Atlanta was a mess on January 28th, 2014. Thousands were trapped on the roads overnight while others managed to get to roadside stores to camp out. Thousands of students were forced to spend the night in their schools and the National Guard was called in to get them home. Many wondered how less than three inches of snow could cripple the city, particularly when Atlanta had experienced a similar storm in 2011?
This traumatic event, the recollection of recent snow storms, and now the current storm prompted some to wonder whether Atlanta has been experiencing more cold and snow than before. How unusual was that January day?
To answer this question, I'll use data from NOAA’s National Climatic Data Center. This long-term data set allows us to put the recent snowy patch into a historical perspective. You can get the NOAA data in this Minitab project file.
The data are from Atlanta's Hartsfield International Airport and range from 1930 to the present. That’s 84 winters! I’ll use Minitab’s time series plots to determine whether the temperatures and snowfall have changed over time.
Low Temperatures in Atlanta
Let's take a look at the temperatures and see how the current winter compare to previous winters. I'll look at both the average low temperature and the extreme low temperature for all months in Atlanta. The green circles represent January 2014.
In the time series plot above, you can see that while January 2014 is on the low end of the regular distribution of temperature data, it's not an unprecedented value. In fact, you only have to go back three years to find a month that is within several degrees. The average monthly lows have not gotten lower over the decades, but have been stable over time.
The extreme low temperature time series plot below tells a slightly different story.
While the average lows follow a consistent pattern since 1930, the extreme lows are slightly different. It appears that up to January 1985 (the purple circle), there were a number of months that had more extreme cold than any month after 1985. In other words, the monthly averages are stable, but Atlanta has avoided the most extreme cold for nearly 30 years.
Snowfall in Atlanta
This time series plot shows Atlanta's total monthly snowfall. January 2014 doesn't stand out as an usual snow month. In fact, you can see three recent months that had more snow! There have been plenty of far worse snow storms. January 1940 had three times the snowfall with 8.3 inches! There is no increasing trend in snowfall.
Why We Need Data
The weather data show how imperfect memories can be. It turns out that Atlanta's winters are no colder or snowier in the recent past than they were decades ago. In fact, 1930-1985 saw more extreme cold temperatures, while the averages stay constant. January's snowfall was not unusual.
Human memory is too imperfect to reliably assess long-term trends. Memories fade, change, and can be selectively retained or forgotten. On top of that, not everyone has experienced Atlanta weather long enough to know what’s normal.
Analyzing trustworthy data gives you the ability to avoid subjective assessment based on imperfect memories. Minitab statistical software and data provide an unbeatable combination that can answer almost any question.