The scariest metric of the climate change era: Wet Bulb Globe Temperature
Global warming means that each summer is hotter than the last and each year deadly heat spreads further north. Yet, heat in itself isn’t…
Global warming means that each summer is hotter than the last and each year deadly heat spreads further north. Yet, heat in itself isn’t what kills. It is rather when the weather interferes with the ability of the human body to get rid of heat. This is called the wet bulb temperature.
Put simply, the wet bulb temperature is the temperature down to which something can be cooled by evaporative means, i.e., by water evaporation. You measure it by wrapping a wet cloth around the mercury bulb of an ordinary thermometer. As long as the cloth stays wet, it will carry heat away from the thermometer, similar to how sweat carries heat away from the human body.
Although digital solutions are preferred nowadays, a simple device called a sling psychrometer can tell the wetbulb temperature:
While the absolute temperature is a factor in wet bulb temperature, so is humidity. The more humid it is the less water the air can absorb. Once you reach 100% humidity, the air cannot absorb anymore and evaporative cooling ceases. At that point, other forms of cooling such as convection currents and radiation continue as long as your body is warmer than its surroundings. These are, unfortunately, far less efficient.
If the body cannot get rid of heat efficiently, the core temperature rises and you get heat stroke which can lead to death.
Wet bulb temperature is always lower than the actual temperature, unless it is 100% humid, in which case it is the same. And it can be deceptive how safe the heat actually is.
The National Weather Service is in the process of rolling out a metric called Wet Bulb Globe temperature, still in the experimental stages, which is like wet bulb temperature but specifically designed for people who are out working or exercising in the sun. They define it here:
The WetBulb Globe Temperature (WBGT) is a measure of the heat stress in direct sunlight, which takes into account: temperature, humidity, wind speed, sun angle and cloud cover (solar radiation). This differs from the heat index, which takes into consideration temperature and humidity and is calculated for shady areas.
A critical difference from apparent temperature is the influence of more factors and the potential for being in direct sunlight, which is the case for many outdoor workers like roofers but also marathon runners and other athletes.
Let’s look at the maximum temperature for today (25 July 2022) in the United States:
It looks pretty hot in Texas and Oklahoma as well as parts of California and the Southwest desert areas, no surprises there. Temperatures along the East Coast look warm but not super hot. The apparent temperature which you can call “how hot it feels in the shade” isn’t too different except that drier areas like the southwest look cooler now:
But, if you look at the WBGT for the nation, it tells a different story:
Now the Southwest looks pretty good and even West Texas is okay. East Texas, Oklahoma, Arkansas, Louisiana, Mississippi, and most of the East Coast, however, are in big trouble today.
Why?
The reason isn’t because it is unusually hot in those areas. That wouldn’t necessarily make a difference. The problem is that it is unusually hot, humid, and sunny all at once.
As the NWS mentions, wet bulb globe temperatures above about 80 degrees are dangerous and require special preparation. Those are the areas on the map that are not green. All those areas in red and gray are particularly dangerous and can kill within hours.
Indeed, a wet bulb temperature above 95 degrees is not survivable for long without non-evaporative cooling like an air conditioner.
Today it is far more dangerous to be outside in Rhode Island than Las Vegas because of the combination of factors, and knowing that can be a life saver, even if you aren’t a roofer or landscaper. If you are going hiking, biking, or working in the garden for a long period of time, you may be putting yourself in excessive danger, as if you were out in a severe thunderstorm, even if the thermometer looks reasonable.
As the climate becomes hotter, high wet bulb temperatures are going to become a more common occurrence and deaths that go along with it are going to rise as well.
During the 2022 heat wave in Europe, parts of Northern Spain reached wet bulb temperatures of 30 degrees Celcius or 86 Fahrenheit. While not that warm for an absolute temperature, this is dangerous territory for the wet bulb metric.
Wet bulb temperature is not only a stay-inside type of metric; it is a metric for how survivable an area is. If about 95 degrees and above wet bulb temperature is not survivable, then areas that regularly see those temperatures would essentially have to go into lockdown and open cooling centers to avoid people dying just from being outside for a few hours.
This is the new future for many places on the planet, particularly the Middle East, Pakistan, India, and Southern Europe where exceptionally high wet bulb temperatures have been observed.
Consider this from a Science article written in 2020:
At wet bulb temperatures above 35°C, researchers estimate that even fit people will overheat and potentially die within 6 hours. Although that temperature might seem low, it equates to almost 45°C at 50% humidity, and what it would feel like 71°C using the U.S. National Weather Service heat index. In the heat wave that ravaged Europe, wet bulb temperatures hit 28°C.
Even higher wet bulb temperatures hit parts of Europe in the 2022 heat wave, suggesting that warming is becoming deadlier by the year. It is only a matter of time before we are all checking the wet bulb globe temperature before deciding to undertake any activity that might take us outside for a few hours. As climate change continues, we might build parts of cities underground, as in the Australian city Coober Pedy. Even if emissions are brought under control, warming will continue for decades, and preparedness means understanding deadly heat much better.