The temps we see in weather forecasts are air temperatures, what meteorologists call dry-bulb temperatures. But they also speak of wet-bulb temperatures. That’s a way of assessing heat stress in direct sunlight. And they do indeed take wet-bulb temperatures with a water-soaked cloth wrapped around a bulb. Wet-bulb temperatures take into consideration air temperature, humidity, wind speed, sun angle and cloud cover. Many believe knowing wet-bulb temperatures will become increasingly important as the world continues to warm.
Many people find themselves in circumstances where they have to be out in the full sunshine on hot days, whether it’s because of their occupation in construction or agriculture, or for some other reason. A wet-bulb temperature can help determine how dangerous the situation that day might be.
Higher wet-bulb temperatures have become more common over the last few decades, with some communities sometimes, temporarily, reaching intolerable levels.
How does wet-bulb temperature work?
Wet-bulb temperatures are cooler than dry-bulb temperatures. The evaporating water drops the temperature a bit, just as how your own body temperature drops when you sweat to cool down. In lower humidity – or air that holds less water – water evaporates faster, dropping the temperature more quickly.
When the humidity is 100% (imagine a steamy summer day), the air is saturated. Evaporation can’t happen in 100% humidity. Only in 100% humidity will the dry-bulb and wet-bulb temperatures read the same.
Staying cool in a high wet-bulb temperature
If you’re stuck outside in 100% humidity, you may sweat, but it won’t cool you. That’s because your sweat needs to evaporate off your skin for cooling to occur. It’s the same as when you step out of a shower and your skin is still wet. It starts to evaporate, which cools you and makes you chilly. If you can’t sweat to cool yourself, you eventually get heat stroke. Then, if you cannot get in air conditioning or the temperature doesn’t lower and you don’t have a way to cool yourself, your brain and internal organs become damaged and death can occur.
Humans can survive in fairly high temperatures as long as the air is dry. That’s why the wet-bulb temperature on hot days can tell you more than a dry-bulb temperature.
What is an intolerable wet-bulb temperature?
A January 2022 study in the Journal of Applied Physiology from scientists at Penn State showed that a fatal wet-bulb temperature is even lower than once believed. The March 2022 Penn State press release said:
It has been widely believed that a 35 degrees C wet-bulb temperature (equal to 95 degrees F at 100% humidity or 115 F at 50% humidity) was the maximum a human could endure before they could no longer adequately regulate their body temperature, which would potentially cause heat stroke or death over a prolonged exposure.
The press release goes on to say:
But in their new study, the researchers found that the actual maximum wet-bulb temperature is lower – about 31 degrees C wet-bulb or 87 degrees F at 100% humidity – even for young, healthy subjects. The temperature for older populations, who are more vulnerable to heat, is likely even lower.
Meteorologists measure the wet-bulb temperature in degrees Celsius (also called centigrade). Few locations in the world still use Fahrenheit for temperatures, but the United States is one of them. So in the U.S., meteorologists translate these temperatures into heat index or “real-feel” Fahrenheit readings. For comparison, a wet-bulb reading of 31 C is equivalent to 125 F on the “real feel” heat index. Or a wet-bulb temperature of 32 C or 90 F is equivalent to a heat index of 55 C (130 F). Technically, the heat index ends at 127 F, so this is unprecedented territory. The temperatures are literally off the charts.
Areas with the most dangerous wet-bulb temperatures
A May 2020 study in Science Advances announced by the Columbia Climate School found that in the past four decades, some locations have already surpassed fatal wet-bulb temperatures at least a dozen times. The press release said:
The study identifies thousands of previously rare or unprecedented bouts of extreme heat and humidity in Asia, Africa, Australia, South America and North America, including in the U.S. Gulf Coast region. Along the Persian Gulf, researchers spotted more than a dozen recent brief outbreaks surpassing the theoretical human survivability limit. The outbreaks have so far been confined to localized areas and lasted just hours, but they are increasing in frequency and intensity.
Heat is already the number one weather-related killer in the United States. The upward-trending temperatures will only make the situation worse.
The interactive world map below shows the highest wet-bulb temperatures according to location. Image via Jeremy Hinsdale; adapted from Raymond et al., Science Advances, 2020. Used with permission.
A warming world
It’s well-known that the world’s average temperatures are climbing. We are also experiencing more extreme weather events in a warming world. According to the Columbia Climate School:
The study found that worldwide, wet-bulb readings approaching or exceeding 30 C on the wet bulb have doubled since 1979. The number of readings of 31 – previously believed to occur only rarely – totaled around 1,000. Readings of 33 – previously thought to be almost nonexistent – totaled around 80.
Scientists have noted that we seem to be much closer to a tipping point than previously believed. They also said some communities are closer to sustained intolerable heat than they expected. Steven Sherwood, a climatologist at the Australia’s University of New South Wales, said that scientists:
… previously believed we had a much larger margin of safety.
Impacts of a high wet-bulb temperature
How will this affect communities moving forward and what can you do to protect yourself? When the wet-bulb temperature is high in your location and you must be outdoors, the National Weather Service recommends that people wear a hat and lightweight, loose-fitting, light-colored clothes. It also says that people should plan to take frequent breaks in shady areas. In addition, drink plenty of fluids to stay hydrated. Staying out of the direct sun is also important, because sunburn makes it harder for your body to dissipate heat.
When the wet-bulb temperature gets dangerously high in a location for long periods of time, people will become more reliant on being indoors in air conditioning. This will, of course, increase the demand and strain on the electrical grid. And if people are confined to the indoors, agriculture and businesses will take a hit with people being unable to work.
Bottom line: The wet-bulb temperature is a measure of heat that considers multiple factors. It’s a good indicator of temperatures that are too dangerous to work in.
Read more: California’s heatwave ranking law, and the global heat-response struggle
Read more: Heatwaves and fires in Europe, Africa, and Asia
The post What is wet-bulb temperature? And what’s it mean in a warming world? first appeared on EarthSky.
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