Wilder Temperature Swings Lately?

Several people have mentioned to me how cold it has been this October.  And it has.  Assuming we reach a high this afternoon of 70, our average temperature for October was 61.2, about 4 degrees below the normal of 65.3.  (These temperatures average the daily highs and lows).

Then again, this summer was hotter than normal.  Our average temperature in June through August was 83, 4.7 degrees above the normal 78.3.  We had this last year too, with a hot summer followed by a cold winter.  But are our temperatures really that much wilder than normal?

To do that, I ran some statistics.  Below you’ll see the average daily temperature for BHM every day since 1/1/2009.

Each dot represents one day, and the blue line is the normal.  Since there is so much scatter in these dots, below is a 10-day moving average of the actual temperature, compared to normal temperatures.

This chart shows something interesting.  In 2009, temperatures oscillated around some in the winter (below and above normal), but averaged out near normal.  Same for spring, summer, and fall 2009.   Winter 2010 is when things started getting wild.  Remember the first 2 weeks of Jan 2010?  Multiple days with highs below 32 and lows near 10.  That was a cold winter, and you can see that as most of the green dots are below the blue line showing normal.  This past winter was cold, too, with our first White Christmas (even though the airport didn’t measure it) and several other snow and ice events.

But, between those two cold winters we had a hot summer in 2010, with most of the green dots above the blue normal line, meaning hotter than normal.  This year, our spring and summer were above normal (severe weather in spring, then hot summer).  And now, we are already running below normal for the fall.  This hasn’t happened before the past 2 cold winters.  Does that mean it will be even colder, as the hemisphere is ahead of schedule on cooling down, or does it mean the Arctic is keeping the cold air moving south more efficiently this year so it can’t build up for the winter?  I don’t know.

The best way to measure the swings in temperature across the year are to look at the 365-day standard deviation of temperature.  In years with hot summers and cold winters, the deviation will be large, and in mild years, the deviation will be small.

The average standard deviation since 1970 has been 15.2 degrees.  We had some wild temperature swings in the late 1970’s (it all started when Spann went on TV in BHM!), more tame years overall in the late 1980’s, but then you see our big jump again lately.  Does all of this mean anything?  I don’t know that either.  But please don’t blame it on global warming, since it has only been going on for 2 years, and it has happened before.

Category: Met 101/Weather History