Wednesday, July 20, 2022

The Wet Bulb Globe Temperature - A Measure of Potential Heat Stress

Several years ago I was watching the U.S. vs. Portugal in a World Cup match played in Brazil and there was a reference to something called the Wet Bulb Globe Temperature (WBGT). It was the first time I had heard of this (or paid attention when it was mentioned). That led to a search for more information and a blog post. With extreme heat being in the news these days and hearing some mentions of WBGT I thought it would be a good idea to revisit the topic with some updated information.

Most of us are familiar with the Heat Index used here in the U.S. to describe the combined effects of temperature and humidity as an "equivalent temperature". The Wet Bulb Globe Temperature (WBGT) is a more elaborate and complex method of measuring heat stress. It measures heat stress in direct sunlight which takes into account temperature, humidity, wind speed, sun angle and solar radiation.

There are three temperature measurements that are part of the WBGT calculation. The wet-bulb temperature is measured using an exposed thermometer with its bulb covered by a cotton wick wetted by distilled water. This measures evaporative cooling which in turn is affected by wind, humidity, and solar radiation.

The second measurement is the black globe temperature. This consists of a thermometer centered in a 6-inch black globe. This measurements represents the integrated effects of solar radiation and wind.

The third measurement is the standard air temperature measured by a shielded thermometer (in a radiation screen). This represents the temperature "in the shade" and is the standard air temperature most of us are familiar with.

These three measurements are used to calculate the WBGT as follows:

Tw = wet-bulb temperature
Tg = black globe temperature
Ta = air temperature

WBGT = (0.7 × Tw) + (0.2 × Tg) + (0.1 × Ta)

Indoors, or when solar radiation is not a factor such as at night, the following formula is used:

WBGT = (0.7 x Tw) + (0.3 x Tg)

Since the formula just weights the contribution of the respective temperatures, temperatures can be in either Celsius or Fahrenheit.

The WBGT was developed in the late 1950s by the U.S Department of the Navy in response to heat stroke cases at the U.S. Marine Corps Recruit Depot at Paris Island, South Carolina. It was recommended as an international standard to measure workplace heat stress in 1989.

One reason we haven't heard much about the WBGT until recently is that it is not easily measured. There are ways to calculate it using wind, solar radiation, temperature and humidity measurements, but wind and especially real-time solar radiation measurements aren't readily available. The Heat Index used here in the U.S. uses temperature and humidity, two easily and regularly measured parameters. However, it represents conditions "in the shade" and does not account for wind and sunshine, both of which can make a significant difference on the heat stress on the body. The availability of digital gridded data sets makes calculation of WGBT for individual locations or large areas easier to do.

WBGT values are comparatively lower than corresponding heat index values. Here is a table of comparisons between the WBGT and the Heat Index. Note how when the wind is stronger the WBGT is a little lower, accounting for the cooling effect of the wind. Solar radiation is approximated by percent of sky cover.

Comparison of WBGT and Heat Index for various weather conditions.
Credit: NWS Tulsa

When the WBGT is 80 to 85°F, working or exercising in direct sunlight will stress the human body after 45 minutes.When the WBGT is above 90°F, heat stress will occur after only 15 minutes.
There are commercially available instruments available to measure WBGT.  They run about $200 and up, a relatively small expense for the workplace or for a large sporting event. The Occupational Safety and Health Administration (OSHA) has guidelines for workplace heat stress using WBGT.

Devices for measuring WBGT


The Southeast Regional Climate Center (SERCC) recently added a new tool to their suite of climate products that provides a forecast of the WBGT for a specific location out five days, including a range of values for sunny vs. shady conditions. The forecast is updated twice daily and is available for the eastern two-thirds of the U.S. Click on the image below to access the WGBT tool.

The National Weather Service produces a map of WBGT for the country.

Heat is one of the leading weather-related causes of death in the country, so be sure you know your vulnerability and how to avoid heat-related illness.

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