Chicago's second heat wave of the season to bring poor air quality Monday, more 90s starting Tuesday

Adriana Pérez, Chicago Tribune on Jul 13, 2026

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CHICAGO — As a heat dome builds in the Plains and “dangerous” record highs become a possibility for parts of the country, the Chicago area is forecast to experience at least three days of 90-degree weather this week, Tuesday through Thursday.

The area will get little overnight relief as temperatures remain in the mid to upper 70s. No rain, mostly sunny skies and light winds will contribute to the uncomfortably warm weather.

Only Lake Michigan breezes will bring some “modest relief” to those living nearby, according to the National Weather Service. Peak heat indices this week are forecast around 100 degrees on Tuesday and Wednesday afternoons, especially away from the lake. The heat index combines atmospheric temperature and relative humidity to determine how heat feels to the body.

This will be the second heat wave of the summer, with the most recent stretch taking place June 30 through July 2, when highs reached 94 to 95 degrees and heat indices were over 100.

“The humidity this time around is not going to be as bad as the heat that we had in June, but nevertheless, it’s still going to be pretty hot out there,” said Brett Borchardt, senior meteorologist at the weather service office in Chicago.

The weather service called this kind of heat normal for the area at this time of year but urged residents to stay prepared. According to historical data, the normal maximum temperature for the Chicago area at this point in July is between 84 and 85 degrees.

With the weather heating up again, and after a week of no rain in the area, the air quality in Chicago on Monday is forecast to become unhealthy for sensitive groups — people with lung disease and asthma, children and teens, and older adults — particularly due to ozone pollution, according to the U.S. Environmental Protection Agency.

Ground-level ozone is produced when two types of pollutants — organic chemical compounds, which are produced in the manufacture of products like paint, pharmaceuticals and refrigerants, and nitrogen oxides from diesel and gasoline engines — react in sunlight. It is produced more effectively and concentrates more in hot, dry and still weather.

While the weather service does not issue air quality forecasts, Borchardt said, “Anecdotally, heat waves and poor air quality do often go hand in hand.”

There is a chance of rain on Friday, he added, which should cool things off.

“So, it doesn’t look like this is going to be a prolonged flash drought-type scenario,” or the rapid onset of dry conditions, he said.

In Chicago, human-made climate change has warmed overall summer average temperatures by 1.9 degrees since 1970 as Midwest summers have become more humid. Average lows on summer nights have increased by at least 2.5 degrees in that same time.

While dry conditions drive up temperatures dramatically, moist air keeps them a bit lower but makes the heat feel worse to humans. This is because, just like greenhouse gases trap heat, moisture holds onto heat in the atmosphere. Rising temperatures, in turn, lead to rising humidity. For every 1.8-degree Fahrenheit increase in temperature, the atmosphere can hold 7% more water.

These kinds of sticky, muggy temperatures that persist into the night can pose a major risk to the health of anyone without access to effective cooling or adequate hydration.

Warmer nights occur when the atmosphere is waterlogged. Clouds reflect incoming heat from the sun into space during the day, but after the sun sets, they absorb heat from the surface and radiate it back toward the ground.

Prolonged exposure to heat 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.

Research has found that the abundance of skyscrapers and asphalt in Chicago traps high temperatures, amplifying the effects of heat by an average of 8.71 degrees, meaning the city has the potential to be that much hotter on a given day, particularly when temperatures are already high.

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