Thursday, August 9, 2012

A Hot July for the US

The National Climatic Data Center reports that this July was the hottest in the instrumented record (1895–present) in the contiguous US, nudging 1936.  Mean July temperatures were more than 6ºF above the 1981–2010 average across much of the interior central United States.

Source: NCDC
In case you are wondering, the mean temperature was 77.56ºF, whereas in July 1936 it was 77.43ºF. Other years with exceptionally high July mean temperatures include 2011, 2006, 1934, and 1901.

Source: NCDC
Before his passing in 2007, Tom Potter, the former Director of the Western Region of the National Weather Service and Weather Coordinator for the 2002 Olympic Winter Games in Salt Lake told me about surviving the 1936 heat wave.  As he put it
"We lived in North Dakota, so it only got to '117ºF in the shade' on the hottest day in our small town. No one had air conditioning in those days, so we all went down in the cellar and my Dad rigged up a homemade evaporative cooler by training a small fan on a wet blanket hung across the cellar doorway. We all survived and so did everyone else among the town's population of 307."
Thanks to rapid advances in our ability to squeeze everything we can out of weather observations, we now have four-times-daily atmospheric analyses extending back to 1871.  These are provided by the 20th Century Reanalysis Project and enable us to compare July 1936 with July 2012, with the caveat that there are greater uncertainties, especially at upper levels, in the July 1936 analysis.

Perhaps not surprisingly, the upper-air pattern over the contiguous US is quite similar.  In July 1936, a broad, high amplitude, 500-mb ridge covered much of the contiguous US, with the Pacific Northwest and Northeast somewhat under the influence of troughing.

A similar pattern occurred this July, although it could be argued that the pattern was somewhat less amplified along the US–Canada border where the ridge over central North America and flanking troughs were a bit weaker.  These minor differences might not lie outside the range of observational uncertainties in the July 1936 analysis.
I think back to Tom Potter's comments to me about his experience in July 1936 and how difficult that month must have been for those who lived in the midwest.  It may not hold the official record, but given the technology at the time, it must have been one of the most miserable months in US history.


  1. What is the scientific explanation for the cool period at the beginning of the 1900's?

    Anomaly? Are the record high temps this year an anomaly, or can they without a doubt be pinned to a specific cause?

    1. The word "anomaly" is often used by scientists to describe the departure from a long-term average. Given that this July was the warmest observed for the contiguous US, it might best be described as "unusual." 1936 is probably the closest analog.

      In part, the high frequency of cool years at the beginning of the 20th century and warm years at the end of the 20th century reflects climate change. Explaining the year-to-year and decade-to-decade variations, however, is a bit more difficult and not something that I'm prepared to answer.