The U.S. National Weather Service's WetBulb Globe Temperature is used to determine safety levels for people working in direct sunlight. It's a way to measure heat stress on the human body and includes data on temperature, humidity, wind speed, Sun angle, and cloud cover.
Though metrics such as this are more holistic than one-dimensional measures such as humidity or the ambient air temperature available with a standard thermometer (also known as dry-bulb temperature), they have their own limitations. Some heat feels different than others and affects the human body in different ways.
In a new study published in the Journal of the Atmospheric Sciences, researchers outlined a new metric to address that shortcoming. And it will sound relatable to anyone who's ever been outside on a hot summer day: stickiness.
Casey Ivanovich, a doctoral student in climate science at Columbia University in New York, was researching climate extremes around the world. The scope of her research included the Persian Gulf, which is extremely hot and generally dry, and Bangladesh, which has higher humidity but lower dry-bulb temperatures. "These are two different types of extremes that might have the same measurement on a wet-bulb temperature thermometer," she said.
A wet-bulb temperature is read from a thermometer covered in a wet cloth. It conveys the temperature a person feels as their sweat evaporates and cools their skin and is affected by humidity. Wet-bulb temperature is one of three measurements that factor into a WetBulb Globe Temperature estimate. (The others are a shaded dry-bulb temperature and solar radiation.)
Stickiness quantifies how heat and humidity contribute to humid heat, seeking to explain why two types of heat that feel so different can have the same wet-bulb temperatures.
"Not all wet-bulb temperatures are created equal."
Nutritional value is a similar concept. Metrics such as volume and carbohydrate levels can explain why two dishes with the same number of calories, such as a small serving of pasta and a much larger serving of zucchini noodles, can feel so different to eat.
Ivanovich compared stickiness to "spice," a term used in oceanography to convey the relative contributions of temperature and salinity to water density. Using the same derivation as a classic paper about spice, Ivanovich and her team created the new stickiness variable by factoring in wet-bulb temperature, humidity, and dry-bulb temperature. Stickiness is designed to be most responsive to changes in humidity and dry-bulb temperature and to be less correlated with changes in wet-bulb temperature.
Jane W. Baldwin, a climate scientist at the University of California, Irvine, who researches the effects of heat on human health, said she was excited to see this study come out because "not all wet-bulb temperatures are created equal."
For instance, a day with a high dry-bulb temperature and low humidity levels could have the same wet-bulb temperature reading as a day with a lower dry-bulb temperature and higher humidity levels. But physiological studies have shown the former presents a greater risk of heat stroke because under such conditions, the human body can't sweat enough to maintain a thermal equilibrium.
"I think stickiness will help us in the long run understand how equivalent levels of wet-bulb temperature might lead to different pathways of adverse health outcomes," Baldwin said.
Humidity's role in human health
Ivanovich said she envisions stickiness being most useful for heat stress preparedness planning, especially as extreme heat becomes more commonplace globally. But, she added, it could also clear up a confusing disparity between physiological and epidemiological findings.
Physiologically, humidity should worsen heat stress for humans by inhibiting sweat evaporation (the main way humans cool themselves), but many population-scale epidemiology studies suggest that the most important predictor of mortality is simply dry-bulb temperature. That is, tracking humidity doesn't help scientists more effectively predict human health effects, including mortality.
"Places that have higher variation in stickiness might be places where we're more able to disentangle the separate impacts of temperature and humidity."
"That's kind of unexpected," Ivanovich said. But these studies may be biased toward outcomes in Western countries, which tend to have more reliable mortality data, she explained. In these regions, it's more common for heat and humidity to be positively correlated, that is, for the same days that exhibit extreme heat to also have high humidity. Some local-level weather stations or governments may choose to track only heat, and it can be difficult for epidemiological researchers to disentangle their effects.
Ivanovich suggested that tracking stickiness could help researchers learn more about how humidity affects people in places that have both extreme dry heat days and extreme humid heat days, such as western Mexico, the Arabian Peninsula, and Australia.
Baldwin suggested the inverse could be useful as well, meaning researchers could use stickiness to determine the most useful places from which to gather data. "Places that have higher variation in stickiness might be places where we're more able to disentangle the separate impacts of temperature and humidity, and so [those] could be places to target gathering epidemiological records from to help reconcile the role of humidity in heat-related mortality," she said.