Why heat wave warnings are falling short in the U.S.

In July 1995, weather reports in Chicago started warning residents about an incoming heat wave. It was going to be hot — around 100 degrees — but nothing that was unheard of for a Chicago summer.

That heat wave turned out to be one of the deadliest in recorded U.S. history.

More than 1,000 people died across the larger region. In Chicago, Black residents made up half the deaths. Many were older people who had succumbed inside their homes, as they tried to ride out the sweltering heat.

The heat wave's heavy toll was largely due to its high humidity. In muggy, humid air, the human body struggles to cool off, because sweat doesn't evaporate as well.

As 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. Many heat waves are deceptively deadly, but traditional weather forecasts often don't capture the full extent of the risk.

The National Weather Service's main heat alert system, the heat index, may be leading the public to misjudge the dangers. 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.

New research also shows the heat index may also be underestimating the effect of temperatures on the human body as they get more extreme, low-balling the hazard from heat.

"We're seeing people die needlessly," says Kristie Ebi, a professor at the Center for Health and the Global Environment at the University of Washington. "We certainly need a better understanding of how to communicate to people that, in fact, they do need to take action."

Dangerous heat is more than just the temperature

Humans have a powerful mechanism to keep themselves cool, not shared by much of the animal kingdom. Sweating reduces our core temperature, since it carries heat away when it evaporates from our skin.

Without evaporation, humans are out of luck. And in humid air, it's much harder for sweat to evaporate effectively.

"Only sweat that evaporates has any ability to cool the body," says Larry Kenny, professor of physiology at Penn State University. "When it gets close to the humidity of the sweat on the skin, it can no longer evaporate."

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.

The heat index shows the full danger, but only for people in the shade

To capture the real impact of a heat wave, the National Weather Service uses its heat index. It portrays what the temperature and humidity really "feel like" to the human body. So if the temperature is 88 degrees and the humidity is at 75%, it really feels like 103 degrees.

The calculations are based on seminal research from 1979, which models how humans physiologically handle heat. But the equations leave out an important factor: sunlight.

The heat index only shows what temperatures feel like in the shade, without the added heat from standing in the sun.

"When you're in direct sunlight, it can feel about 15 degrees warmer," says Kimberly McMahon, public weather services program manager at the National Weather Service. "If you're doing vigorous physical activity and you are in direct sunlight, not to mention, say you are over blacktop as opposed to standing above grass, the heat can have a different impact."

That means the heat index isn't applicable for outdoor workers, sports teams and other groups who must spend hours in the sun. Using the current heat index could lead them to underestimate the danger they're in.

The heat index is also modeled on how a healthy person responds to heat — and a specific one at that: 5 foot 7 inches and 147 pounds. Other groups — older people, pregnant people and those with chronic health conditions — have a much harder time coping with heat and are much more susceptible to its effects.

Heat index also lowballs the impact of higher temperatures for everyone

As pivotal as the heat index research was, it had a flaw. It models how humans physiologically feel heat, but at higher temperatures, the model broke down and it couldn't calculate the effects of high heat and humidity. To complete the heat index, the National Weather Service extrapolated using the lower temperature to fill in the gaps for the higher temperatures.

New research suggests that method doesn't capture how much more dangerous higher temperatures can be. UC Berkeley researchers David Romps and Yi-Chuan Lu worked with the original model to allow it to calculate higher temperatures.

They found the National Weather Service's current heat index is underestimating the effect of high heat by as much as 28 degrees. One example: in the 1995 Chicago heat wave, the heat index at the time showed the temperature and humidity felt like 124 degrees. Romps says using a corrected heat index, conditions actually felt like 141 degrees, putting the human body under an immense amount of cardiovascular stress.

"Using the correct heat index would allow us to identify those handful of times where the heat is so severe that it is pushing our bodies close to the breaking point," Romps says. "What's so important about it is that we can identify the times where the warnings really need to be made with clarity, and people really need to pay attention."

The National Weather Service says it's currently reviewing the results of Romps' research.

"We're trying to always learn more and take into consideration how we can improve not just our communication on heat, but how we can improve the different heat stress indicators," McMahon says. "So we are working with the CDC, EPA and as well as many other of our federal partners to continue to try to find better and more widespread ways of alerting the general public, our emergency managers and our decision makers."

Convincing the public that heat is more than a nuisance

For many, heat is all too common in the summertime and seems like more of a nuisance than a real danger. But climate change is making heat waves hotter, longer and more frequent.

"These are outside of people's envelope of experience and they don't expect them," says Ann Bostrom, professor of environmental policy at the University of Washington. "So in those kinds of contexts, it's very difficult for people, understandably, to understand the risks they're exposed to."

In addition to the heat index, the National Weather Service releases an "excessive heat warning" when a heat wave gets dangerous. But critics say that language is too general and not specific enough for vulnerable groups.

"There is a big difference between knowing it's hot and knowing what I need to do individually," Ebi says. "We do need to work better on the messaging."

The weather service is piloting a new kind of heat alert in the Western U.S., known as HeatRisk. It provides heat alerts at four different levels, with specific warnings for who is at risk. It also takes into account how long a heat wave has been going on, as well as whether people are enduring high nighttime temperatures, giving them little respite.

Last week, California also approved a first-of-kind bill that requires the state to develop a heat wave ranking system, which will establish warnings based on the health impacts of heat on vulnerable populations.

Disaster experts say even the most targeted messages aren't useful unless they're actually reaching people. A key step is working with local groups to reach vulnerable populations, like senior centers, neighborhood groups or church groups.

As temperatures keep rising, even cities that aren't known for blistering summers will need to begin that kind of planning.

"It's not just the hottest cities that need to be addressing heat," says Sara Meerow, associate professor at Arizona State University who works on heat. "Communities everywhere do. Places that have not had to worry as much about excessive heat need to now."