For all its images of ski resorts and snow-capped peaks, Colorado is experiencing short winters and hot summers that increasingly put people at risk for heat-related illnesses. Yet until this year, the National Weather Service had not issued a heat advisory for the Denver metropolitan area in 13 years.
That’s because the heat index that the weather service typically uses to measure the health risks of hot weather depends on temperature and humidity. Colorado’s climate is so dry that reaching the threshold for this type of heat advisory is nearly impossible.
But this year, the National Weather Service in Colorado adopted a prototype heat warning index known as HeatRisk, which is used in California and other parts of the western United States and relies on local climate data to determine how much warmer than normal temperatures will be. And what could be the danger to humans.
The result is a more defined standard for warning people about heat and a higher likelihood of issuing an advisory in Denver and other areas of the state. Since adopting the heat risk index earlier in the year, the weather service has issued five heat advisories for the northeastern part of the state.
“We could never have issued them based on the old way of looking at the effects of heat,” said Paul Schlatter, science and operations officer for the National Weather Service in Boulder. “Now with HeatRisk it’s much easier. It looks really nice and has real impact for Colorado.”
On July 18, for example, the high temperature in Denver was 100 degrees and the relative humidity was 13%. That puts the traditional heat index at 94 degrees, below the threshold for a heat advisory, Schlatter said. But the heat was high enough under the new system to warrant a heat advisory.
The weather service has three levels of weather warnings — advisory, watch and warning — but the Denver area has not reached the most severe level. Still, even heat advisories are critical to public health. In Denver, a heat advisory triggered the opening of cooling centers and warned residents to avoid exertion during the hottest parts of the day.
“If you look back before 2010, the average summer in Denver was less than one day of 100 degrees or more,” said Greg Thomas, director of environmental quality at Denver’s Department of Public Health and Environment. “Now it looks like we’re at 100 or above on those handful of days. That’s consistent with what projections are saying for climate change with hotter and drier summers.”
Denver has four triple-digit temperature days so far in 2022 The record was set in 2012, when the city had 13
Boulder’s weather service office began evaluating the HeatRisk Index three years ago and found that the incidence of emergency room admissions for heat-related illnesses increased on the same days the index indicated high risk. However, that data may underestimate the true health impact of extreme temperatures because heat can exacerbate other conditions, such as kidney disease or diabetes, that are not counted in the heat illness count. And studies have shown that mental health conditions can flare up during hot weather.
According to the Centers for Disease Control and Prevention, extreme heat in the United States kills more Americans than any other type of weather hazard, with more than 67,000 emergency room visits, 9,000 hospitalizations, and 700 deaths.
“People see heat as a nuisance,” said Kimberly McMahon, public weather service program manager for the National Weather Service. “The biggest challenge — regardless of the indicator we’re using — is getting people to recognize that heat is dangerous and can lead to death.”
The HeatRisk Index began in 2013, when the California Office of Emergency Services approached the National Weather Service’s Western Region headquarters about providing a more consistent heat advisory tool than the Heat Index.
Heat index criteria for issuing an advisory were lower in Northern California and the mountains than in Southern California. State officials were looking for a simple system they could use for the entire state. After initial HeatRisk Index testing in California, it adopted the rest of the West in 2017.
Colorado is in the weather service’s central region. When weather service officials in Boulder learned of the new index in 2019, they contacted other Colorado weather service offices in Grand Junction and Pueblo, and they all agreed to use the heatrisk index to issue weather advisories beginning this year.
McMahon emphasized that the heat index is one way to assess extreme heat and that weather service offices in the West Region and Colorado can use a combination of heat risk, traditional heat index and maximum temperature to determine when to issue an advisory.
The heat index was developed in 1979 by Robert Stedman, a physicist working in the textile industry, to measure how hot it feels when both temperature and humidity are high.
Recent research from the University of California-Berkeley suggests that traditional heat indices may underestimate the health risks of extreme temperatures by as much as 20 degrees. David Rumps, a Berkeley professor of earth and planetary sciences who conducted the study with graduate student Yi-Chuan Lu, said they mapped the heat index to human physiology and found that skin blood flow is so enhanced at extreme temperatures that Their bodies were almost incapacitated. compensation.
Once the skin temperature equals the core body temperature of 98.6 degrees, the core temperature begins to rise. The maximum surviving core temperature is believed to be 107 degrees.
“So we’re closer to the edge than we previously thought,” Romps said.
The researchers modified the traditional heat index formula and then applied that correction to past heat waves. They, for example, during a 1995 heat wave in Chicago that killed at least 465 people, the National Weather Service reported the heat index as high as 135 degrees when the temperature really felt like 154 degrees. Romps said he sent the study to the weather service.
The purpose of the HeatRisk index is to show how much warmer temperatures are than normal. For example, it accounts for whether a hot day occurs in early summer, before people are used to the heat, and for sustained hot weather. The threshold for a heat advisory using the HeatRisk index is higher in mid-summer than in May or September.
Heat risk is also caused by nighttime temperatures dropping below 70 degrees, giving people and buildings a chance to cool down. There are rarely nights in Colorado where the temperature gets above 70.
Such factors are combined to determine a heatrisk index score ranging from 0 to 4, corresponding to a color scale from green to magenta. A score of 3 will trigger an advisory and 4 a heat warning.
A healthy person may be fine if there is heat risk in the yellow zone, but older people, young children and pregnant women may want to take precautions. Also, some medications can affect a person’s ability to regulate their body temperature, putting them at high risk even at a low risk threshold.
It is up to the local health jurisdiction to determine how to respond to a weather service heat advisory. Despite the warming trend — a 2-degree Fahrenheit increase in average Colorado temperatures over the past 30 years — most counties in the state lack extreme heat mitigation plans.
When Grace Hood joined the Boulder County Public Health Department as a public health planner in October, she was tasked with putting together an extreme heat advisory plan. He presented the plan to the county board of health on June 13, just three days before the weather service issued the first heat advisory for Boulder since 2008.
“Holy cow,” he thought. “Here we go.”
Boulder has issued four heat advisories this summer. When public health officials tracked who showed up to emergency rooms with heat-related illnesses during those days, two groups stood out: older people and outdoor athletes.
The health department then works with the parks and recreation department to identify trails with high sun exposure and post extreme heat safety information at trailheads.
Denver public health officials recently adopted an extreme heat plan. This includes advising people to move to cooler places, mainly recreation centers and libraries, if they lack air conditioning at home. According to Denver public health officials, about 75% of the city’s housing was built before 1980, when summers weren’t as hot. An estimated 30% to 40% of homes lack air conditioning.
The National Weather Service is gathering feedback on the heatrisk prototype, accepting public comments until September 30.
“So far, I’d call it a win,” Schlatter said. “We have a better idea of which days are really going to be problem days for health department people to focus on.”
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