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Sunday, February 22, 2015

More on TOBS, Unfortunately

Zeke Hausfather now has a post on Climate Etc. that says the same thing I did but draws a totally different conclusion.

Watch this, "Until the late 1950s the majority of stations in the U.S. record recorded temperatures in the late afternoon, generally between 5 and 7 PM. However, volunteer temperature observers were also asked to take precipitation measurements from rain gauges, and starting around 1960 the U.S. Weather Service requested that observers start taking their measurements in the morning (between 7 and 9 AM), as that would minimize the amount of evaporation from rain gauges and result in more accurate precipitation measurements."

A shift from later afternoon to morning, so let's say 6 pm to 8 AM okay?

A liquid in glass max/min will record the past 24 hours if reset every 24 hours.  At 8 am you would have recorded yesterdays' high this morning's low.  At 18:00 hrs (6pm) you would have recorded today's high and today's low.  You have a one time shift from yesterday' high to today's high by changing from 6pm to 8 am.  There is a very small probability that a max or min would occur at either 6pm or 8am.   With daylight savings time though a 7am reading could become a 6am astronomical reading gumming up the works.  That would increase the possibility of "split" or double recording of a lower than normal lower.  

" Between 1960 and today, the majority of stations switched from a late afternoon to an early morning observation time, resulting a systemic change (and resulting bias) in temperature observations."

If you tell a network of volunteers to please shift to 8 am readings , the vast majority would shift to 8 am readings over a few months not a few decades.  If there is any TOBS impact due to the shift it would most likely be a step change, something noticeable.

There is a step change around 1950 but no step change around 1960.  The odds of any TOBS adjustment being required due to an extremely gradual changing of observation time over 20 plus years is vanishingly small. 

From the start of the chart to ~1990 TOBS adjustments never needed to be more than +/-0.1 C degrees.  

Interestingly enough, the time of observation adjustment developed by Karl et al 1986 is not strictly necessary anymore. Changes in time of observation in station records show up a step changes in difference series compared to neighboring stations, and can be effectively removed by the pairwise homogenization algorithm and similar automated techniques"

Zeke even points out that TOBS issues would show up as step changes.  The LIG max/min thermometers were nearly completely phased out by 1995 after which there is a bigger need for TOBS adjustment?  I don't think so.  TOBS is most likely being confused with some other need adjustment do to instrumentation, mmts, local area changes, parking lots, trees, runways, clutter and shelter aging.  

Zeke just cannot get the concept that TOBS, the unneeded adjustment, is a kludge covering other issues.   When you consider adjustments on an instrument by instrument basis, the way it should be done, you are more likely to find out more about those "other" issues like land use change and such.

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