We’re in the midst of our third stretch of sustained below-average temperatures this month…in fact, July of 2014 will likely be one of the 20 coolest on record. (The Nashville record book extends back to 1871, so that’s significant.) All in all, while there have been some hot and humid days, and some strong thunderstorms here and there, the weather this month has been pretty great.





Because no one can ever be satisfied with enjoying pleasant weather while it’s here, I’ve been getting a LOT of questions about what our current cool weather portends for next winter’s weather. (Yes, I used the word “portends.” Don’t judge.)



My experience a meteorologist tells me that short-term weather patterns are rarely reliable indicators of long-term climate trends…



…but any halfway-decent scientist wants proof. So, I dove into the record books to look for answers. First, here are the numbers I’ll be referring back to throughout the rest of this post. These are the 40 coolest Julys on record, along with the average December-January-February temperatures for the following winter (and their overall rank in the 143 years of Nashville records):



A few things of interest within those numbers:

1) The “normal” winter temperature over the last 143 years is 40.4 degrees. The 20 coolest Julys on record were followed by 13 colder-than-average winters…leaving 7 as above-average.

2) Of the 20 coolest Julys in Nashville, 6 were followed by Top-20-coldest winters.

3) Expand that a bit, and 10 were followed by Top-40-coldest winters.

4) Looking at the flip side, of the 20 coolest Julys, only 2 subsequent winters ranked in the Top 20 warmest.

5) The Top 20 coolest Julys were followed by a Top-40-warmest winter only 5 times.

So based on those facts, we can reasonably expect below-normal temperatures next winter, right?



Those five facts above are fun bits of trivia, but they’re picked from a small sample size…they’re not the entire picture. If you want to definitively state that there is a direct correlation between July temperatures and following-winter temperatures, you have to look at all the data, and plot every July temperature against every winter temperature. If there’s a direct relationship, the chart would look like this:



Here’s what the chart really looks like for July vs. Winter temperatures:



Quickly glance at that, and you probably notice the line through the dots — that’s the “best-fit” line that attempts to summarize all the data points in a straight line. “AHA!” you say, “that line goes UP! Therefore, a cooler July equals a colder winter, and a hotter July equals a warmer winter!” And Moriarty says…



There’s a way to measure the accuracy of that “best-fit” line…it’s called the “Coefficient of Determination,” and it’s abbreviated as R-squared. (Why is it R-squared instead of CD? Why isn’t it called something simpler? These are questions I asked my statistics professor in graduate school, and the answers were…unenlightening.)



Anyway…that “Coefficient of Determination” tells us whether the best-fit line does a good job of summarizing the data, or whether it’s just the software doing it’s best to pound a square peg into a round hole. An R-squared value of exactly 1 means that it’s a perfect fit, while values closer to zero mean that there’s really no relationship between the two things you’re trying to compare.

For July temperatures vs. Winter temperatures, the R-squared value is 0.019. That’s really really low, and tells us that with almost 150 years of data, there is virtually no predictive relationship between temperatures in July and temperatures the following winter.