Researchers at the University of Illinois in Urbana-Champaign said the average temperature for the entire state in July 2009 was 70.4 degrees. That's more than 5 degrees below normal.
The previous record, set in July 1924, was 71.5 degrees.
In Chicago, this July came in 7th since records were kept. However, that rating needs further explanation, according to ABC7 meteorologist Phil Schwarz, because of changes in the way the temperature was recorded.
"Until 1942 the observation was near the lake, so if you got any wind it was cool," said Schwarz. The observation was then moved inland.
"(This July's) the second coolest since the site moved inland from Midway and then to O'Hare," said Schwarz.
The observation remains at O'Hare now. But, had there been an even playing field- without the change in observation sites- Schwarz thinks Chicago could have come close to a record low, too.
"It's the second coolest in 100 years," said Schwarz, who also said Chicago's records go back into the 1800s-- further than most cities.
There are normally at least five days where the temperatures hit 90-degrees or higher across the state. Most of Illinois did not see any temperatures that high in July.
Researchers say the cool down resulted in lower energy demand and poor growing conditions for some farmers.
Will the cool down continue?
"While August started off cool, things start to heat up by this weeekend," said Schwarz.
Meanwhile, statewide precipitation was 1.1 inches above normal in July, at 4.9 inches.