Toward the middle of the day Sunday, as tens of thousands of runners race in the Bank of America Chicago Marathon, temperatures are forecast to climb into the low 70s to 80 degrees.
It’s not the worst kind of weather for a long run. But it is unseasonably warm — the normal high for Oct. 11 is 65 degrees — and “it’s less than ideal,” said George Chiampas, chief medical and safety officer for the marathon since 2007 and an emergency physician at Northwestern Memorial Hospital.
“Everybody should respect the weather,” he told the Tribune. So organizers are asking participants to heed their warning system’s yellow flag for Sunday by adjusting their pace, slowing down and staying hydrated.
Research has identified a temperature “sweet spot” that can optimize runners’ performance. But an analysis by nonprofit Climate Central last year found that 190 of 221 global races will see a decline in odds for optimal weather conditions by 2045 — including the eight Abbott World Marathon Majors, one of which is Chicago’s — as global temperatures rise from human-caused climate change.
Heat has already disrupted races elsewhere in the country and the world. Most recently, in May, several people needed medical attention during multiple marathon events in the Netherlands due to high temperatures, with at least two participants requiring resuscitation, local media reported.
Andrew Pershing, chief program officer at Climate Central and lead analyst in the study into marathon heat, said that it represents the kind of decision-making that society is starting to grapple with because of climate change.
“We all are going to have to think about the fact that the weather conditions that we expect are shifting on us,” he said. “So whether it’s the weather conditions when you start school or when you start football practices in the summer, they’re very different now than they used to be, and they’re going to be even more different in another 20 years.”
Scientists studying how temperature affects a marathoner’s speed have found that elite female runners perform their best at around 50 degrees, while elite male runners do so at 39. Elite runners are those in the top 1%. Recreational female and male runners are fastest at 44 and 43 degrees, respectively. The favorability of race day conditions varies across different types of runners as weather factors interact with their body composition, biological sex and athletic skill.
The probability of having these optimal daily average temperatures on race days for elite men in the Chicago Marathon is projected to remain at 14% through 2045 under climate change, the study found. Elite women will experience a 3% decrease from a 57% probability in the same time span.
The chances of optimal running temperatures will remain at 23% for recreational male runners in the Chicago Marathon over the next 20 years, and only drop from 25% by one percentage point for recreational female runners.
While projected decreases in the likelihood of good weather for future Chicago marathons are small or even nonexistent compared to other races, it doesn’t mean race day temperatures can’t turn out to be worse — or better, depending on how you look at it — for certain groups.
“With climate, you’re always dealing with probabilities, and climate change is ultimately about how those probabilities shift because of greenhouse gases in the atmosphere,” Pershing said. “We will look at … how frequently do we expect to get temperatures near 70, near 50, near 40, at a location? And then we model how those probabilities are going to shift as the planet heats up.”
Runners have also already experienced abnormal heat for the October race in previous years.
On the day of the 2021 Chicago Marathon, the average temperature was 18 degrees above normal at 75 degrees, according to National Weather Service data. In 2007, the marathon had to be stopped as high temperatures reached the upper 80s, some 20 degrees above the normal high for that date. A 35-year-old runner with a heart condition died after collapsing on the course.
“There really wasn’t a precedent in stopping a race because of heat,” recalled Chiampas. “I think our decision in 2007, without question, established what we now have as, really, best practices for marathon medicine.”
Beyond the high temperatures, he said, it was a “fairly excessive” humidity — in the 80-85% range — that worsened running conditions that Sunday in October 2007.
“Runners, including elites, were having a very difficult day. We basically stopped the event so that we don’t overwhelm the hospitals,” Chiampas said. “You know you can’t beat the humidity, as a runner.”
Now, he feels the race is operationally able to support runners through a warm race day, with aid, water and medical stations, and a Biofreeze cooling zone 5 miles from the finish line.
In the last several years, Chiampas said, Chicago Marathon organizers have taken a closer look at the course and realized that, on a given day, temperatures can vary along the route.
“It’s not the same across all 26.2 miles,” he said.
Specifically, they’ve found temperatures can be between 3 and 5 degrees warmer between miles 14 and 19 — which include the West Loop, Greektown and Near West Side neighborhoods. This can be due to a variety of factors like being away from the cooling effect of the lake, a lack of trees and the predominance of heat-trapping asphalt and concrete. For instance, an early week forecast from the NWS office in Chicago noted it would feel cooler Sunday near the Illinois shore.
“On Marathon Sunday, (runners) should be cognizant that when they go west, on the western part of our course, it’s going to get a little bit warmer, especially if the sun is out,” he said. “They’re gonna be feeling good — ‘I’m halfway across the marathon.’ But it actually may get a little bit warmer.”
Heat, humidity and forecasts
Alex Smith, 43, from Elmhurst, can attest to the difference humidity can make. He has kept a careful log of the dew point — a measurement of moisture in the air — alongside his pace and heart rate during runs.
“The dew point really became more my guiding metric,” Smith said.
He recalled an 18-mile run he did while he trained this year, when temperatures were 72 degrees, and the dew point was 71.
“It was the hardest run I’ve had the entire (training) block. It felt like I was just fighting through the air,” he said. “Every mile was a battle.”
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Dew point, like the heat index, can offer people an idea of how comfortable it will feel outside. According to the weather service, a dew point of 55 or less is “dry and comfortable”; between 55 and 65 feels “sticky”; anything 65 or above feels oppressive, or in Smith’s words, like “death.”
Smith created a local guide of sorts, chi.run, as an independent resource (not affiliated with the official Bank of America Chicago Marathon) after running his first one in 2024. He writes it’s meant for sharing “the stuff that actually matters when you live and train in Chicago. How to survive winter miles. How to deal with summer humidity.”
When it’s humid outside, the process of sweating becomes less efficient given that the atmosphere is already waterlogged. So sweat stops evaporating well, sticks to the skin, and the body can’t cool off. The body then tries to increase skin blood flow to release heat, meaning the heart pumps faster.
“You send blood to your skin to cool off. If you do that, then that’s less blood for the rest of your body: your muscles, your heart, … your brain,” said Ted Shieh, executive board member of Illinois Clinicians for Climate Action and an emergency physician with Duly Health and Care, formerly DuPage Medical Group.
This is why runners have to monitor their heart rate even more closely when it is hot and humid.
“I know on really high dew point days, I have to slow down,” Smith said. “And kind of actively reduce my pace to keep the heart rate in check.”
Kevin Daliva-Paleczny, 34, has been keeping a close eye on the forecast as one of tens of thousands of people from all over the country and the world who are slated to run in the Chicago Marathon that day.
He didn’t grow up an athlete, the Wicker Park resident will readily say in conversation. And the anxiety that tends to accumulate in the lead-up to big days has sent him down rabbit holes before; the week of his wedding, his phone camera roll was chock-full of screenshots of weather forecasts. He remembers constantly refreshing his email inbox for weather updates before races, as if waiting to hear back from a job interview.
Now with nine marathons under his belt, Daliva-Paleczny feels a little differently, and encourages those following his journey as a runner on Instagram and TikTok to let go of that anxiety as well. “I still screenshot the weather. I just don’t let it consume me,” he said.
“We’ve trained through some, like, really horrible conditions. It’s been hot and so humid. … And, yeah, the marathon is going to be the hardest thing that we’ve done all cycle,” he said. “But if we just keep psyching ourselves out or thinking too much about this one thing or worrying too much about this one thing, we kind of end up worrying about it twice.”
Chiampas said that preparing for the Chicago Marathon in October allows runners from the Northern Hemisphere to train in the summer months, giving their bodies time to acclimate to warmer temperatures.
While runners should still check the forecasts, doing so many days in advance could cause unnecessary anxiety — especially in places like Chicago, where the weather can often feel frustratingly unpredictable.
In 2024, the Washington Post analyzed federal data to assess the accuracy of weather forecasts — specifically daily high temperatures — across the country, and found that the Great Plains and parts of the Midwest have some of the most unpredictable weather. Forecasts in Illinois are accurate within 3 degrees only two days out, though cool months are easier to predict than warm months.
This is because, in “the vast middle of the country,” there are no moderating factors — such as big oceans that absorb the sun’s warmth or arid deserts that discourage disruptive weather systems — and air masses converge with one another frequently, some warm from the Gulf of Mexico and some cold from Canada, for example, causing quick temperature swings.
Future of racing and records in Chicago
Because of Chicago’s flat landscape and its fall weather during the annual race, Chiampas said, many recreational runners come to set personal records — and elite ones come to chase world records, too.
In 2023, the late Kenyan long-distance runner Kelvin Kiptum set a new record here with a 2:00:35 time, which was broken earlier this year in London.
In many other races, however, rising temperatures are decreasing the odds of achieving record-breaking runs, according to Climate Central. For instance, Sabastian Sawe, who broke Kiptum’s record in May, could not break his own record in Berlin last month, thwarted by unseasonable warmth, the Associated Press reported.
The chances of optimal temperatures in Berlin last year were very low for most runners, and 1% for elite men. That chance is going to drop to zero by 2045, according to Climate Central’s analysis.
Even if favorable running conditions likely won’t get that much rarer in Chicago, past experiences with above-normal heat have led other host cities, even in Great Plains and Midwest states, to move future event dates.
The AP reported that, beginning in 2027, the Twin Cities Marathon in Minneapolis-St. Paul will be held later in the month instead of the first Sunday in October after the race was canceled in 2023 due to record heat and humidity, and after starting temperatures of 70 degrees in 2025. It also reported that the Sioux Falls Marathon in South Dakota similarly moved the race from August to September last year after canceling the 2024 race because of heat.
These moves, though necessary, might nonetheless create logistical headaches for runners trying to complete different races. A rule of thumb runners use after a long race is to rest at least one day per mile run, so about 26 days after a marathon, to allow for physiological and cellular repair.
As some host cities move their event dates, it can create conflicts in a stacked race schedule that usually allows runners to participate in different marathons. For instance, the Chicago and New York City marathons are usually within three weeks of each other.
“There’s a huge running boom, especially post-COVID, of people (who) are just willing to run as many marathons,” Daliva-Paleczny said. “Marathoners will find a way to make it happen. I know people who ran Berlin like two weeks ago and now are running Chicago this week. So, we’ll find a way.”
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Regarding whether hopeful and returning Chicago Marathon runners will have to adjust their plans and strategy to any changes in the race’s operations remains to be seen.
“We obviously are looking at all components and variables. What to change (in) the course or not. But, most importantly, we have the operations and the systems in place to hopefully be able to manage warm days, cold days, windy days,” Chiampas said. “For right now, I think October is a good date for us, but obviously we’ll be following all those things that are happening in the industry.”
The 48th annual Chicago Marathon will start and finish in Grant Park downtown beginning at 7:30 a.m. on Sunday. Read more of the Tribune’s marathon coverage here.