Heat Index vs. Wet-Bulb Globe Temperature

There are two measurements we use to reinforce how dangerously hot the weather is: heat index and wet-bulb globe temperature (WBGT). But what's the difference between the two?

The heat index is a measure of how hot it feels outside to the human body. To do so, the heat index uses two pieces of information: the temperature and the relative humidity. As either one of these individual variables rise, so does the heat index. The heat index utilizes temperatures measured in the shade, not temperatures in full sunlight. So, if standing in full sunlight, the heat index for that particular sunny spot would be even higher than the official heat index temperature. In some cases, the heat index can be increased by up to 15 degrees if you are exposed to direct sunlight.

Humidity plays a major role in cooling the body. If the humidity is high, sweat doesn’t evaporate from your skin as quickly, slowing the cooling of your body. If the humidity is low, sweat can evaporate too fast, dehydrating your body further.

WBGT is similar to the heat index in that both measurements indicate the amount of heat stress on your body. Unlike heat index, however, WBGT is calculated under direct sunlight. The WBGT system was developed in the 1950s, in direct response to heat-related illnesses that affected the U.S. Armed Services in the 1940s. After the system was implemented, there was a notable reduction in heat related illnesses during basic training.

To calculate the WBGT, you need five variables: temperature, humidity, wind speed, sun angle and cloud cover. A more holistic approach, WBGT provides more specific guidance for working or exercising under direct sunlight on a hot day.

A WBGT that is less than 80 degrees offers no restrictions for working or exercising in the sun. A WBGT between 80 and 85 degrees indicates that working in the sun will stress your body after 45 minutes of continuous work or exercise and recommends taking 15 minutes of breaks each hour. A WBGT between 85 and 88 degrees will stress your body in 30 minutes, and would require an equal 30 minutes of rest for every 30 minutes in the sun. A WBGT between 88 and 90 will stress your body in 20 minutes, requiring 40 minutes of rest, while a WBGT of 90 degrees or more will stress your body in 15 minutes of direct sunlight, making it necessary to break for at least 45 minutes each hour.

For any outside activities, the WBGT will be a more valuable measurement than the heat index as it accounts for the effects of direct sunlight and has clearer guidelines for managing an outdoor workload. Use the WBGT this summer to keep yourself safe when exercising or working outdoors.

Sources: NWS, NIH
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