Was last winter unusually cold?

I've heard several people around our area comment about how cold our last winter was. When one remembers the days in January when the high temperature was barely above 0°F, or the 95 days we spent below 50°F, it seems like the fair-weather crowd has really suffered. Compared to the historic averages for Peoria, Illinois, has it been colder than average? No.

For those readers that are averse to data, graphs, math, and any technical tools that enable us to actually understand our surroundings, I advise you to stop reading right now. You will only be bored and confused, and will likely leave comments about how bored and confused you are. Leave now after you leave happy comments about how warm the weather is. The rest of us will examine the data that led me to the conclusion that our winter in Peoria was warmer than typical.


This first chart shows average mean temperatures for our area from the beginning of August 2009 to the present. That data is represented by the blue curve. The red curve represents the actual mean temperatures we've experienced over that period. When the red curve was lower than the blue curve, the mean temperature was lower than average, while means that are warmer than average are represented by the data that shows the red curve above the blue one. The mean temperature is the average of the daily high and low temperatures, regardless of what time of day those extremes occurred or how long the temperature lingered near those values.

An overview of the chart shows a period at the beginning of October when the mean was well below average followed by about a month and a half that was almost always warmer than average. Likewise there were stretches in January and February that were consistently colder than average, and significant periods in March and April that were warmer than average. How can we tell whether the season as a whole has been colder or warmer than typical?

Answer: the Heating Degree Day. Heating degree days (HDD) are defined relative to a base temperature, and give an indication of how much energy would be required to heat a building on any given day. A common base temperature is 65°F, although almost any base would give you the same kind of relative measure. Given the base temperature of 65°F, our heating season would not technically begin until the average mean temperature drops below 65°F. For Peoria, that condition does not occur until about September 19. The chart below shows data from August 2009 since temperatures were cold enough at the end of that month to actually record HDD at that time. In terms of HDD, our heating season is still ongoing, and won't officially end until about the middle of May. Even though we haven't used our furnace the last few weeks, our historical average mean temperature is still below 65°F.

The purple bars on the chart indicate the difference between the actual HDD and average HDD for each day. The positive bars on the chart indicate the mean temperature was colder than average, and the negative bars indicate mean temperatures warmer than average. The orange curve is the cumulative difference, and is an indication of whether the season to date has required more or less heating energy than average. As you can see, we are over 200 HDD warmer than average for this time of year, although as recently as March 18 our heating season was colder than average.

At this point, a good cold snap in May could bring us right back in line with an average heating season, but I don't know many people that are wishing for that. Thanks for visiting!