Record-Wettest August in Illinois, Illinois State Water Survey

ISWS Press Release

For Immediate Release September 1, 2016
Record-Wettest August in Illinois
Jim Angel, Ph.D. - (217) 333-0729,
Lisa Sheppard - (217) 244-7270,

CHAMPAIGN, Ill. - The statewide average rainfall for August was 6.89 inches, 3.30 inches above normal and the wettest August on record, according to Illinois State Climatologist Jim Angel, Illinois State Water Survey, University of Illinois.

Multi-sensor Precipitation: Departure from Normal (inches)

This year's rainfall just beat the old record of 6.86 inches set back in 1977. The highest rainfall total was in Sterling, IL with 14.01 inches.

A damp July with a very wet August combined equals the wettest July-August on record. The rainfall total of July-August was 13.74 inches, which is 6.07 inches above normal. It beat the old record for July-August of 12.83 inches, set back in 1915.

The highest two-month rainfall total was in Downers Grove, IL with 22.93 inches of rain.

The two-month average rainfall was above normal for almost the entire state, except for small portions of Lake and McHenry counties. Southern Illinois, south of Chicago, and northeast of St. Louis were 9 to 12 inches above normal.  Some parts of southern Illinois experienced 12 to 15 inches above normal.

The statewide average temperature for August was 75.9 degrees, 2.3 degrees above normal. That is tied with 1943 as the 15th warmest August on record. The average high temperatures were close to normal.

In fact, most of Illinois never exceeded the low to mid-90s in August. However, high humidity levels did not allow temperatures to cool off at night. As a result, nighttime temperatures were 3 to 5 degrees above normal.

The dewpoint temperature is the temperature you would have to cool the air for condensation or dew to form. The average dewpoint temperature for August ranged from the low to mid-70s in southern Illinois and the mid-60s to low 70s in much of the rest of the state. That is about 1 to 3 degrees above normal; thus it was more humid than usual in August.


The Illinois State Water Survey at the University of Illinois at Urbana-Champaign, a division of the Prairie Research Institute, is the primary agency in Illinois concerned with water and atmospheric resources.

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