Winter is Here

The arrival of December marks the beginning of climatological winter (astronomical/calendrical winter begins Deceber 21st this year). We in the USA have seen some cold winters recently, especially in the northern midsection of the country.


I was looking at temperature trends in the different climate divisions of the U.S., particularly trends in winter temperature since 1970 (basically the last 50 years).


Red dots mark regions which are warming up, blue dots regions which have been cooling down. There aren’t any blue dots; the whole country has warmed up, although the trend has been slight in the western USA (and not everywhere statistically significant). We can get a better picture of the differences between different regions, by plotting not the warming rate itself, but its difference from average. Now, red dots are regions warming faster than average, blue dots regions warming more slowly than average:

The strongest warming was in the northeast and the northern midwest, the weakest in the northwest.

I also decided to look at short-term “trends.” They aren’t really trends (nowhere near statistical significance) but do tell us about more short-term fluctuations. So I estimated “trends” over a mere 20 years (since the year 2000) and found this:

Now we see actual cooling in the upper midwest, along with warming in the southeast (I’ll emphasize that these are fluctuations, we can’t call them actual trends). I even estimated “trends” over the last decade, since 2010:

Now we see no sign of cooling in the upper midwest, but very strong warming in the southeast.

Perhaps it’s because the upper-midwest cooling happened from the 2000s to the 2010s, while the southeastern warming was mostly during the 2010s. So, I looked at the difference between the average temperature during the 2000s (2000 through 2009) and during the 2010s (2010 through 2019):

And now we get to the crazy idea behind this post: perhaps this pattern (the difference from the 2000s to the 2010s) is really an imprint of an increased tendency toward incursions of the “polar vortex.” I say that because when I’ve seen stories about the polar vortex over the last several years, they seem concentrated on that region of the upper midwest.

It’s only speculation, and very speculative speculation at that. But it’s interesting…


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6 responses to “Winter is Here

  1. Reblogged this on Don't look now and commented:
    The OpenMind blog gives some support to the idea that Arctic weather is escaping to bother us with dangerous. Looks like Jennifer Francis is right. See Why Climate Change is Bringing the Polar Vortex South
    https://www.discovermagazine.com/environment/why-climate-change-is-bringing-the-polar-vortex-south

  2. David B. Benson

    Being that blue dot in southeastern Washington state, subjectively no! There haven’t been colder winters, on average, in the last decade.

    But I am not a thermometer…

  3. Interesting, I’d never heard of winter being anything other than DJF in the Northern Hemisphere/JJA in the Southern Hemisphere!

    • FWIW, I was always taught that the seasons begin on the relevant astronomical marker–the equinoxes for spring and fall, and the solstices for winter and summer. (Though I think the primary school version simplified that to the 21st of each respective month.)

      But yes, that’s not strictly compatible with December normally being considered a (NH) “winter month” even though 3/4s of its days fall in, well, fall. Conversely, March is usually thought of (hereabouts) as a “spring month,” even though it mostly falls within the purview of winter according to the criterion in the first paragraph above. Chalk it all up to the analog squishiness of natural language and usage, I guess.

      • @Doc Snow,

        And don’t even start with the astronomical complications of getting the Sun into the mix! That gets complicated. And, here, I’d say, empirical evidence is not much help. The trouble is the support for a good sample is too large … You need to go to a lot of latitudes at different times of year. This is a geometry problem.

        And, if a simple measure that seems plausible is tried, for example, the insolation we get on our solar panels, that’s highly misleading. While configurations of sources might be similar in September and March equinoxes, we’ll see a lot more illumination in March. It turns out that’s because (a) we have deciduous trees all about us, and in March all their leaves are gone, and (b) all our solar panels are oriented slightly away from major compass points and, indeed, 20 of them are faced most East-West with 14 facing ENE and 6 facing WSW. This is deliberate: the 14 pick up early morning leafless illumination in March and the 6 pick up late day illumination in late Spring through Summer, as well as Summer peaks.

        Yeah, that’s off topic …

      • EQ–“You need to go to a lot of latitudes at different times of year.”

        As a Canadian from Snow Ste. Marie, er, Sault Ste. Marie transplanted to South Carolina, all I can say is “Tell me about it!”

        In my old life, March was pretty consistently a winter month. Now, February tends to mark the coming of spring.

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