Temperature falls a degree short of 100 in Chicago on Tuesday, crosses threshold in Rockford

Despite forecasts indicating Chicago could reach 100 degrees for the first time in a decade on Tuesday, temperatures were one degree shy of the triple-digit mark.

The temperature recorded at O’Hare International Airport, which marks the official location for Chicago’s weather data, reached 99 degrees in the mid-afternoon, National Weather Service meteorologist Kevin Doom said. The record for June 21, set in 1988, remained 101 degrees.

Early Tuesday morning, forecasts called for a potentially record-setting day of heat in the area, with highs of 100 degrees expected.

Temperatures at Rockford Airport meanwhile did crack 100 degrees. This was the first time Rockford hit 100 degrees since July 17, 2012, and tied the June 21 daily high record for the airport.

The last time temperatures reached 100 degrees at O’Hare, similar to Rockford, was back in July 2012, weather service data shows. On July 5 and 6, 2012, temperatures peaked at 103 degrees.

While the heat index, also called the “feels-like” temperature, passed 100 degrees Tuesday. But this is not uncommon, as it did so last week too.

The heat index this time this week wasn’t as high, as humidity is much lower than it was last week, Doom said. Estimates showed it could land between 100 and 102.

The dew-point temperature, which helps measure how much water is in the air, was projected to be in the 60s. Last week, that marker was in the 70s, driving the feels-like up to between 105 and 110, Doom said.

“That’s what really drives up the heat index is how much water is in the air,” he said. “Our body cools down by sweating and that sweat evaporating from our body. But when the air is humid, it doesn’t evaporate effectively … and almost acts as an insulator for the heat.”

At Midway International Airport in Chicago last week, 100-degree weather was recorded on both June 14 and 15, according to data from the weather service.

Despite the heat Tuesday, forecasts have temperatures dipping back to the 80s Wednesday and Thursday, with temperatures near Lake Michigan possibly in the 70s on Thursday, Doom said.

Friday could be a similar story, according to forecasts from the weather service. Saturday could see its high get into the lower 90s before Sunday and Monday see a cool down with temperatures expected to be in the mid-70s.