1. Why Japan's Heat Warnings Used Only Air Temperature Until 2020
Until the summer of 2020, the Japan Meteorological Agency issued "High Temperature Caution Information." This was issued when the maximum temperature for the next day or the current day was expected to be roughly 35°C or higher (or 33°C or higher in Aomori Prefecture). They looked only at the maximum air temperature [1]. In 2021, this was replaced by the Heat Stroke Alert. The new standard is a WBGT of 33 [1]. The Ministry of the Environment and the Japan Meteorological Agency began using this nationwide on April 28, 2021. It had been tested earlier in the Kanto-Koshin region the previous summer [2].
The WBGT is shown in "°C," just like air temperature. However, it is not actually air temperature [3]. Let us see what goes into this new number.
2. How the WBGT Mixes Three Thermometers in the 7-2-1 Rule
The WBGT is made by mixing the values from three different thermometers. The first is the dry-bulb thermometer, which is an ordinary thermometer showing the usual "air temperature." The second is the wet-bulb thermometer. This has gauze soaked in water wrapped around its bulb. It shows a lower temperature when humidity is low because water evaporation takes away heat [1]. The third is the black-globe thermometer. This measures the temperature at the center of a thin copper ball painted black. It matches how hot it feels in the sun on days with little wind [1].
Outdoors, when there is sunlight, the WBGT equals 0.7 times the wet-bulb temperature plus 0.2 times the black-globe temperature plus 0.1 times the dry-bulb temperature. Indoors, without sunlight, it is 0.7 times the wet-bulb plus 0.3 times the black-globe [4]. The wet-bulb has the heaviest weight at 70%. The air temperature (dry-bulb) is only 10% outdoors and 0% indoors. This shows that the WBGT is not just a corrected air temperature; it is a different measure where humidity and sunlight are the main actors.
3. How the WBGT Was Developed at a US Marine Training Camp
The WBGT was developed at the Marine Corps Recruit Depot on Parris Island in South Carolina, USA. It was created to judge heat stroke risks in advance [4]. This training camp has high humidity and strict rules for uniforms and equipment. Because of this, it is explained that heat strokes were likely to happen there [4].
According to the Ministry of the Environment, the proposal was made in 1954 [3]. The guidelines from the Japanese Society for Biometeorology cite a paper by Yaglou and Minard from 1957 [4]. Since the years look slightly different, this article lists them as "proposed in 1954" and "paper in 1957," exactly as the sources state. It is important to note that this was not about war, but about preventing heat stroke during training. Under these conditions, a method to judge danger in advance by turning humidity and sunlight into numbers was sought.
4. How the WBGT Can Differ by 5 Even When Air Temperature Is 30°C
Let us compare two days where the air temperature is both 30°C. These numbers are examples for calculation practice, not actual observations. The black-globe temperature is set to 40°C for both. On a muggy day, the wet-bulb is 28°C. The calculation is 0.7 × 28 + 0.2 × 40 + 0.1 × 30 = 30.6. On a dry day, the wet-bulb is 21°C. The calculation is 0.7 × 21 + 0.2 × 40 + 0.1 × 30 = 25.7. Even though the air temperature is the same, the WBGT differs by about 5 [5].
There is a reason humidity matters so much. Even if relative humidity is 60%, air at 30°C holds about 18g of water vapor per cubic meter. Air at 10°C holds less than one-third of that amount [6]. So, the same "humidity %" means different amounts of water depending on the temperature. The National Institute for Environmental Studies notes a study from the University of Tokyo. It says that in Japan, where the link between air temperature and humidity is weak, the WBGT (which includes humidity) is more strongly linked to death risk than air temperature alone [6]. Another number to remember is 28. Data from major cities in 2005 showed that when the daily maximum WBGT exceeded 28, the rate of heat stroke patients suddenly increased [1].
5. The Four WBGT Risk Levels and Two Types of Heat Alerts
The Japanese Society for Biometeorology divides the WBGT into four levels in their "Guidelines for Prevention of Heat Stroke in Daily Life." The levels are: Danger (31 or higher), Severe Caution (28 to less than 31), Caution (25 to less than 28), and Attention (less than 25) [5]. At the "Danger" level, elderly people are at high risk even when resting. The guidelines advise avoiding going out and staying in cool indoor spaces [5].
The standard for the Heat Stroke Alert is 33. This is higher than the "Danger" level of 31 in the daily guidelines. An alert is issued when the predicted maximum WBGT reaches 33 at any one information point within a prefecture forecast area or similar zone [7]. Alerts are announced around 5:00 PM the day before and around 5:00 AM on the day [1].
On April 24, 2024, the Heat Stroke Special Alert began operation. This is a legal notification under the Climate Change Adaptation Act. It is issued when the predicted maximum WBGT reaches 35 at all information points in a prefecture (excluding some points defined by law) [8]. A key difference is that the regular alert needs "any one point" to reach 33, while the special alert requires "all points" to reach 35. When a Special Alert is issued, cooling shelters designated by mayors are opened [8]. Both 33 and 35 are predicted values, not measurements of what actually happened.
6. Why Most Published WBGT Values Are Estimates, Not Measurements
The number of locations changes each year. Some are measured, but many are estimated [9]. Only a few locations measure black-globe temperatures directly. Most locations use a formula to calculate it from air temperature, relative humidity, solar radiation, and wind speed [9]. Predicted values also come from the Japan Meteorological Agency’s numerical forecasts using the same formula [9].
Commercial WBGT meters fall into three types according to the Japanese Society for Biometeorology guidelines: those using natural wet bulbs (expensive and rare), electronic ones using humidity sensors instead of wet bulbs (cheap and common), and simple ones without black globes (not suitable for outdoor sun) [5]. Published values represent a location. So, the heat in a sunny schoolyard may feel different from the shade under a tree. However, all these values include information about humidity and sunlight, unlike simple air temperature.
7. How to Check the WBGT and Air Temperature Yourself
If you want to try this on a day when the WBGT is published, here are some steps. Make a table for several days. List the maximum air temperature from weather forecasts and the daily maximum WBGT for your local point from the Ministry of the Environment’s "Heat Stroke Prevention Information Site." Do you see days where the air temperature is almost the same, but the WBGT is different? Compare the WBGT numbers with the levels 25, 28, 31, and 33. Count how many days fall into each level.
For reference, the Japanese Society for Biometeorology’s "Guidelines for Prevention of Heat Stroke in Daily Life" is available in Japanese. It has careful charts and tables, including guidelines on what to do at each WBGT level [5]. On hot days, you do not need to go outside to observe. Looking at the official website’s numbers from inside your home is enough.