Friday, June 27, 2014

TOBS nailed.


OK, that's a bit triumphalist. Sorry. But I've been arguing, at WUWT and elsewhere, about why adjustment of USHCN is necessary. And I get a chorus of - no you can't alter original data, not if it increases the trend. And I point in vain to my earlier analytic justifications for TOBS (here and here). See Zeke for context, and Victor Venema for a much fuller explanation of min/max thermometers and TOBS.

I think I eventually worked out the right counter, so I thought I'd write it down here before I forget.

  • The min/max data that you see in a record is not (usually) original data of daily min/max. It is typically a record of the location of min/max markers on a thermometer at a specific time of day (when it was then reset).
  • An assumption must then be made to connect that with records of specific days. In the old style, you might assume that a max marker at 5pm Tuesday (example) was the daily max for Tuesday. If it was at 9am, you'd assume it was the max for Monday (and at some time in between, you'd have to switch).
  • Repeat, this is an assumption. It is not original data. And it won't always be right. Many of those 5pm Tuesday readings would have been set the previous Monday. That would arise from a warm afternoon when 5pm, not the max for Monday, was warmer than all of Tuesday to 5pm.
  • This is double counting, and 5pm creates a warm bias. Warm afternoons can get counted twice. Cold mornings don't.
  • Repeating again, an assumption was made and is inevitable. It creates a bias. People raised objections about how the bias can't be measured exactly. I emphasised here that there was a huge amount of data to base an estimate on; that the analysis was straightforward. Oh no, they say, how do you know that people actually read when they said they did (answer - see DeGaetano in that link). Etc. But anyway, the key thing is there is a bias, and it's a scientific duty to estimate and allow for its effect. The objectors want to say it is zero. That's an estimate, baseless and bad. We can do much better.
  • The original data is not data about daily temperatures. To get that requires interpretation. And you have to do it right. Laziness won't wash. We can do better. Over the years, NOAA has done better. And yes, for reasons explained in link above, that had a warming effect.

10 comments:

  1. Nice post. The people that complain about the TOBS corrections, do want to take the effect of urbanization taken into account. It seems somewhat inconsistent to only remove some non-climatic effects from the estimates of the global mean temperature and not others. That gives the impression that the reason is not science, but politics.

    Nick, did you see my old post on TOBS? A bit more technical.

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    1. Victor, thanks. Yes, I did see your old post, but have been forgetting to link to it as a much fuller explanation. I'll remedy that here.

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  3. An excellent summary, Nick.

    I was intrigued at Watts' recent conspiracy theorising about you: a 45-65 year old being payed to troll at "skeptic" sites such as his own. My fury at the willful ignorance and hyprocisy of Watts and his yes-folk would I think lead me to do likewise had I more time . As it is I'm left to give the occasional word of thanks for those who do attempt to clean out these Augean Stables.

    Concerning Victor's comment my undersatnding is that Watts wants to excise (censor?) data from any weather station that might possibly be subject to the UHI effect, rather than carrying out corrections on affected stations. If that is done for TOBS-biased, UHI-biased and any other biased stations then I would guess there would be no data to use. This would seem to be the only consistent position for Watts and co. to hold:

    "We know nothing, so let's carry on with BAU in the absence of any evidence for or against".

    That said Watts holds so many mutually inconsistent positions from day to day that he probably has held this one in the past.

    Incidentally, Nick, has Anthony made any more visits to your blog?

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    1. "Incidentally, Nick, has Anthony made any more visits to your blog?"
      Thanks, Bill. No, the blog has been fairly quiet lately. It may not last. Anthony has followed through on the Goggard stuff.

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    2. Bill, on average a station series has one non-climatic change every 20 to 15 years. Just look back 100 years, how different the world was. The first World War just started. It is nearly impossible to keep a station unchanged over such a period. So I think you are right, if you would remove all stations with an inhomogeneity, there would not be enough stations to compute a global mean temperature.

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    3. Thanks for that, Victor. Steven Goddard (real name we now know to be Tony Heller) continues to rail about the ludicrousness of TOBS, assuming that any half-way competent weather station volunteer would automatically make an extra trip to the weather station to reset the maximum/minimum if he or she noticed later in the evening that the weather were suddenly changing. Hmm, what if it were after dark, and the station was a mile or more away? A lot fewer street lights back in the 1930s and far fewer car owners.

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    4. Bill,
      Yes, that is a particularly silly line for Goddard to take, and should disqualify him from ever being an observer. Observers weren't asked to report the min/max for a calendar day. They were asked to report what they read on their instrument at a particular time. To have a thousand observers ad-libbing Goddard style would be of no use at all.

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  4. all of the above agreed, however , what is the justification for applying tobs to stations that are now infilled ? surely if no physical observation is being made ,it is not required ? again apologies for the lack of technical knowledge on my part ,but if i do not ask,i will never know.

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    1. Most stations reported for most of the 125 year history. If a tOBS change is needed, it will affect part of that reporting history. If a station has no raw data, there is nothing to adjust, so no TOBS; neighbouring values are used instead.

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