Saturday, April 18, 2015

Central Illinois Winter Temperature Summary, 2014-2015

Now that winter is officially finished, we can look back at the temperature data and compare reality to what we remembered. I think everyone here in Central Illinois perceived our latest winter to be warmer than the one before it, but prolonged cold (and warm) spells have a way of making comparisons difficult from year to year. The data is presented again this year in the form of combined bar and line charts. The blue bars represent the deviation of the daily mean temperature from the historical average where each bar represents one day. The red curve represents the cumulative deviation from the historical average, and indicates whether the season as a whole was colder or warmer than what we'd typically expect.

The first observation I'd like to bring to your attention is the value of the red curve at the right side of the chart. Including data from March 20, the cumulative deviation from average was -3 degrees. That's almost as close to "normal" as one can get. That means, on average, the daily mean temperature was about 0.03°F colder than the historical average each day. That's a temperature difference most of us can't perceive. It's also probably a smaller number than the error in the measuring equipment at the National Weather Service in Peoria, not to mention the fact that the daily mean temperatures I use are reported as integers that have already been rounded a fraction of a degree each day.

It's also interesting to note that the winter as a whole was warmer than normal until the last week of February, indicated by the red curve crossing the x-axis into negative territory on February 26. February 27 added momentum to that move by recording the largest deviation from average of 28 degrees F below the historical mean. The deviation of 23 degrees F above normal on March 23 recorded the largest deviation on the warm side.

By comparison, we can look again at the data from the winter of 2013-2014 and notice that the cumulative temperature that winter was above normal only a few days all season. I think most of us are happy not to repeat those temperatures this year.

To illustrate just how unusual our temperatures were last winter, I've plotted the cumulative deviation curves for both winters on the same scale on this chart. In both cases, February contributed the largest cumulative degrees below normal, but this year a significant warming trend began on March 7 that carried through the end of the season. Another significant difference this winter was the 18 consecutive days of warmer than normal temperatures beginning on January 15. Those days averaged 10.5 degrees above normal and contributed greatly to counteracting the colder than normal periods.

Since our historical mean temperatures are still low enough to expect heating degree days this time of year, we are technically still in our heating season. We'll continue to compile data to see whether this heating season as a whole was warmer or colder than average.


  1. I thought the winter was cold.

  2. Yes, Kathryn, even though the temperatures here were near the historical average, that doesn't mean it wasn't cold.