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.
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| 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.
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| Devices for measuring WBGT |
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