The graph below shows US summer afternoon temperatures since 1895, sorted into two groups based on the time of day when their reading was taken on July 15, 1936.
The time of day data was taken from this 2008 archived USHCN metadata file, which has been deleted from the current USHCN web site.
Archived from : http://cdiac.ornl.gov/ftp/ushcn_monthly/station_history
Note that both sets of data show the same cooling trend since 1895, and that the 1936 morning recorded stations are cooling slightly faster than the afternoon recorded stations. This is the opposite of the assumptions made by USHCN in their TOBS “correction” – indicating that TOBS adjustment severely corrupts the US temperature record.
Last night I was remembering what I did when I was seven years old, I religiously recorded temperatures on my min/max thermometer and always did the right thing – I reset the maximum in the morning, and reset the minimum in the afternoon. Otherwise the data is tainted. This is the obvious way to avoid TOBS problems, and most likely what many station operators did. In fact you would have to be very stupid and irresponsible to not do something similar.
People have asked why the morning stations are warmer than the afternoon stations. My guess is that people in hotter climates are more motivated to report morning readings than people in cooler climates.
Here is the list of stations used for morning and afternoon readings.
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