One of the primary excuses used by NOAA to hide the decline in the US temperature record, is Time Of Observation Bias (TOBS.) The theory is that people used to be incredibly stupid and reset their max/min thermometers only once per day – in the afternoon. This would cause double counting of some high temperatures, and result in historical temperatures which were too high.
This theory is easy to test. Half of the stations in Missouri took their min/max readings during July 1936 in the afternoon, and the other half took them at morning or night during that month. I separated those groups out and found that it makes no difference to summer temperature trends.
The group of morning/night stations has been cooling at almost exactly the rate as the group of all stations. Also note that summer temperatures in the 1930’s are essentially identical in the two groups. No matter what tricks NOAA uses to hide the decline in US temperatures, it doesn’t change the facts.
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Another important test is the percentage of days over 100 degrees, comparing morning and afternoon stations. The result of this interesting – the morning/night stations actually have a higher frequency of hot days than the afternoon stations. This is the opposite of what TOBS theory would predict, and most likely due to the fact that the morning stations are generally further southwest than the afternoon stations.
There is no indication that TOBS has a serious biasing effect, and no indication that the NOAA adjustments being done are legitimate.