![]() ![]() The heat index shows the full danger, but only for people in the shade When high heat and humidity pass a threshold where almost no evaporation takes place, people can die within a matter of hours, even just sitting in the shade. "When it gets close to the humidity of the sweat on the skin, it can no longer evaporate." "Only sweat that evaporates has any ability to cool the body," says Larry Kenny, professor of physiology at Penn State University. And in humid air, it's much harder for sweat to evaporate effectively. Without evaporation, humans are out of luck. Sweating reduces our core temperature, since it carries heat away when it evaporates from our skin. Humans have a powerful mechanism to keep themselves cool, not shared by much of the animal kingdom. It shows how humidity makes the temperature feel hotter, but only for a person sitting in the shade, leaving out outdoor workers and others who spend hours in the sun.įor rescue workers, like these firefighters who do mountain rescues in Phoenix, extreme heat makes responding to distress calls particularly dangerous.ĭangerous heat is more than just the temperature The National Weather Service's main heat alert system, the heat index, may be leading the public to misjudge the dangers. Many heat waves are deceptively deadly, but traditional weather forecasts often don't capture the full extent of the risk. In muggy, humid air, the human body struggles to cool off, because sweat doesn't evaporate as well.Īs heat waves get more frequent, longer and more intense with climate change, disaster experts say the country's current heat warning system is falling short. The heat wave's heavy toll was largely due to its high humidity. Many were older people who had succumbed inside their homes, as they tried to ride out the sweltering heat. ![]() In Chicago, Black residents made up half the deaths. More than 1,000 people died across the larger region. That heat wave turned out to be one of the deadliest in recorded U.S. It was going to be hot - around 100 degrees - but nothing that was unheard of for a Chicago summer. In July 1995, weather reports in Chicago started warning residents about an incoming heat wave. High humidity made the heat deceptively dangerous. In 1995, a heat wave in Chicago required morgue technicians use to rows of refrigerated trucks, after 1,000 people died the broader region. ![]()
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