One of the most common terms we hear during the summer when discussing the weather is Heat Index. While the Heat Index can give you an idea of how dangerous the heat can be, there is another lesser known variable that may give a complete description, known as the Wet Bulb Globe Temperature.

The Heat Index is a measure of how hot it feels to the human body, taking into account the temperature and the relative humidity. The higher either one value is, the higher the heat index and the greater the stress exerted on the body. Heat Index utilizes the temperature measured in the shade and therefore will likely be greater in the sun, where many activities take place. For temperatures taken in the full sunlight, Heat Indices can increase as much as 15°F. Humidity plays a major effect on cooling the body. If the humidity is high, sweat on the surface of the skin does not evaporate as quickly, slowing the cooling of the body. If the humidity is low, sweat evaporates too fast, leading to dehydration. Below is a Heat Index chart.

Wet Bulb Globe Temperature (WBGT) is similar to the Heat Index in that it is an indicator of the stress on the body from the heat. While it is not as well-known as the Heat Index, it’s not a new term. WBGT was developed in the 1950s after heat related illnesses affected the US armed services during the 1940s. After its implementation, there was a reduction in heat related illnesses during basic training. One fundamental difference between the Heat Index and WBGT is that the latter is calculated with the temperature measured in the sun. While Heat Index only takes temperature and Relative Humidity into account, WBGT takes several variables into account. These variables include:

• Temperature
• Humidity
• Wind Speed
• Sun Angle
• Cloud Cover
• Physical Activity

WBGT is often used in marathons. One study on the effects of on marathons resulted in two significant findings:

• Higher WBGTs have more effect on performance
• WBGT has more of an impact on slower runners

Based on the results of the study, a formula was devised to estimate the effect on performance. A few conclusions were made based on this formula:

• For each 5°C increase in WBGT, the performance in the top three runners decreased by 0.9%
• Those in 25th place experienced a 1.1% decrease in performance for every 5°C
• Runners who finished 50th, 100th, and 300th, experienced a decrease of 1.5%, 1.8%, and 3.2% respectively in performance for each 5°C increase in WBGT

Below is a chart that can be used to estimate the WBGT. The values in this table may differ from the actual value depending on other parameters listed in the above list.

In this chart, you can see the guidelines for athletes and those working outside depending on the WBGT.

While the WBGT may be a more accurate representation of the heat stress on the body, there are also a few caveats to keep in mind. When using it to plan outdoor activities, some factors that should be considered include the duration of activity compared to the length of breaks, the number and length of breaks, and at what WBGT outdoor activity will be canceled. For any outside activities, work, or sports, WBGT will be more valuable than using Heat Index as the recommendations are more specific to being active in full sun.