July 2012 Hottest Ever in the U.S.? Hmmm….I Doubt It

August 8th, 2012 by Roy W. Spencer, Ph. D.

Using NCDC’s own data (USHCN, Version 2), and computing area averages for the last 100 years of Julys over the 48 contiguous states, here’s what I get for the daily High temps, Low temps, and daily Averages (click for large version):

As far as daily HIGH temperatures go, 1936 was the clear winner. But because daily LOW temperatures have risen so much, the daily AVERAGE July temperature in 2012 barely edged out 1936.

Now, of course, we have that nagging issue of just how much urban heat island (UHI) effect remains in the data. The NCDC “homogenization” procedures are not really meant to handle long-term UHI warming, which has probably occurred at most of the 1218 stations used in the above plot.

Also, minimum temperatures are much more influenced by wind conditions and other factors near the surface…Max temperatures give a much better idea of how warm an air mass is over a deep layer.

Also, I thought one month doesn’t make a climate trend? If we look at the 5-year running mean of the daily averages for July’s over the last 100 years, we see that while recent Julys have indeed been warm, it is questionable whether they rival the 1930s:

And if we do the same 5-year averaging on July maximum temperatures, the 1930s were obviously warmer:

So, all things considered (including unresolved issues about urban heat island effects and other large corrections made to the USHCN data), I would say July was unusually warm. But the long-term integrity of the USHCN dataset depends upon so many uncertain factors, I would say it’s a stretch to to call July 2012 a “record”.

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