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Scientists use ‘wet temperature’ to assess the effects of extreme heat on the body. But what is it?
Extreme heat waves can cause dehydration, exhaustion, heat stroke and even death. But air temperature isn’t the only factor that determines how dangerous hot weather can be. There’s another measurement called the wet-bulb temperature, Live Science reported. The wet-bulb temperature takes into account the temperature of the air (called the “dry-bulb temperature”) and the water vapor, or humidity, in it. The wet-bulb temperature is measured using a thermometer wrapped in a cotton cloth soaked in water. If the air around the bulb is less than 100 percent humid, some of the water in the cloth evaporates and cools the thermometer, Kathryn Fisher, a doctoral student in physical physiology at Penn State, told Live Science. In more humid environments, less water evaporates from the cloth because the air is already saturated. Therefore, the thermometer can’t cool as well as it would in a drier climate. The higher the humidity, the higher the wet-bulb temperature, and vice versa.
Wet-bulb temperature is calculated using a specific equation that takes into account dry-bulb temperature and relative humidity. It can also be easily inferred by reading a psychrometric chart like the one shown below. Wet-bulb temperature indicates how hot weather affects the body. For example, an air temperature of 40 degrees Celsius and a relative humidity of 30 percent results in a wet-bulb temperature of about 26 degrees Celsius. However, a cooler air temperature of 25 degrees Celsius but a higher humidity of 90 percent would produce a similar wet-bulb temperature of about 24 degrees Celsius. Wet-bulb temperature helps understand how hot weather affects the body because a similar evaporation process occurs when people sweat. The amount of warmth you feel at a given temperature can be different depending on whether you are in a humid or dry environment. Jennifer Vanos, an associate professor at Arizona State University’s School of Sustainability, told Live Science that you can either be in Houston, a swampy city, or in Phoenix, an arid city. Even though the temperature is the same in both places, you may feel hotter in Houston because it’s more humid and sweat can’t evaporate to cool you down. In this example, Houston has a higher wet-bulb temperature than Phoenix.
The highest temperature the human body can withstand Scientists originally deduced from basic physiological principles that 35 degrees Celsius was the maximum wet-bulb temperature that humans could survive. They predicted that once that temperature was reached, the body would no longer be able to cool down to safe levels. However, new research is beginning to question that assumption. “If we assume that 35 degrees Celsius of wet blood is the limit of human survival in hot weather, then we shouldn’t be seeing the deaths from heat in Phoenix or other arid areas of the world, and we’re seeing a lot of them,” Vanos said. For example, in very hot and dry places, people may sweat more to cool their bodies. However, the sweating should not be too much. For example, older people may not sweat as much as younger people, and therefore their survival rate is lower, Vanos said. Exercise and exposure to direct sunlight can also increase a person’s risk of overheating, as can not drinking enough water to replace the water lost in sweat. “We have to get away from black and white thinking, because we can look at our neighbors and see, oh, they suffer in this hot weather, and we are fine – why is that?” she said. In very dry environments, wet-bulb temperature can also be misleading, Fisher said. For example, if the temperature outside is 50 degrees Celsius and the humidity is 10 percent, the wet-bulb temperature is about 26 degrees Celsius, she said. If people focus only on the wet-bulb temperature, they may mistakenly believe the weather is less extreme than it actually is, which could be dangerous, she added.
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