What happened to hot June and July days?

Unnoticed in a media environment that treats any anomalous weather event as evidence of global warming is that fact that the frequency of extremely hot days during June and July in Kentucky and Tennessee since 1999 is less than at any time in the past 100 years.

Using data from Bowling Green (BWG), Louisville (SDF), Lexington (LEX), and Nashville (BNA) I found that the occurrence of days with a high temperature >95F (an extremely hot day in my book) during the months of June and July has fallen dramatically since the hot summer of 1999. The following table shows the number of days hotter than 95F by decade for the four stations through their common period of record (1960-current).

Decade | BNA | BWG | SDF | LEX
1960s | 42 | 31 | 7 | 5
1970s | 21 | 39 | 9 | 1
1980s | 65 | 44 | 33 | 23
1990s | 37 | 50 | 37 | 13
2000s* | 10 | 5 | 7 | 1

*Through 2007

Even more remarkable is the fact that the first half of the 20th century was much more likely to experience extremely hot temperatures during June and July. Here are the numbers of days >95F for Bowling Green (BWG) since 1900.

Bowling Green
1900s | 119
1910s | 147
1920s | 137
1930s | 163
1940s | 72
1950s | 78
1960s | 31
1970s | 39
1980s | 44
1990s | 50
2000s* | 5

* Through 2007

The 40-year period from 1900-1939 was nearly 3 times as likely to experience a day with a high temperature > 95F than the recent 40-year period from 1960-1999.

I want to make clear that this analysis does not suggest that global warming is not occurring. Quite the contrary, as the evidence presented in the latest IPCC report provides an overwhelming argument that global warming has occurred and is primarily caused by human activities. What is important to note is that of all land areas, the Southeastern U.S. has shown among the least amount of warming over the past 100 years and can expect a relatively small amount of projected warming in the next couple of decades. This analysis fits nicely with the idea that the climate of the Mid-South is not changing nearly as much as most places.

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