Over a month ago, this blog addressed the question, "Was last winter unusually cold?" and in the process we also wondered whether 2009 was colder than average in Peoria, Illinois. When we were living through the weather of 2009, the year seemed colder than normal to us: we received 50% more precipitation than average that year, which probably meant more cloudy days, and a general feeling that more days were colder than average than warmer. Not one to be satisfied with subjective feelings in matters like this, I dug up the data to uncover the truth about 2009.
The vertical bars on the chart below indicate daily temperature deviations. If the actual average temperature was warmer than the historic average for that date, the bar projects upward indicating a positive temperature difference, and vice versa for colder. You can count the bars on the chart, or you can trust me that there were 167 days warmer than average, 178 days colder than average, and 20 days in which the deviation from the historic average was zero degrees. By that measure, there were more days colder than average than days that were warmer than average. But since a day that was 1 degree colder than average counts as much as a day that was 30 degrees warmer than average, does that really tell the whole story?
Another way to look at the data is to look at the cumulative degrees above or below average. That data is reflected in the purple curve that wanders above and below the 0°F line in the chart above. As you can tell, the curve ends the year on the right side of the chart in positive territory -- 42 degrees above average to be exact. Dividing that number by 365 days tells us that each day in 2009 was, on average, 0.12 degrees above the historic average. That may be a small enough number to be statistically insignificant, or at the very least, indiscernible for most people.
So, was 2009 colder than the historic average? That, like most conclusions about weather trends these days, depends on which criteria you use to make the decision. Thanks for visiting, and thanks to Weather Underground for the historic data.