What is Wet Bulb Globe Temperature?

WBGT is a "simple index of the environment that is considered along with metabolic rate to assess the potential for heat stress among those exposed to hot conditions...[which] combines the measurement of two derived parameters: natural wet-bulb temperature (t_nw) and black globe temperature (t_g). Where the sensors are influenced by direct incident radiation from the sun (solar load), either outdoors or indoors, the weighting of the globe temperature is reduced by including air temperature (t_a)."

In plain English, this means that WBGT is a weighted measure meant to capture the effects of air temperature, sunlight, humidity, and wind on people working/exercising in hot conditions.

Source: ISO 7243:2017, "Ergonomics of the thermal environment — Assessment of heat stress using the WBGT (wet bulb globe temperature) index"

How do I interpret the colors and values?

They vary based on which guidelines you're following and which activity you're engaging in. There are guidelines for athletics, military training, and workplaces.

The colors used in the current edition of the map are based on the Japanese scale, with the addition of two extra categories at the low and high ends.

What makes WBGT better than the Heat Index/just looking at air temperature?

WBGT takes into account the way your body exchanges heat with the surrounding environment. Instead of only considering heat and humidity, it accounts for the effects of sunlight and wind (or lack thereof) as well (and is used as an ISO Standard because of this). How many times have you sought out refuge from the heat of the sun in the shade? Or felt the relief from a cool breeze? WBGT accounts for those effects.

How is WBGT calculated?

The simple answer is:

T_nwb(0.7) + T_g(0.2) + T_a(0.1),

where Tnwb = natural wet-bulb temperature, Tg = black globe temperature, and Ta = air temperature. Of course, getting the first two values can require more intensive calculation. Note that this equation is only valid for outdoor scenarios.

Why estimate WBGT instead of just measuring it?

WBGT can be measured, and many workplaces and militaries operating in hot environments use this specialized equipment. However, the meters are costly and often require special instructions and upkeep to ensure accurate readings. Over the past two decades, many researchers involved in meteorology and public health have come up with surprisingly accurate formulae based on atmospheric physics that can be used to estimate WBGT and other heat stress indices. As temperature extremes and heat waves have become more severe due to climate change, this information will become of increasing value not only to workers and athletes, but also to the public in general.

Why are you using Celsius? This is America and I'm used to Fahrenheit!?

After getting used to inflated temperature values from apparent temperatures used by television meteorologists (like the Heat Index), the values that WBGT produces in Fahrenheit are downright unimpressive. For example, a WBGT value of 92°F means that it can be hazardous for even acclimatized, healthy, young people to exert themselves in the heat. That's a pretty standard summer high temperature here in North Carolina. On the interactive map, Fahrenheit values are listed next to the Celsius values. The data section only contains temperatures in Celsius for the time being.

How is WBGT different than the Wet Bulb Temperature?

Weather forecasters sometimes reference the wet bulb temperature when dealing with potential frost conditions and a few other scenarios. You may have come across an article referencing wet bulb temperatures in relation to extreme humid heat events (and potential issues of survivability in these conditions, which are already occurring), because wet bulb temperature is a good way to quantify humid heat extremes. The difference between the two is that wet bulb temperature expresses the temperature achievable by evaporation alone by considering air temperature and atmospheric moisture, while WBGT adds a wind (air movement) and solar radiation component. A slightly different form of wet bulb temperature makes up 70% of the WBGT value.

Why use the "natural" wet-bulb instead of the psychrometric?

The difference between the two can be simplified as such: natural is a person sitting on the beach wearing a wet t-shirt and psychrometric (sometimes called "aspirated" or "forced" for this reason) is a person riding a bicycle while wearing a wet t-shirt. Psychrometric wet-bulb temperature represents the lowest temperature achievable through evaporation alone. If the wind speed is high enough (about 10 knots), the difference between the two largely evaporates, but wind speeds on hot days are usually quite a bit lower than that, so using psychrometric would fail to model the natural evaporation processes that WBGT works to approximate.

What is the globe temperature?

Black globe temperature primarily responds to changes in sunlight and, to a lesser degree, wind. The "globe" is essentially a 150mm toilet float ball painted matte black with a thermometer inside (see the ISO standard), but meters using smaller ping-pong sized balls are often also used with appropriate corrective measures. The globe is black to ensure effective absorption of the sun's shortwave radiation. Black globe temperature has a smaller weight in the WBGT equation, but due to its wide variability compared to the natural wet-bulb temperature, it can have a large effect on the final value.

Why don't the heat advisories and warnings line up with WBGT sometimes?

Depending on the atmospheric conditions, it's possible for thermally hazardous conditions to exist without an accompanying high WBGT and vice-versa (see G.M. Budd's Wet-bulb globe temperature (WBGT)—its history and its limitations if you'd like to learn more). The method(s) for determining advisories and warnings are much more sensitive to air movement than the WBGT methods are. It is possible, for example, to have a heat warning and a moderate WBGT if conditions are right.

How are heat advisories and warnings derived?

See the user guide for more information.