Finally, temperatures starting to cool
Dave Adams empties water bottles into a cooler Saturday at the Lincoln Park Arts and Music Festival. (Brian Cassella/Chicago Tribune / July 7, 2012)
It appeared unlikely that Chicago would have an unprecedented fourth consecutive day of 100 degrees. Although the day warmed up quickly, with an official reading of 97 degrees at O'Hare International Airport by 9:51 a.m., the mercury there had dipped to 83 degrees by 2:51 p.m.
A cold front moving south toward the Chicago region from northern Lake Michigan is forcing the temperature swing, and it will also bring winds that could reach 30 mph, according to WGN meteorologist Tom Skilling.
"This flow is to deliver cooler air to the area setting the stage for a precipitous temperature drop," Skilling wrote in his Chicago Weather Center blog.
He advised boaters on Lake Michigan to monitor changing weather conditions today "since a comparatively quiet lake Saturday morning is likely to grow more challenging later in the day and Saturday night as wave heights build." Waves could reach 6 feet by Saturday night into Sunday, he noted.
An extreme heat warning for the Chicago area had been set to expire at 6 p.m. Friday but was extended to 4 p.m. today. The weather service said the heat index -- how hot it actually feels -- could reach 105 today.
Chicago-area residents are being warned again about high levels of air pollution. The Illinois Environmental Protection Agency has declared an Air Pollution Action Day in the region today because of elevated ground-level ozone.
Ozone is an unstable gas created when volatile chemicals and exhaust from smokestacks and tailpipes "cook" in the heat and sun.
The pollutant can be harmful to people with heart and lung problems and to those who exercise outdoors. Health officials recommend that everyone limit outdoor activities on Saturday and try to stay in a cool place. They also discourage activities that can contribute to ground-level ozone, including driving vehicles and mowing lawns.
On Friday the city and suburbs baked in record-setting heat for the third straight day. It was only the third time since records have been kept that there had been three days in a row in the 100s.
The temperature peaked at 103 degrees at O'Hare International Airport at 3 p.m., breaking the record of 99 degrees set in 1988. It then dropped to 101 by 4 p.m. and to 90 by 10 p.m.
Only two other times in Chicago's record books have there been three straight days of temperatures in the 100s: July 3-5 in 1911 and Aug. 4-6 in 1947.
The forecast calls for a high near 100 on Saturday before a cold front brings temperatures down to the 80s on Sunday.
The Cook County medical examiner's office has linked six deaths so far to the heat and is investigating four others.
The Associated Press contributed to this report.