Wet bulb globe temperature (WBGT) is a calculated temperature that professionals use to determine the amount of heat stress on an individual. It has been proven to be the most accurate predictor of heat stress ever developed. WBGT takes into account the air temperature, humidity, wind speed, and energy from the sun to calculate a person’s heat stress.
So why are we concerned about heat stress? Heat is the number one cause of weather deaths in the United States. On average, there are three deaths a year from exertional heat stroke in football alone. If preventing death is not enough reason to care about WBGT, the legal implications of ignoring heat illness in athletes is severe. For example, two coaches in Georgia were charged with murder after one of their athletes died of heat stroke during practice. If you are still not convinced that heat illness needs to be taken seriously, especially by those in powerful leadership positions, then maybe money can convince you. If a coach ignores heat stress and an athlete dies under their watch, they will be sued, and lose millions.
Wet bulb globe temperature has been researched and guidelines have been developed over the past 65 years. WBGT was created by the United States Navy Bureau of Medicine and Surgery between 1954 and 1955 in response to several Marines dying from heat illness in the early 1950s. Using the newly developed heat stress monitor and recommendations on how to combat heat stress, heat-related illnesses at Parris Island Marine Base dropped significantly. By 1982, WBGT had become so common that ISO adopted it as a standard. During the 2012 school year, the Georgia High School Association adopted a heat policy that has been widely regarded as the standard for high school athletics. Georgia went from leading the country in heat-related deaths of interscholastic football players to not having any heat related football deaths for six years.
How to Calculate Wet Bulb Globe Temperature
Wet bulb globe temperature is calculated by taking three measured temperatures and weighting them by their importance in their contribution to heat stress. Here is the calculation for WBGT outdoors:
WBGT = 0.7(Tw) + 0.2(Tg) + 0.1(Td)
WBGT = wet bulb globe temperature
Tw = wet bulb temperature
Tg = black globe temperature
Td = dry bulb temperature
Wet Bulb Temperature
Seventy percent of WBGT is the natural wet bulb temperature. The wet bulb temperature is the lowest temperature that you can obtain from evaporative cooling. In other words, the natural wet bulb temperature tells us how well sweating will help cool us down. All else being equal, on less humid days, sweating cools us down more than on high humidity days. To obtain the wet bulb temperature, a thermometer is placed in the shade with a cotton wick on the bulb. The wick is long enough to drop into a glass of water. The water goes up the wick, and as wind blows across the wet bulb, the temperature drops. After allowing the thermometer to come to equilibrium you can read the thermometer to get the natural wet bulb temperature. The wet bulb temperature will always be less than the dry bulb temperature, unless the air is at 100% relative humidity. If the relative humidity is 100%, the wet bulb and the dry bulb temperatures will be equal. The wet bulb temperature will never be higher than the dry bulb temperature.
Black Globe Temperature
Twenty percent of WBGT is the black globe temperature. Black globe temperature, also called globe temperature, is a measure of the amount of heat stress caused by the sun. To determine this temperature a six inch diameter copper sphere is painted matte black. A thermometer is inserted into the sphere with the bulb in the middle. The whole apparatus is then placed in an unshaded area. After allowing the thermometer to come to equilibrium, the thermometer will display the globe temperature.
Dry Bulb Temperature
The final 10% of WBGT is the dry bulb temperature. The dry bulb temperature is the ambient air temperature, or what we normally refer to as the temperature outside. Even though this is a very easy number to obtain, you have to do it correctly to prevent errors. To get an accurate reading the thermometer has to be shielded from the sun and any moisture.
How to Measure the Wet Bulb Globe Temperature
Measuring a true WBGT is difficult and expensive. You need all three of the previous thermometers set up precisely, and you have to read them almost simultaneously. To get the most accurate gauge of heat stress on humans, the readings of these three temperatures should be made about four feet (1.2 meters) off the ground. The idea behind this is that this is the height where the main mass of humans is located. Also, the thermometers need to be in place for a minimum of 15 minutes before they are read so the thermometers can equilibrate to their surroundings. This whole process is time consuming and labor intensive. Here is a diagram of a device used to measure true wet bulb globe temperature:
But have no fear! In the past decade or two, there have been great advances in our knowledge of WBGT and in technology. This has allowed us to accurately produce a reading that is very close to, if not identical, to true wet bulb globe temperature very quickly. The different methods and devices that are currently available to obtain WBGT readings will be in a blog post coming soon.
TL/DR: WBGT has been around a long time and has been proven to save lives.
Until next time, stay cool! 😎