Despite all the images of ski resorts and snowy peaks, Colorado is experiencing shorter winters and hotter summers that put people at increased risk of heat-related illnesses. Until this year, however, the National Weather Service had not issued a heat advisory for the Denver metro area in 13 years.
This is because the heat index commonly used by the weather service to measure the health risks of hot weather is based on temperature and humidity. Colorado’s climate is so dry that getting to the thresholds for that type of heat warning it’s almost impossible.
But this year, the Colorado National Weather Service adopted a prototype heat advisory index, known as HeatRiskwhich is used in California and other parts of the western US and is based on local weather data to determine how much higher than normal the temperature will be and what the dangers to people might be.
The result is a more defined standard for warning people about the heat and a greater likelihood that a warning will be issued in Denver and other areas of the state. Since adopting the HeatRisk index earlier this year, the weather service has issued five heat advisories for the northeastern part of the state.
“We’ve never been able to cast them based on the old ways of looking at heat impact,” said Paul Schlatter, science and operations officer for the National Weather Service in Boulder. “Now with HeatRisk, it’s much easier. It shows very well and gets to the real impact for Colorado.”
On July 18, for example, Denver had a high temperature of 100 degrees and a relative humidity of 13%. That put 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 issue a heat advisory.
The weather service has three levels of weather alerts: advisories, watches and warnings, but the Denver region has not reached the most severe levels. Still, even heat advisories are crucial to public health. In Denver, a heat advisory triggers the opening of cooling centers and alerts residents to avoid exertion during the hottest hours of the day.
“If you look back before 2010, Denver would have an average of less than one day per summer of 100 degrees or higher,” said Gregg Thomas, director of the Division of Environmental Quality at the Denver Department of Public Health and Environment. “Now it looks like we’re in that handful of days at or above 100. It’s in line with what projections have been saying for climate change with hotter, drier summers.”
Denver has had four triple-digit temperature days so far in 2022. The record was set in 2012, when the city had 13.
The Boulder Weather Service office began evaluating the HeatRisk Index three years ago and found that emergency room admissions for heat-related illnesses tended to increase on the same days the index indicated high risk. However, those data may underestimate the true impact of extreme temperatures on health because heat can exacerbate other conditions, such as kidney disease or diabetes, that are not counted in heat illness figures. And research has shown that mental health conditions can flare up during hot weather.
According to the Centers for Disease Control and Prevention, heat causes more than 67,000 emergency room visits9,000 hospitalizations and 700 deaths each year in the US Extreme heat kills more Americans than any other type of weather hazard.
“People see heat as a nuisance,” said Kimberly McMahon, public weather services program manager for the National Weather Service. “The biggest challenge, no matter what index we’re using, is getting people to recognize that heat is dangerous and can kill.”
The HeatRisk Index began in 2013, when the California Office of Emergency Services approached the National Weather Service Western Region Headquarters to provide a more consistent heat warning tool than the Heat Index.
The heat index criteria for issuing an advisory was lower in Northern California and in the mountains than in Southern California. State officials were looking for a simpler system that they could use statewide. After the initial HeatRisk index was tested in California, it was adopted by the rest of the Western region in 2017.
Colorado is in the central region of the weather service. After weather service officials in Boulder learned about the new index in 2019, they contacted the other Colorado weather service offices, in Grand Junction and Pueblo, and they all agreed to use the HeatRisk index to issue weather advisories starting from this year.
McMahon emphasized that the heat index is just one way to assess extreme heat and that weather service offices in the western region and Colorado can use a combination of HeatRisk, the traditional heat index and maximum temperature to determine when to issue a warning. warning.
The heat index was developed in 1979 by Robert Steadman, a physicist who works in the textile industry, to measure how hot you feel when both the temperature and humidity are high.
recent research from the University of California-Berkeley suggests that the traditional heat index can underestimate the health risks of extreme temperatures by up to 20 degrees. david rompsA Berkeley Earth and Planetary Sciences professor who conducted the research together with graduate student Yi-Chuan Lu, said they mapped the heat index to people’s physiological states and found that skin blood flow was so high in extreme temperatures that their bodies were almost unable to compensate.
Once the skin temperature rises to match the body’s core temperature of 98.6 degrees, the core temperature begins to rise. The maximum survival core temperature is believed to be 107 degrees.
“So we’re closer to the edge than we thought before,” Romps said.
The researchers modified the traditional heat index formula and then applied that correction to past heat waves. They found, for example, that during a July 1995 Chicago heat wave that killed at least 465 people, the National Weather Service had reported the heat index as reaching 135 degrees when the temperature was actually 154 degrees. Romps said he forwarded the investigation to the weather service.
The HeatRisk index aims to show how much hotter than normal the temperature is. For example, it takes into account whether a hot day occurs in early summer, before people get used to the heat, and consecutive hot days. The thresholds for a heat advisory using the HeatRisk index are higher in midsummer than in May or September.
HeatRisk also takes into account if nighttime temperatures drop below 70 degrees, giving people and buildings a chance to cool down. Most of Colorado rarely has nights where temperatures stay above 70.
These factors are combined to determine a HeatRisk Index score from 0 to 4, combined with a color scale, from green to magenta. A score of 3 would trigger a warning and 4 a heat warning.
A healthy person may be fine when the heat risk is in the yellow zone, but older people, young children, and pregnant women may want to take precautions. Also, some medications can affect people’s ability to regulate their body temperature, putting them at higher risk even at lower risk thresholds.
Then, it’s up to local health jurisdictions to determine how to react to weather service heat advisories. Despite the warming trend, a 2 degree Fahrenheit increase in Colorado’s average 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 developing an extreme heat advisory plan. She presented the plan to the county board of health on June 13, just three days before the weather service issued its first heat advisory for Boulder since 2008.
Holy cow, he thought. “Here we go.”
Boulder has had four heat advisories this summer. When public health officials tracked who showed up to the ER with heat-related illnesses on those days, two groups stood out: seniors and outdoor athletes.
The health department then worked with the Department of Parks and Recreation to identify trails with high sun exposure and posted extreme heat safety information at trailheads.
Denver public health officials recently adopted an extreme heat plan. It includes advising people to go to cool places, mainly recreation centers and libraries, if they do not have air conditioning in the home. According to Denver public health officials, about 75% of the city’s housing was built before 1980, when summers weren’t as hot. It is estimated that between 30% and 40% of homes lack air conditioning.
The National Weather Service is collecting feedback on the HeatRisk prototype, taking public comments through September 30.
“So far, I would call it a win,” Schlatter said. “We just have a better understanding of what days are really going to be problem days for people in the health department to focus on.”
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