“If you think that the weather in Illinois this summer has been cool and wet, you’re right.
These conditions are not unique to Illinois, however. Temperatures this summer in most of the
eastern two-thirds of the United States also have been cooler than normal as frequent intrusions
of Canadian air have dominated the region. By contrast, warmer than normal temperatures have
affected the northwestern United States. While the magnitude of the cool temperatures this year
has been extraordinary, the pattern is not unique to 2004. Following a period of frequently very
hot summers in the 1930s and early 1950s, the period since the late 1950s has seen relatively cool
summers on average in Illinois with no trend over that period. At the same time, summers in the
western United States have become progressively hotter. This regional difference in trends is
probably caused in large part by a change in the overall atmospheric circulation patterns. The
ultimate causes of these shifts are not clear at this time,” 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.
“Temperatures in Illinois this summer are averaging 3.1 degrees below normal statewide,
with even colder August temperatures, 4.9 degrees below normal. Both summer and August rank
as the 5th coldest since 1895. In recent years, only 1992 was ranked colder, 3rd coldest summer
and August since 1895. The 1915 record still stands for the coldest summer (3.9 degrees below
normal) and coldest August (5.8 degrees below normal),” says Angel.
Temperatures this August are averaging 68.7 degrees, with precipitation averaging 5.45
inches, 149 percent of normal. Temperature extremes this month have ranged from 40 degrees at
Congerville on August 15 to 97 degrees at Kaskaskia on August 19. Grayville reported the
largest one-day precipitation total, 6.87 inches on August 26.
“June–August temperatures are averaging only 70.7 degrees, and precipitation is
averaging 12.87 inches, 111 percent of normal,” says Angel.
Although the National Weather Service is calling for an increased chance of cooler-than-normal
temperatures for September, the temperature outlook for November–January is neutral,
suggesting a return to more seasonable temperatures.
“The type of summer we’re having this year does not necessarily foreshadow an early
frost. For example, an analysis of the 10 coldest summers in Champaign-Urbana yields a first fall
frost date only 6 days earlier than the 1971–2000 average of October 17. The first frost typically
occurs on October 7–14 (northern Illinois) and October 14–21 (central and southern Illinois),”
concludes Angel.
Also See: Summer 2004 in Illinois - Where Did It Go?
Illinois Frost Dates and Growing Season
