It comes as no surprise that this July was one of the warmest on record. The statewide average temperature for Illinois was 80.1 °F. That is 4.3 °F above average and the 6th warmest on record (tied with 1955). Here is how the top six Julys look:
- 83.1 in 1936
- 81.7 in 1901
- 81.3 in 1934
- 80.4 in 1916
- 80.2 in 1921
- 80.1 in 2011 and 1955
While the daytime temperatures were impressive, it was the very warm nighttime temperatures that pushed this July into the top 10 list. Here in Champaign-Urbana, we were the 7th hottest July in terms of daytime high temperatures but we were the 2nd hottest July in terms of nighttime low temperatures. Why so hot, especially at night? The high humidity levels experienced in July prevented the nighttime temperatures from cooling off.
Looking at stations with records of at least 30 years, we had 168 broken and 71 tied daily record high low temperatures. Meanwhile we had only 28 broken and 24 tied record daily high temperatures.
At least 38 sites reported temperatures reaching the 100°F mark. The hottest temperature reported for July was 105°F at Dixon Springs on July 13 and Streator on July 25.
Does this July indicate climate change?
Below is the graph of July temperatures for Illinois from 1895 to present. While July 2011 was outstanding compared to recent decades, we have had other stretches of hot Julys. In my opinion, the most interesting feature is the dramatic rise in July temperatures in the 1920s and first half of the 1930s that maximized in 1936 before returning to values closer to the long-term average.
As the shading indicates, we were more often warmer than average (red shading) in the first half of the 20th century. We were more often cooler in the second half of the 20th century and in the early 21st century. Another thing to note is that 2009 was the coldest July on record for Illinois with an average of only 70.2°F and now this year with the 6th warmest.
At this point, this July does not indicate a pattern of hotter summers in July. The large year to year variability as well as the tendency for trends of up to 10 years to appear and disappear show just how hard it is to detect long-term (i.e., multi-decade) climate change in the Illinois records for summer.