Sunday’s air quality is unhealthy for sensitive groups and the heat index will break 100 degrees on Monday and Tuesday, prompting federal, state and city agencies to urge Chicagoans to stay indoors when possible.
The Illinois Environmental Protection Agency declared an Air Pollution Action Day for Chicago-area counties Sunday due to high ozone levels, which are caused by pollutants emitted by cars, industrial sources and wildfires such as those currently blazing in Canada chemically reacting with sunlight.
Then, the National Weather Service in Chicago issued an excessive heat watch from Monday afternoon to Tuesday evening due to the “dangerously hot and humid conditions.”
Poor air quality and heat waves frequently happen together because particulate matter lingers in stagnant masses of hot air, said National Weather Service meteorologist Casey Sullivan.
Heat index values could reach 105 degrees Monday and 115 degrees Tuesday. Meanwhile, overnight temperatures are only expected to drop to the mid-to upper-70s.
Climate change is making the Chicago area more humid, which increases the “feels like” temperature, or heat index, and makes the nights hotter. Average precipitation has increased by 15% in Illinois between 1895 and 2019. Consequently, average summer temperatures have warmed by 1.5 degrees over the last six decades, with average lows increasing by 2.2 degrees.
Heat becomes dangerous if it lingers for more than a day, and warm nights after hot days do not give the body adequate time to cool down, according to the National Weather Service.
Hot weather also does not affect everyone in Chicago equally. Dense residential and industrial areas retain heat more than communities neighboring natural, open spaces like the lakefront, according to a 2023 city-sponsored study. Temperatures collected in different neighborhoods at the same time of day varied by as much as 22 degrees. These findings did not account for humidity, which increases the heat index, meaning that the temperature differential likely felt even greater.
The state EPA advises those with asthma, children and older adults to avoid prolonged outdoor activity Sunday. Meanwhile, the National Weather Service and the city of Chicago encourage residents to drink plenty of fluids and seek air-conditioning. Children and pets should also never be left in unattended vehicles for any length of time, as this week’s extreme heat can cause car interiors to reach lethal temperatures in minutes.
The city also has public cooling centers — which include library branches, park district facilities and community college buildings — to provide residents relief from the heat. See here for a full list of locations and hours.
The National Weather Service is monitoring temperatures and may elevate the excessive heat watch to a warning Sunday afternoon as confidence in heat index values increases, according to Sullivan.
Scattered thunderstorms Tuesday evening and Wednesday are expected to cool the city back down mid-week.
