperature paired with 40% relative humidity “feels like” 83º. In desert regions, such as Arizona and New Mexico, we often see “feels like” temperatures lower than the actual temperature. Because of this, a place with a 100º air temperature with a 52% relative humidity “feels like” 120º, while a drier region with a 120º temperature and a 10% relative humidity feels like 115º. While this process works well when comparing two separate locations, it doesn’t answer the question of what 120º actually *should* feel like.

So, if the heat index is so flawed, then what could we use instead? Here’s where the Wet Bulb Global Temperature (WBGT) comes in handy. The WBGT is simply a measure of “heat stress” while in direct sunlight. Not only does it measure what the heat index assesses, heat and humidity, but it also takes wind speed, sun angle, and cloud cover into account, as well. For people who work outside or spend the majority of their time in the great outdoors, this is a great supplement, or even alternative, to the heat index. As you can see below, the way to calculate WBGT is much simpler:


No exponents, no parentheses, just simple addition and multiplication. Now that’s something anyone who’s passed 3rd-grade math can calculate! Let’s break down what exactly those variables in the formula mean. WBGT, as we’ve covered before, means Wet Bulb Global Temperature. Tw, as defined above, is the wet-bulb temperature. The wet-bulb temperature is found by wetting a cloth and then drying it by passing air through the cloth. As the cloth dries, the temperature will fall, similarly to how you feel cooler as the water evaporates off of you when you exit a pool. While the wet-bulb temperature is not the same as the dew point, it is half of the equation to calculate relative humidity, which should be somewhat familiar. Next is Tg, which is the globe thermometer temperature. A black globe thermometer is self-explanatory, but I attached a picture of what it is below. It’s just a thermometer with a black ball (globe) on it. The color black is very effective at absorbing sunlight (which is why black cars heat up quicker than other colored cars in the sun) so the more direct sunlight there is, the hotter the thermometer gets. Conversely, this black globe thermometer will have a cooler reading on cloudier days. Lastly, we have Td, which is, simply put, just the air temperature.

Black Globe Thermometer

A black globe thermometer.

While people may not have easy access to a black globe thermometer, we here at WCCB have our very own WeatherSTEM station, which can easily measure black globe temperature and WBGT. You probably see where I’m going with this. As the only TV station in the Charlotte area to have a WeatherSTEM site, WCCB has access to life-saving information on potentially dangerous heat exposure. According to the National Weather Service, extreme heat is the deadliest weather phenomenon in the United States, so having all possible resources to track dangerous temperatures is critical.


Heat safety guidelines using WBGT, as per the Georgia High School Association.

So, you may ask, why is the wet bulb global temperature seldom used over the heat index? The first reason seems pretty obvious to me: name recognition. Not only has the heat index been around for decades longer than the WBGT, but it also has a much easier name to remember. Secondly, while the formula for the heat index is complicated, what goes into that formula is not. If you ask a random person on the street to define ‘temperature’ and ‘humidity,’ they’ll likely give you an acceptable definition. If you ask that same person what a black globe temperature is, they may have some difficulty. Lastly, more eye-popping numbers are produced when using the heat index versus the WBGT, which may make it more likely for the heat index to catch someone’s eye. Looking at the tables above and below, you see that the “dangerous” range for WBGT is typically 90º or above, which may not sound as serious as a heat index of 108º, despite the fact they generally depict the same conditions.

Wbgt Vs Heat Index

A quick comparison between WBGT and heat index. (NWS Tulsa)

Let me be clear: I agree that the WBGT can be confusing, especially since we’ve been using the heat index for a long time. However, the heat index’s inability to calculate what outdoor conditions feel like in direct sunlight could lead to some critical mistakes. As we enter the heat of the summer, sun angle and intensity are both crucial aspects that can’t be discounted in the tropics and the mid-latitudes. Given these shortfalls, there’s a reason why sports leagues, public sector administrations, and even the military use WBGT.  While the heat index may be more commonly used and understood by weather watchers for now, hopefully, we can switch over to WBGT, which is a much more effective tool for measuring treacherous heat.