According to the National Centers for Environmental Information (NCEI), the average April temperature in Illinois has warmed by 2 degrees over the last century. In addition, the most recent decade of 2006-2015 was the warmest set of Aprils on record. Statewide records go back to 1895.
Here are the statewide average temperatures for April in Illinois plotted by year. The purple line and black dots show the year-to-year variability in April temperatures in Illinois. The blue line shows the upward trend. You can find the original data and graph here.
Here are some other interesting features of April temperatures in Illinois. There is a 15.6-degree temperature difference between the coldest April (1907 at 43.1 degrees) and warmest April (2010 at 58.7 degrees) on record. The five coldest Aprils occurred on or before 1950. On the other hand, the five warmest Aprils are spread out across the historic record. Even after the last two cold winters, the April temperature in 2014 was close to average and 2015 was above the 1981-2010 average.
Below is a bar plot of the same data averaged by 10-year period from 1896 to 2015. Looking at the April average temperature by decade, the upward trend is still clear even without the year-to-year variability of the first graph. And as I mentioned earlier, the most recent decade of April temperatures was the warmest on record.
What are the pros and cons to warmer Aprils? Using this this April as a guide, a warmer April can translate into lower heating costs. For agriculture, it can mean an earlier start to the growing season (as long as there isn’t too much rain). However, it also means an earlier start to weeds and insect pests. And for those of us with allergies it can mean an earlier start to the allergy season.
One final comment is that while this particular dataset from NCEI goes back to 1895, we have more limited data from the 19th century. That data indicates that Illinois was as cold, if not colder, than the early decades of the 20th century. I will show some of that data in a subsequent post.