Chicago broke a century-old record Monday when the low temperature only dipped to a hot and humid 79 degrees.
At this time of the year, the normal low is 64 degrees. So having a low of 74 on Sunday, 79 on Monday and another potentially record-breaking low in the high 70s on Tuesday is unusual in the area for July — and even more so for August, said Gino Izzi, a meteorologist with the National Weather Service office in Chicago.
“You have a hot day, let’s say, in a drier environment, where you have a high temperature of 95 and then at night, a low of 65 — it’s enough that even just opening windows, you can cool off and let the body recover,” said Todd Kluber, another weather service meteorologist. “But when we have these consecutive nights that are also warm, there isn’t much relief in terms of the daytime stress of heat … and then it kind of builds over multiple days.”
Sweltering summer nights are the most compelling evidence of climate change in the Midwest, experts say.
Overall summer average temperatures have increased by 1.5 degrees between 1970 and 2022 in Chicago, but average lows have warmed at a higher rate of 2.2 degrees in that same time.
Low temperatures from Sunday into Monday offered little respite from the oppressive daytime heat, dropping from the mid- and high 80s overnight to 79 degrees at 7 a.m. Previously, the highest minimum at around this point in the late summer was 82 degrees on Aug. 21, 1916.
Similarly, after midnight on Tuesday, temperatures remained in the low 80s before briefly dropping into the high 70s during the morning commute. According to the weather service, Tuesday was the hottest day of this week’s heat wave, with a 114-degree heat index and a record-breaking 99-degree high for Aug. 27.
Monday also marked the warmest low this summer since June 18 when overnight temperatures during a record-breaking 90-degree heat wave were 78 degrees.
Humidity makes heat persist for longer into the night.
Kluber said this week’s temperatures are being caused by a shift to the northeast of a heat dome that was building in the West and Southwest, but humidity from agriculture and the Gulf of Mexico is also contributing. AccuWeather experts said temperatures are climbing 8 to 14 degrees above historical averages in parts of the Midwest due to this moisture in the air.
In Illinois, a major corn-producing state, corn sweat can also make heat — whether it breaks records or not — feel even worse: Rising average temperatures due to climate change are increasing the rates at which corn crops release water vapor into the atmosphere as they mature in the summer and saturate the air with moisture. For context, 1 acre of corn can release 3,000 to 4,000 gallons of water daily, and Illinois planted 11.2 million acres of the crop in 2023.
That sticky, persisting humidity, in turn, poses more risks to public health as it extends human exposure to uncomfortable temperatures. This prolonged exposure can lead to insufficient or poor sleep, compromising the immune system, increasing the risk for cardiovascular disease and diminishing cognitive performance. Heat-related illnesses like heat stroke can also be fatal.
Sticky air and hot nights: How climate change is subtly shifting Midwest summers
In addition, hotter summers are driving up cooling demand and electricity bills across the nation; in parts of the Midwest, including Illinois, average household costs of cooling this summer are predicted to be $581, up almost 10% from $524 last summer.
But as energy bills rise, extreme summer heat can force low-income families into a precarious choice: keep their homes at unsafe temperatures, stay cool and incur high utility debt, or forgo other basic needs, including medicine and food to pay electricity bills.
An already-strained power grid becomes overloaded during hotter, longer heat waves. Outages during these periods can further expose vulnerable individuals.
Between 2000 and 2023, the state of Illinois had the third most major power outages in the country from May to September with a total of 48 — though that is still only about half the blackouts that chart-topping Texas and Michigan experienced during that same timeframe, according to climate science nonprofit Climate Central. Each major power outage leaves more than 50,000 customers, including homes or businesses, without power.
Chicago Tribune’s Karina Atkins contributed.
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