Historical Climate Data Indicate More Hotter Days Likely in Mid-July, Illinois State Water Survey

Press Release

For Immediate Release June 27, 2002
Historical Climate Data Indicate More Hotter Days Likely in Mid-July

Jim Angel - (217) 333-0729, Fax: (217) 244-0220, jimangel@illinois.edu
Eva Kingston - (217) 244-7270, Fax: (217) 333-6540, eva@sws.uiuc.edu
"With planting delays due to the wet spring, there is growing concern about the effects of July's typically hot weather on crops with less mature root systems. Hot weather and associated slight drops in rainfall are more likely to occur in mid-July, almost a 50 percent higher likelihood than in the first and last week of July," says State Climatologist Jim Angel of the Illinois State Water Survey (http://www.sws.uiuc.edu), a division of the Illinois Department of Natural Resources.

Using historical data across Illinois for July, the number of days at or above 90oF and the average rainfall total per week are as follows: 1.9 days at or above 90oF, 0.99 inches (June 30-July 6); 2.7 days, 0.80 inches (July 7-13); 2.8 days, 0.87 inches (July 14-20); 2.1 days, 0.95 inches (July 21-27); and 1.8 days, 0.78 inches (July 28-August 3).

For Illinois, average high temperatures in July typically range from 90oF in the far south to 82oF in the far northeast where Lake Michigan has cooling effects. As a result, days with temperatures 90oF or higher are more common in southern Illinois than in northern Illinois. However, data indicate the greatest likelihood of hot weather during the three middle weeks of July, regardless of the location in the state.

Looking across the state, historical data show a fairly even distribution of precipitation throughout July with some tendency to be drier in the second week of July. The average amount of rainfall during July is 4.05 inches says Angel.

So far for June, cool, wet conditions early in the month have been balanced by warm, dry conditions later on. As a result, June is near average in terms of both temperature and precipitation. Soil moisture levels are adequate at present, although the warm, dry weather has dried out the surface layer at some locations.

The National Weather Service forecast calls for equal chances of above average, below average, and average temperatures for July. Historical records also indicate that hot days in June do not necessarily lead to hot weather in July and August.

"Historical records give us an idea when heat waves are most likely to occur. Unfortunately, specific heat waves are hard to predict more than a few days in advance," says Angel.

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