July was one of the hottest and driest on record. So was the year to date. Here are how things stack up so far.
How July Ranks
This July was the second warmest and fourth driest on record in Illinois, based on preliminary numbers for the month. The statewide average temperature was 81.8 degrees, 6.4 degrees above normal (1981-2010 average). The statewide average precipitation was 1.47 inches, 2.58 inches below normal or 36 percent of normal.
Statewide Average Temperature Rankings for July in Illinois
1936: 83.1 ºF
2012: 81.8 ºF
1901: 81.7 ºF
1934: 81.3 ºF
1916: 80.4 ºF
Statewide Average Rainfall Rankings for July in Illinois
1930: 1.02 inches
1916: 1.23 inches
1936: 1.24 inches
2012: 1.47 inches
1914: 1.51 inches
I think July 1936 still wins the prize as the most miserable month in Illinois records with the hottest temperatures and the third driest rainfall total. I’m sure 1916 was no fun either as the fifth warmest and second driest July on record. Of course, air-conditioning was pretty much non-existent in 1936. My dad says he remembers sleeping outside that July on the family farm in western Illinois (near Nebo) because the house was too hot.
How the Year-to-Date Ranks
This year so far is the warmest and third driest on record. The statewide average temperature for January-July 2012 was 56.9 degrees, 5.5 degrees above normal. The statewide average precipitation for January-July was 14.05 inches, 9.82 inches below normal or 59 percent of normal.
Statewide Average Temperature Rankings for January-July
2012: 56.9 ºF
1921: 56.1 ºF
1987: 54.1 ºF
1998: 54.0 ºF
2006: 53.9 ºF
Statewide Average Precipitation Rankings for January-July
1936: 12.22 inches
1934: 13.55 inches
2012: 14.05 inches
1988: 14.60 inches
1914: 15.20 inches
This rainfall map was generated by our GIS specialist Zoe and uses NWS radar and rain gauge data. The result is a more detailed picture of the rainfall in July 2012. A few lucky areas in northeastern and far southern Illinois received 2 to 4 inches of rain. Most of the state got an inch or less.
The statewide average rainfall for July in Illinois was 4.12 inches, only 0.29 inches above average. However, rainfall across the state varied widely from too dry to too wet.
Rainfall amounts in the northern third of the state were impressive with widespread areas in excess of 6 to 8 inches. The heaviest rains fell around Galena. A CocoRaHS observer (IL-JD-2) reported a monthly rainfall total of 19.21 inches while the nearby Galena NWS Coop observer reported 17.78 inches. In nearby towns:
The middle third of Illinois was exceptionally dry. Some of the smaller rainfall totals for July were just south and west of the heavy rainfall in northwest Illinois. One of the drier sites was Aledo with only 0.55 inches. Amounts of only 1 to 2 inches were common in central Illinois. This equates to rainfall departures that were generally 50-75 percent of average (or worse) across the region. Champaign-Urbana experienced its 20th driest July on record with only 1.58 inches, 3.12 inches below average. Springfield reported 1.09 inches and Peoria reported 1.66 inches.
The southern third of Illinois was wet, where amounts of 4 to 8 inches or more were common. The largest July rainfall total was at the NWS Du Quoin site with 8.88 inches.
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.
The National Weather Service (NWS) has issued a variety of heat warnings and advisories for Illinois this week. The combination of temperatures in the upper 90s and high levels of humidity mean a greater risk of heat-related illnesses and death.
One measure that combines both the effects of temperature and humidity is the heat index. Explanations of the heat index can be found on Wikipedia and NWS. Below is a chart showing the heat index for a given temperature and relative humidity. If you like to do your own calculations, here is the NWS heat index calculator.
While the heat index incorporates relative humidity to give a better idea of what the temperature feels like, there are some important underlying assumptions. It assumes a person who is 5′ 7″, 147 lbs, walking at 3 mph, wearing long pants and a short-sleeve shirt, in the shade with a light breeze. It is estimated that working out in the sun would increase the heat index by 15°F.
Here are some resources to consider for monitoring the heat and what to do during the current heat wave: