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California moves toward launching nation’s first heat wave ranking system

Categorizing heat waves could help communities better prepare and reduce deaths.

Athlete Sam Richardson uses a UV-blocking umbrella while speed-walking in Elysian Park in Los Angeles on July 7. (Damian Dovarganes/AP)

The narrative is becoming all too familiar: A severe heat wave builds and, days later, people die. Now, legislators, scientists and a think tank are convening to better adapt to the most lethal weather phenomenon — by categorizing and naming it in major U.S. cities.

In January, California Insurance Commissioner Ricardo Lara and other delegates will formally introduce legislation to rate and name heat waves in Los Angeles — potentially establishing the nation’s first ranking system for such occurrences. The categorization would help communities take measures to reduce the number of heat-related fatalities.

Heat waves have a particularly impact in Southern California, where they have become more frequent, intense and longer-lasting over the past five decades. Los Angeles is predicted by 2050 to experience an average of 22 extreme heat days annually — up from six days in the period from 1980 to 2000.

“This is a climate adaptation and resilience strategy,” said Kathy Baughman McLeod, director of the Adrienne Arsht-Rockefeller Foundation Resilience Center, which is helping spearhead the initiative. “We’re doing what we can do now to protect people’s lives and livelihoods. And that means being aware of the risks they face and what they can do to protect themselves.”

Los Angeles is one of six pilot locations in which the Arsht-Rockefeller Center is working. The others are Kansas City, Milwaukee and Miami-Dade County; and Athens and Seville, Spain. The center aims to improve communication and public recognition of heat-wave threats. In August 2020, the center formed the Extreme Heat Resilience Alliance, a group of emergency-response organizations, researchers, cities and nonprofit entities addressing the dangers of urban heat.

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The concept is similar to the rating of hurricanes, but this ranking system would instead categorize heat waves on the basis of projected health outcomes rather than meteorology.

“This is, if not the only one, at least one of the very few meteorological warning systems that is based on the outcome,” said Larry Kalkstein, chief science adviser to the Arsht-Rockefeller Center. “That is, how many people are going to die rather than saying it’s going to be 105 degrees.”

Kalkstein and his team are creating three heat-wave categories to use in major U.S. cities. category 1 would indicate a relatively low number of expected deaths, perhaps a zero to 10 percent increase in daily mortality. Category 3 would indicate a larger potential increase in the number of deaths — such as occurred in the Pacific Northwest heat wave in June.

Each category would be tied to specific actions that can help alleviate the effects of the extreme heat. For instance, a category 3 designation could trigger the opening of municipal swimming pools and air-conditioned shelters, and even door-to-door checks on the elderly and other vulnerable members of the community. Local utilities could be prevented from cutting off air conditioning or power for nonpayment during intense heat events. Other protections could include altering the schedules of outdoor workers.

Kalkstein and his colleagues are awaiting the approval of the National Weather Service, the only government agency that could announce official heat-wave alerts and categories, to conduct pilot studies. The goal is create an interactive website, accessed by the Weather Service and stakeholders, that automatically looks for forecast updates about every 15 minutes.

“We want to be able by next summer to try this system out in these different cities. … Then if they’re successful, the goal, obviously, is to do this nationwide,” Kalkstein said.

The team has permission from local governments to run pilot programs in Athens and Seville next summer.

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How to categorize and name a heat wave

A heat wave originates from a large body of air, sometimes described as a heat dome. But not all air masses are the same, and they can pose different threats. Some are hot and humid, created by offshore warm tropical wind. Some are hot and dry, originating in deserts in the U.S. Southwest.

“We identify those air masses that kill a larger number of people from all causes, not just heat stroke, because it’s not only heat stroke that goes up with mortality. It’s respiratory failure, heart attacks and strokes that go up,” said Kalkstein, who has helped the Weather Service and others develop heat alert systems.

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Kalkstein and his team have conducted detailed statistical analyses on how meteorology affects mortality in two of the pilot cities, Kansas City and Milwaukee. In Kansas City, they found 41 heat waves that resulted in higher mortality since 1975 by using this air-mass approach as well as other meteorological factors, such as the persistence of the air mass.

For instance, their calculations showed that a 17-day heat wave in July 1980 was associated with an average mortality increase of 425 percent over the period — or about a 25 percent increase in mortality per day.

In Milwaukee, they found 31 such heat events since 1975. For a heat wave in July 1995, they calculated an average mortality increase of 212 percent over the six day event, or a daily average increase of 35 percent.

In general, Kalkstein said, the hot, humid air masses and hot, dry air masses are typically associated with higher mortality.

Kalkstein said these retroactive analyses are vital for actual heat-wave forecasting and categorization. If an approaching air mass is similar to past observations, the team can classify the upcoming heat wave according to the historical data. The goal is to work with the Weather Service to look at its data and potentially make heat-wave forecasts up to five days ahead of the events.

“We can tell stakeholders up to five days in advance, ‘Hey, a category 3 heat wave is coming your way’ and work with them as to what needs to be done when a category 3 heat wave occurs — open air-conditioned shelters, go door to door and knock on doors of people who we know are vulnerable,” Kalkstein said.

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Kalkstein said three categories would be used across U.S. cities for consistency, but the thresholds for each classification would vary depending on local factors. For instance, the profile of the local population or the amount of concrete infrastructure could increase or reduce the danger posed by a heat wave.

That means these retrospective analyses would need to be conducted separately for each city.

“If one city is basically consisting of a larger proportion of elderly people, they’re going to be more sensitive than a city that has younger people,” Kalkstein said. “Even if the weather’s the same, all those things need to come into account.”

In addition to ranking heat waves, the pilot cities will experiment with naming heat waves. Baughman McLeod said that deciding what to name heat waves is more of a human behavior study. A social marketing firm has been helping test names to focus groups. For instance, some rejected the idea of naming heat waves after local flora and fauna, saying they would not take those heat waves seriously.

“My prediction is that a human name or a set of Greek names actually that reflect human beings is going to be where people land,” said Baughman McLeod. “But we’re setting up a process for answering that question.

Category 1 Heat Wave Henry may be coming soon to a city near you.