Thursday, April 22, 2010

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!


  1. If I wasn't already in awe of your awesome smartness, I would be now... cuz you are. Awesomely smart, I mean. yeah.


  2. Ok... so I am confused... (but not, I believe anyway, because I don't understand your graphs etc.... but, maybe I'm wrong, and I really, really don't understand the graphs :-)

    I did enjoy reading about HDD and the mean/average temps etc, and the graphs as well! But I don't understand two things:

    First, why it was started in August instead of the beginning of winter?

    Second, why it is you used HDD instead of the historical mean temperature for Peoria, and then compared your data to that? Especially if the purpose of the study was to find out whether or not the weather was 'unusually' cold. (and I presume unusually is referring to historically unusual, but correct me if I'm wrong)

    Or perhaps there was going to be another post comparing the historical data to the present data?

    Really enjoyed the post though!

  3. Thanks for the compliment Diane, but since you don't see my errors every day I won't burst your bubble...

    Emil -- first, the chart was started in August since we started recording consecutive HDD at the end of that month last year. So, in my opinion, the heating season began with those "cold" days.

    Second, the charts really do reflect deviations from mean temperatures but only to the extent that a HDD was recorded for that day. In other words, if the actual mean temperature was colder than the average mean, but the actual was 65 or above, it was ignored on the second chart since it didn't affect heating requirements.

    I have also tracked the deviation from mean that you refer to, and over the past year (all seasons included) our cumulative difference from the mean is about 208 degrees above average. That total turned positive on about March 30.

  4. Let me guess..... the second paragraph was written to my sister. :-)

  5. I took your advice and stopped reading before I hit graph #1. I am not going to leave any negative, "I'm bored" comments (be proud of me). I would like to inquire, however... where do you find the time to do all this extensive research??? It seems to me that in your spare time you would want to do something relaxing. Then again, maybe I just answered my own question and you somehow find this relaxing?

  6. OK, now I get it! Thanks!

    Using the deviation from HDD instead of a raw deviation from average temperatures seems to have a more direct practical application anyway, due to how it ties directly into how you need to heat your house etc.

    Global warming occurring in Peoria? :-) Or maybe Climate Change would be a better name for it? (what a thought... the weather/temp. actually changes!!! ;-)

  7. Tara -- was it that obvious?

    Kathryn -- I am proud of you! This project started when I offered to try to find a correlation between daily temperatures and how often my friend's heater runs in his garage. I spent quite a few lunch hours at work gathering historical temperature data and putting the graphs together. I wouldn't call it relaxing as much as satisfying to be able to understand something better after analyzing the data.

    Emil -- I don't have all the data yet with which to draw a conclusion, but it may be interesting to see what the cumulative deviation from the means is for the calendar year 2009 since that one "seemed" colder than average.

  8. Well.... the other thing you might want to take into consideration is the dew point, because that can have a very large impact upon how cold/warm the weather 'feels'. And, if I remember correctly, 2009 was a very wet year, which may have contributed to how that year felt colder on average.