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By Erik Hare

December is a special time of year, as people all over the nation get themselves into the Holiday spirit.  Minnesota is no exception, but we have our own traditions that come from the topic we love to hate the most:  the weather.  Sure, everyone talks about it but no one actually does anything about it.  We are wallowing in below zero Fahrenheit nights right now, struggling to keep warm as winter closes in around us.  But what is less well known is that what we are experiencing is perfectly “normal” because this is a special time of year for the great Minnesota winter as well.

One of the many special things about Minnesota is that records of temperature have been kept continuously since Fort Snelling opened in 1820.  We know exactly what the pioneers faced in those long, dark winters without central heat or cars to move them around.  This mountain of data is useful not only for reminiscing, however, but also helps us to understand just what we can expect today.

The chart below is from an excellent site of many charts and graphs that explain just about everything you might every want to know about Minnesota weather.  They are provided by Meteorologist Charles Fisk, who graciously allowed them to be used here.  The first chart shows, in dark bands, the average high and low by date, with the record high in red and the record low in blue.

If you look at it closely, the temperature slowly but steadily moves down from its high on July 28th to a low point right around early December.  From there, things are rather constant into the beginning of February.  Winter really does come on suddenly in December, and holds on tightly until after Winter Carnival.  This is the time of year when we can expect the icy grip that we are experiencing now, and it’s been that way at least since the land was first settled by Europeans.

But there’s more to the story than that.  If you look carefully, you can see that the average temperature we should expect is about 20F.  Some people may call that “normal”, but that is incorrect.  If we look at the data compiled over the last 187 years we can see that some years are colder than others.  When you’re in the middle of a big continent, you have to expect some variation as winter moves in earlier some years than others.

That’s where the next graph comes in, the standard deviation in temperature from the average.  This is nothing more than a measure of the variability in what you can expect on any given date.

Not only is the end of July the warmest time of the year, it’s also the least variable.  While we all know the temperature usually takes a wintry nosedive in early December, the variability spikes up as well.  Using normal statistics to these numbers, we know that for a large set of data for any date (which we have) that you can expect the temperature to be plus or minus 1.96 times the standard deviation 19 years out of 20 – the best definition of “normal” you can have.  That means that while the average temperature in early December is about 20F, we can expect anything from about –5F to 45F as a perfectly “normal” temperature at this time.

Yes, that means that 19 years out of 20 we’re going to have weather just like we are having right now – killer cold and all.  It’s all perfectly “normal” in Minnesota.

That’s the real story of Minnesota weather, and why we tend to talk about it as much as we do.  It’s not just the cold, which we expect, but the long history that tells us that we should expect just about anything.  That may not sit well with everyone, but it’s true.  December is a special time in Minnesota, and like the Holidays you should be ready for just about anything in your stocking.  This year, we got a lump of coal.  Hey, it happens!