Cool Mr Watts

Last year, NOAA issued a comment indicating that the temperature record produced only by the weather stations that Watts’ project classified as good or best was statistically equivalent to the whole temperature record. This indicated that the question Watts had asked about the reliability of the surface temperature records (and it was a good question) was answered; it just wasn’t the answer Watts was looking for. According to the criteria set out by Watts and his project the surface record was  reliable.

There was just one problem, the comment made by NOAA wasn’t very detailed. Now the paper On the reliability of the U.S. Surface Temperature Record (Menne 2010) has been published in the Journal of Geophysical Research looks at the issue in greater detail, and the results are definitely not what Watts has in mind. The poorly sited weather station show a slight cooling bias!

Poor sites show a cooler maximum temperature compared to good sites. For minimum temperature, the poor sites are slightly warmer. The net effect is a cool bias in poorly sited stations. Considering all the air-conditioners, BBQs, car parks and tarmacs, this result is somewhat a surprise.

The question is why?

The reason why the poorly-sites stations measured cooler temperatures lies in the predominant types of thermometers used at the two types of sites. An electronic Maximum/Minimum Temperature System (MMTS) is used at 75% of the poor sites. These MMTS sensors are attached by cable to an indoor readout device, and are consequently limited by cable length as to how far they can be sited from the building housing the indoor readout device. As a result, they are often located close to heated buildings, paved surfaces, air conditioner exhausts, etc. It turns out that these MMTS thermometers have a flaw that causes them to measure minimum temperatures that are slightly too warm, and maximum temperatures that are considerably too cool, leading to an overall cool bias in measured average temperatures. In contrast, only 30% of the “good” sites used the MMTS sensors. The “good” sites predominantly used Liquid in Glass (LiG) thermometers housed in wooden shelters that were more easily located further from the buildings where the observers worked. Since the poorly-sites stations were dominantly equipped with MMTS thermometers, they tended to measure temperatures that were too cool, despite their poor siting.

Here is the paper’s conclusion:

These results underscore the need to consider all changes in observation practice when determining the impacts of siting irregularities. Further, the influence of non-standard siting on temperature trends can only be quantified through an analysis of the data. Adjustments applied to USHCN Version 2 data largely account for the impact of instrument and siting changes, although a small overall residual negative (“cool”) bias appears to remain in the adjusted maximum temperature series. Nevertheless, the adjusted USHCN temperatures are extremely well aligned with recent measurements from instruments whose exposure characteristics meet the highest standards for climate monitoring. In summary, we find no evidence that the CONUS temperature trends are inflated due to poor station siting.

This underlines three main points:

  1. Photographs (the main data gathered by are no substitute for data analysis. Watts has never done a proper analysis of the temperature data produced by the poorly sited weather stations.
  2. There are many non-obvious factors that may influence temperature records, assuming bias does not demonstrate bias exists.
  3. The adjustments made to the raw data are shown to reduce the discrepancy between temperature measurements from poorly sited weather stations and well sited stations, increasing the reliability of the surface record.
  4. The surface temperature record is reliable despite not being perfect.

Despite the fact that Watt’s had a clear bias towards finding an artificial warming trend, the effort did ask some important questions and did produce valuable data.

One might reasonably question whether the goal of was to lead us into greater scientific truth or merely to sow doubt about the temperature record. Nevertheless, their efforts to rate each individual weather station enabled scientists to identify a cool bias in poor sites and isolate the cause. A net cooling bias was perhaps not the result the volunteers were hoping for, but improving the quality of the surface temperature record is surely a result we should all appreciate.

Anthony Watts deserves thanks (and indeed was thanked in the paper) for his efforts.

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