In July 2012, the US National Oceanic and Atmospheric Administration (NOAA) reported.

The average temperature for the contiguous U.S. during July was 77.6°F, 3.3°F above the 20th century average, marking the warmest July and all-time warmest month on record for the nation in a period of record that dates back to 1895. The previous warmest July for the nation was July 1936, when the average U.S. temperature was 77.4°F.:

The Daily Caller reports on a blog article by Anthony Watts that claimed that the historical figures reported by the NOAA have been changed in the meantime, without explanation, reinstating July 1936 as the hottest month.

Watts ran the same data plot again on Sunday and found that NOAA inserted a new number in for July 1936. The average temperature for July 1936 was made slightly higher than July 2012, meaning, once again, July 1936 is the hottest year (sic) on record. [Annotations in the graph are from Watts]

and provides these graphs:

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enter image description here

Is it true that NOAA now reports July 1936 as the hottest (July) month on record? If so, why was this changed?

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    Hmm... The graphs are actually significantly different. Notice the delta in average temperature and then if you look at individual points, they all seem shifted by some amount. (The linked-to article claims fraud on the part of NOAA.) – Larry OBrien Jun 30 '14 at 23:42
  • I've changed the language while trying to stay true to the controversy. Is that better? – Oddthinking Jul 1 '14 at 0:18

Just to add to @Henry's answer (+1): If you go to the NOAA webpage for downloading the data, at the top of the page it says:

NCDC transitioned to the nClimDiv dataset on Thursday, March 13, 2014. This was coincident with the release of the February 2014 monthly monitoring report. For details on this transition, please visit our public FTP site and our U.S. Climate Divisional Database site.

which suggests that the reason for the change in record is due to a change in the dataset (set of stations) used to compute the mean temperature, and that the change was announced and clearly indicated to those downloading the data. It is a shame that the blogs and media didin't bother to investigate the reasons for the change before assuming that NOAA has made an adjustment without explanation. The announcement is here.

  • But why did this notable reversal not deserve a PR mention? Why wasn't it a "climate highlight"? – user20862 Jul 1 '14 at 13:54
  • It isn't a notable reversal, IMHO, and isn't worth a PR mention. Why is it important in which year an area comprising (IIRC) about 2% of the globe was marginally warmest (ahead of 2012)? Blogs pay a lot of attention to this sort of thing, I suspect largely to distract attention away from trends, which are of far greater importance. If you have two very hot years, with little to choose between them, small changes in the dataset will change which is reported as the hottest, but so what? – Dikran Marsupial Jul 1 '14 at 13:58
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    NOAA isn't a political entity, it is a federal agency with responsibility for weather data etc. They also gave a clear notice of the change of dataset at the top of the page. Suggesting that the change was made without explanation (when clearly an explanation was provided) arouses my suspicion/skepticism, it ought to arouse yours as well. – Dikran Marsupial Jul 1 '14 at 14:16
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    Perhaps that is because July hasn't finished yet, and that would be the next time it would be appropriate to discuss the record for July. NOAA is not exclusively controlled by politicians. I work for a government funded institution, but that doesn't mean my work is in any sense politically motivated. – Dikran Marsupial Jul 1 '14 at 14:31
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    Oddthinking, yes indeed there is a lot to be said for Hanlon's razor (en.wikipedia.org/wiki/Hanlon%27s_razor)! Or at least more moderately worded versions ;o) @Gracchus the change in dataset used by NOAA was made in March this year, the wording of the last paragraph of the announcment (ncdc.noaa.gov/news/…) suggests that it predates the changeover, so yes, the announcement does predate the blogs. – Dikran Marsupial Jul 1 '14 at 15:26

I am unclear what is in doubt.

You can follow the links and get July data from NOAA at http://www.ncdc.noaa.gov/cag/time-series/us/110/00/tavg/1/07/1895-2014.csv?base_prd=true&firstbaseyear=1901&lastbaseyear=2000 which includes the lines

Contiguous U.S., Average Temperature, July
Units: Degrees Fahrenheit
Base Period: 1901-2000

and this confirms Anthony Watts's most recent graph. He previously used an earlier version which is consistent with what NOAA had turned into sentences at http://www.ncdc.noaa.gov/sotc/national/2012/07 saying

The average temperature for the contiguous U.S. during July was 77.6°F, 3.3°F above the 20th century average, marking the warmest July and all-time warmest month on record for the nation in a period of record that dates back to 1895. The previous warmest July for the nation was July 1936, when the average U.S. temperature was 77.4°F.

The two points here are that NOAA revises its historical numbers from time to time when it has new information or better methods, and that July 1936 and July 2012 have similar figures with the relative ranking sensitive the the precise details. I do not see any issue with either of these points.

What is controversial at the moment is how NOAA moves from raw local reports to final data, how it deals with missing or estimated values and with breaks in local reports, and whether its methods lead to a bias over time. But that is a different question

  • The point that you seem to accept and Anthony Watts seems to reject is that it is acceptable to revise historical data (especially without notice). As this is the key part of your answer, please provide references to support that they do this, and have done so here. – Oddthinking Jul 1 '14 at 7:46
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    @Oddthinking: The question was whether NOAA had revised data, to which the answer is yes. I do not see any point in trying to supply references as to whether it is acceptable to make such revisions (if you want to see what somebody else thinks about revisions, try the ONS). Anthony Watts has observed that NOAA does revise data and indeed he appears to be in discussion with them with a view to getting them to revise again. – Henry Jul 1 '14 at 8:28
  • You are right; that is an answer to a very literal interpretation of the question, following the references provided in the question. However, the only reason this question is notable enough to be reported on is the further claim: that NOAA is unreliable, "not being honest with the public" and "not scientific", with the implication that it is misleading the public about climate change. You appear have taken a stance on this political point ("I do not see any issue") without references. – Oddthinking Jul 1 '14 at 8:59
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    @Oddthinking: I do not see any suggestion in the question that NOAA is "not being honest with the public" and "not scientific". As for its numbers being reliable, they have changed in the past and will change in the future. As I said in my reply, whether NOAA's adjustments to the raw data are biased over time is a very different question. – Henry Jul 1 '14 at 9:23
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    @DikranMarsupial: I think we now have two half-answers that together would make a decent answer. An opportunity for someone to post a summary answer, or for someone to merge. – Oddthinking Jul 1 '14 at 14:46

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