Thursday, February 13, 2014

Has this been a normal winter?

Most of us here in Central Illinois have noticed that the temperatures have been running a bit colder than average over the last few weeks, such that some of us have wondered, "Is this winter colder than normal, or are we just spoiled by the last two winters that were milder than average?"  For the sake of this investigation, "normal" temperatures are the simple average of the historical average high and low temperatures for each day.  For instance, the historical average high for February 13 is 36°F and the low is 19°F in Peoria, Illinois, so the "normal" mean temperature for today is 27°F.  Naturally, it's unusual to experience both the average high and low on any given day, so comparing the mean is helpful in determining whether the weather has been warmer or colder than average.

Figure 1 below shows the daily temperature deviation from average since the beginning of winter on December 21.  The blue bars represent the degrees F above or below the daily average, while the red curve is the cumulative deviation from the average over the same period.  Of the 54 days recorded so far, only 17 have been warmer than the historic average, and of the 35 colder days, 25 are 10 or more degrees colder than their average.  January 6 deserves special recognition for recording a mean temperature 31 degrees colder than the historical average!  The mean that day was -9°F, and the high for the day reached -5°F, but alas, that was not the coldest January 6 on record.

Figure 1: December 21 through February 12

The cumulative deviation from average briefly wandered into positive territory twice in December, but has been solidly colder than the average ever since.  Currently, our winter is a cumulative 339 degrees colder than average, which means the daily mean temperature has averaged over 6 degrees colder than average. Since the beginning of 2014, we've averaged over 7 degrees colder than average, and the month of February is averaging over 16 degrees colder than average per day even though February 1 was warmer than the historical average!

Figure 2: September 21 through February 12

In some respects, the entire heating season since last September looks warmer than just the winter data, primarily due to the warm weather from the end of September to the middle of October.  The cumulative deviation is even greater over this period than over the winter period, however, which means our fall season was actually colder than average despite the warm start.  Our 362 degree cumulative deviation works out to about two and a half degrees colder than average per day for the 145 days of fall and winter so far.

So, back to the question in the title, has this been a normal winter?  Yes, it has been normal in the sense that we've had some days warmer than average, and some days colder than average.  But no, it has not been normal in the sense that the temperatures have averaged much colder than the historical average so far. Winter isn't finished, however, as we have 36 more days until spring and a warming trend in the forecast. In order to finish the winter even with the historical average, each of the remaining 36 days would have to average over 9 degrees warmer than average. Plenty of people around here would welcome a trend like that!

(Editor's note: a variation of the word "average" appears 38 times in this post)


  1. You are a geek, my friend. That's just one thing I appreciate about you.

  2. I can judge how cold our winters are by how often I have to wear a coat. Last winter, I had to wear one three times, this winter I have had to wear one more than 10 times. So we are having a colder winter than normal. My system is far less scientific, but it works for me. :)
    Stay warm up there.


  3. Thanks, James. Back at ya!

    Victoria, having to wear one's coat only three times in a winter is inconceivable to me.

  4. I'm just.... I'm just.... falling asleep.