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All-time daily temperature records have been smashed in Colorado this summer, but here’s why the rising overnight minimums are so concerning


by Allen Best

Whew – it was another hot night, the worst in a string of hot July nights in metropolitan Denver. The temperature, according to my cell phone, did not fall below 80 degrees until 1 a.m. In my office, it was even hotter, approaching 84.

Yes, you are correct. I have no air. I have a full-house fan, which draws colder air in through windows and ushers the hot air out through the attic of this 133-year-old bungalow. To work effectively, night-time temperatures must get down into the 60s. The phone told me that overnight it got down to 74.

Summer nights have been warming. We pay attention to the record highs, and we’ve had some of those. But cooling off at night can make all the difference.

“In general, we know that summer minimum temperatures are rising,” said Peter Goble, a climatologist with the Colorado Climate Center. This summer’s overnight lows so far in 2022 haven’t been setting new high marks – but they’re very, very close.

“It’s not unprecedented but it’s right up there with the all-time maximums (for overnight lows) that have been observed,” he said after checking data for several locations.

For example, Denver’s Central Park Station – this is where the former Stapleton airport was located – had a temperature of 69 the night when supposedly it got only to 74 here at my home/office in Olde Town Arvada. No record – but it was pushing the extreme. Only 12 times have temperatures exceeded 70 degrees.

These night-time hot temperatures serve as a strong reminder of the relatively narrow band of temperatures at which humans can feel comfortable — and function. Older people – I guess that includes me – are less accommodating of both heat and cold. We’re also at more risk.

“It’s not a single hot night that hurts. It’s the accumulation of multiple hot days and nights. That’s where we are right now,” said Nolan Doesken the morning after that hot, hot night. “Heat waves don’t start claiming human lives until they’re three or four days in a row.”

The former Colorado state climatologist, Doesken has a vivid memory of the hottest night on record in Fort Collins, where he lives. He had driven solo that day 1,100 miles from Muskegon, Mich. That’s a grueling drive for anybody, but for Doesken, a lover of all things weather since a child growing up in the Midwest, most notable was the complete absence of clouds.

Doesken has an air conditioner as backup but tries to rely upon a whole-house fan. “When we get down to 62 or 63 overnight, we can be very comfortable with our full-house fan.”

Some people have no air conditioning. Swamp coolers work well in dry climates, but they, too, have their limits.

Elizabeth Babcock, the climate team manager for the city of Denver, reports she weatherized her older home – an imperative for keeping cool temperatures in and hot temperatures out – and installed an evaporative, or swamp, cooler. They can cool the interiors of buildings, but they do a poor job of filtering impurities such as come with wildfire smoke.

The better answer? Air-source heat pumps, which can filter the air while cooling homes in summer and warming them in winter.

“I think we have to be strategic in how we think about cooling technologies,” said Babcock.

Denver, along with the rest of the globe, has been heating up. A 2017 study by the Rocky Mountain Climate Organization found the number of 100-plus days per year had more than doubled in the 21st century. With continued high emissions of greenhouse gases, Denver should expect mid-century high daytime temperatures 2 degrees hotter on average than experienced in El Paso, Texas, in the latter decades of the 20th century.

By century’s end, Denver’s high temperatures will be on par with those of Tucson in recent years, according to the study.

Babcock believes that the nighttime warming temperatures experienced in Denver already make that study look dated. “We are seeing impacts of climate change here today, and we really weren’t built for this new climate.”

In making Denver more resilient to rising temperatures, Denver conducted a study to identify neighborhoods most vulnerable to extreme heat, both day and night.

Vulnerable populations can be identified in various ways, including physical disabilities and age. Children under 5 and those over 70 tend to be most vulnerable. So are people with diabetes. And do they have access to transportation?

Even if transportation is available, the better option is to make homes less vulnerable. “There are lots and lots of reasons that people would not want to leave their homes to go to a cooling shelter,” said Babcock.

See Denver’s heat-vulnerability mapping.

Globeville, Colo. Allen BestGlobeville, which is bifurcated by Interstate 70, is one of Denver’s neighborhoods most vulnerable to extreme heat. Photo/Allen Best

The city is rolling out several programs to enhance resiliency to extreme heat. One will yield 2,000 trees over the next three years in the Westwood, Globeville, Elyria, and Swansea neighborhoods, among the city’s most vulnerable. The trees and plantings along with care will be provided for several years.

Another measure to temper what is called the heat-island effect of a city is a requirement governing roofs of 25,000 square feet or more. Instead of black tar, which absorbs and retains heat, the new roofs must use materials that have greater reflectivity. They should cool off more quickly at night.

The city is also aggressively pursuing electrification to lower emissions, but that also reduces the heat in urban areas by replacing engines that generate heat with electricity. “When you think about things like internal combustion vehicles, they produce a lot of heat. Electrification will temper the heat somewhat.

“We’re looking at all potential tools available to us to address the extreme heat,” Babcock said.

No data are readily available about how many deaths in Colorado can be attributed to heat. Axios this week reported more than 1,900 people had died in Spain and Portugal from the heat there during the preceding week. The Environmental Protection Agency points to some statistical approaches that more than 1,300 deaths occur per year in the United States due to extreme heat. The New York Times reported that 100 million Americans this week were under heat advisories or warnings. That included Austin, the capital of Texas, where temperatures had reached 100 or more for the 40th straight day on Wednesday.

In a June story titled, “How Extreme Heat Kills, Sickens, Strains and Ages Us,” the Times told of research by scientists. “One thing is for sure, scientists say: The heat waves of the past two decades are not good predictors of the risks that will confront us in the decades to come,” the newspaper’s Raymond reported. Their research, he went on, has now focused on the effects on ordinary people.

Like many meteorologists, Doesken was not immediately sold on climate change. The accumulating evidence of hot nights persuaded him. Goble, at the Colorado Climate Center, explains why night-time high temperatures are so important to understand.

“Summertime minimum temperatures do not vary naturally from year-to-year as much as summertime maximum temperatures, or temperatures in other seasons. For this reason, it is easier to spot long-term trends, such as our current warming trend, looking at summertime minimum temperature data,” he explains.

Other times of year, including winter, weather is altogether more variable from day to day and week to week.

In summer, there’s more variability in daytime temperatures than at night. So when we have a marked increase in nighttime temperatures, that is a strong indicator of a warming climate consistent with the theory of global warming.

Theory in this case means not a hypothesis, but rather a cohesive and complex idea that explains much. Einstein’s theory of relatively, for example, remains intact after a century of people looking for flaws. Similarly, theory of global warming explains much of what is being observed.

“The easiest way to identify long-term trends is in summer nighttime temperatures,” says Goble.

“We are seeing significantly warmer night-time minimum temperatures in summers of the 21st century as compared to the 20th century.”

SEE ALSO: A story on Climate Central report on cities, extreme heat and humidity:

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