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.
Temperatures in Illinois this week have ranged from the upper 90s to the low 100s. At times the night-time lows have been in the upper 70s and low 80s as a result of the high humidity.
Emerson Nafziger, University of Illinois Extension, has a thoughtful discussion about the effects of these conditions on crops in Illinois. In general, high day-time temperatures are not a major concern for corn until they get above 100 degrees. Other potential problems include: high night-time temperatures leading to higher losses of sugars available for crop growth; high humidity levels increasing the risk of foliar disease; and the lack of rain in parts of Illinois since the beginning of July leading to reduced photosynthesis.
You can read the full story on the University of Illinois web site “The Bulletin: Pest Management and Crop Development Information for Illinois” High Temperatures and Crops.
We officially hit 101 degrees today here in Champaign-Urbana, Illinois, according to the long-term site at the Illinois State Water Survey. That’s the warmest we have been since August 18, 1988. However, it does not beat the record for this date, which was 104 degrees set in 1901.
Based on preliminary reports, Geneseo and Bentley Illinois reached 101°F yesterday. Another nine places reached 100°F. They include Illinois City, Moline, Mt. Carroll, Rockford, Prairie City, Normal, Rantoul, Streator, and Urbana. The Chicago Botanical Garden was close with 99°F.
Here is the list with the last time they saw 100°F:
Geneseo: July 25, 2005
Bentley: August 11, 2010
Illinois City: July 24, 2005
Moline: July 17, 2006
Mt. Carroll: August 18, 1988
Rockford: July 10, 1989
Prairie City: July 26, 2005
Normal: July 26, 2005
Rantoul: July 22, 2002
Streator: June 26, 2009
Urbana: July 13, 2005 1995 (thanks Chris G.)
By the way, the last time Chicago at O’Hare reported 100°F was on July 24, 2005.
You notice all these sites were in central and northern Illinois instead of southern Illinois. Much of central and northern Illinois have been dry. As a result, more of the sun’s energy is devoted to warming up the surface and lower atmosphere and less for evaporation and transpiration in plants.