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A Hot Topic: Graphing Temperature Trends

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Statistically, it's growing warmer.

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Recently, as I was working with a spreadsheet that has all of my weather forecasts for The Old Farmer鈥檚 蜜桃恋人 for the 23 years that I have been making them, it occurred to me that here was a database that contains a great deal of information that might be useful in looking at trends.

To do this, I moved the data around within the spreadsheet to be able to create graphs showing how temperatures have changed over the past 23 years. These graphs show some interesting trends that seem to fit, at least in a general sense, with the idea that it鈥檚 growing warmer. 

Above-Normal Temperature 鈥淣ormals鈥

Let me explain these graphs. As you may realize, each year we forecast the coming year鈥檚 weather for 18 United States regions and seven Canadian ones. The maps of these regions that appear in The Old Farmer鈥檚 蜜桃恋人 print editions and on 蜜桃恋人.com show several cities within each; it is these cities for which I have compiled data on actual temperatures and how they have departed from normal. 

For example, when I say that the temperature departure for December 2018 was 2.61 degrees F (see Figure 1, below), this means that the average of all of the cities shown on all of these regional maps for the United States and Canada was 2.61 degrees F warmer than their most recent official 30-year normals, which cover the period of 1981鈥2010. The 鈥渘ormal temperatures鈥 that I use are the official ones updated once each decade by the United States and Canadian government meteorological services, based upon smoothed averages of daily temperatures for the 30-year period covered.

Figure 1: Monthly temperature departures from normal, 1996鈥2018
Figure 1. Monthly temperature departures from normal, 1996鈥2018.

Thus, if I say that the 2019 average temperature is 1 degree above normal, this temperature is going to be different from what it would be if I were referring to the 2009 average temperature being 1 degree above normal, because over the ensuing decade the 鈥渘ormal鈥 temperatures would have been recalculated and changed. 

Since temperatures generally have been rising, this means that each decade, when the new 鈥渘ormal鈥 temperatures are computed, they are higher than the previous 鈥渘ormal鈥 temperatures. The effect of this is that a graph that shows how temperatures have departed from normal over the years actually understates the real increase in temperatures, since the normals are rising along with the actual temperatures.

When we examine the graph showing the monthly departure of temperature from normal (Figure 1, above), we see that most鈥攂ut not all鈥攎onths in the past 23 years have had temperatures above normal. However, there does not seem to be any clear trend in these temperature departures, or at least not one that I can readily see. The greatest positive departure was 8.67 degrees F back in January 2006, while the greatest negative departure was 4.78 degrees F in December 2000.

12-month_moving_average_departures_from_normal_large_full_width.jpg
Figure 2. 12-month moving average of monthly temperature departures from normal, 1996鈥2018.

When we look at the 12-month moving average of these departures (Figure 2, above), their pattern becomes more apparent, as there does seem to be a general rising trend in the temperature departures, with nearly all above normal. The only times when this 12-month moving average of temperature departures for the United States and Canada were below normal were in 2001 and 2014.

36-month_moving_average_departures_from_normal_large_full_width.jpg
Figure 3. 36-month moving average of monthly temperature departures from normal, 1996鈥2018.

Examining the 36-month moving average of temperature departures (Figure 3, above) seems to show a trend emerging. Although there are still peaks and valleys, a general upward trend seems more apparent, with each peak higher than the previous one.

What Causes these Changes?

Meteorologists and climatologists have proposed various explanations for the month-to-month and annual variations, including teleconnections such as El Ni帽o, changes in ocean currents, and changes in solar output. 

As for the clear trend of rising temperatures over the past decades, I have seen only three viable proposed explanations: (1) natural variability (perhaps combined with changes in land use and population), (2) an expected general rise in average temperatures as we are still coming out of the last ice age, and (3) warming temperatures due to human activities that increase the levels of carbon dioxide and other atmospheric greenhouse gases.

Find predicted temperatures for the upcoming year in .

About The Author

Michael Steinberg

Mike Steinberg is Senior Vice President for Special Initiatives at Accu蜜桃恋人 Inc. in State College, Pennsylvania. Read More from Michael Steinberg
 

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