What's Happened to January?
Our coldest month -- an average of more than three degrees colder than February, and almost five degrees colder than December -- has been mostly a nonperformer of late. The DC area, as measured at Reagan National Airport, is currently facing its fourth consecutive January of above-average temperatures, and the 16th warmer-than-average January out of the last 20.
Why has our coldest month been a dud? Or has it? The devil is in the details.
Official monthly (and daily) averages are based on 30 years of data, which currently spans the period from 1971 to 2000. (Averages are updated every 10 years; starting in 2011 the 30-year period will shift forward to 1981 to 2010.) So, the average January temperature is simply calculated by adding up the average temperatures for all 30 Januarys and dividing by 30, right? Well, not exactly.
It turns out the averages are smoothed, such that data from unusually warm January days -- like Jan. 7 and 8 of the current month, which saw highs at Reagan National reach 69 and 73, respectively -- are adjusted so the daily average meshes more closely with surrounding daily averages. For example, let's say the average high for Jan. 15 over the 30-year period is 43 and the average low is 27. And that for the next day, Jan. 16, the average high is 48 and average low is 30 (a significant difference for two consecutive days), because it just so happened that unusual warmth struck on that date a couple times over the 30-year period. In this case, the data for Jan. 16 would be tweaked downward to fit better with the surrounding data.
Since January temperatures tend to be unusually high more often than they are unusually low, the official January average is a bit colder than the "actual" average. Officially, our 1971-2000 average temperature for January is 34.9 degrees. Yet if we calculated the true average for the 30 Januarys over that period, the result would be about a degree higher. Smoothing has made the official January average colder than it really is. What does all this mean for our current streak of above-normal Januarys?
Januarys that came in officially at slightly above average (e.g., 2000, 2001, 2005) are actually right around the true 30-year average, and perhaps slightly below. While there is no question that January has skewed warmer rather than colder in the last 20 years, the trend is not as significant as it looks. So when the weather pundits declare "another warm January," we need to assess how warm. Because calculating average temperatures isn't as straighforward as it may first seem.
Of course, this January will likely finish four to five degrees above average. Warm by anyone's standards.
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