NOAA massively tampers with US temperature data to turn a 90-year cooling trend into a warming trend.
Their main excuse for doing this is Time of Observation Bias (TOBS.)
NOAA says that station operators in the past used to reset their min/max thermometers in the afternoon, and now they reset them in the morning. The theory being that resetting thermometers in the afternoon causes double counting of hot days, and resetting thermometers in the morning causes double counting of cold days. So NOAA cools the past and warms the present to compensate.
This is easy to test. I split the stations up into two groups – those that took readings in the morning during July 1936 and those that took readings in the afternoon during July 1936. I chose July 1936 because, it was an extremely hot month, which NOAA’s adjustments massively cool.
NOAA is correct that most stations took their readings in the afternoon during that month: 937 currently active USHCN stations were afternoon stations in 1936, and only 140 were morning stations. So let’s see how the trends compare.
The two groups of stations show identical trends in temperature anomaly. The TOBS adjustment is fake. There is no indication of double counting.
Additionally, the trends in the frequency of hot days are also identical for the two groups, but the morning stations tend to have more hot days. This is because people in warm climates tend to work earlier in the morning.
This is confirmed by looking at the latitude of the stations. Morning stations average about one degree further south than afternoon stations.
Station history was obtained from this archived NOAA link : USHCN ORNL/CDIAC-87 NDP-019
In order to test out double counting of hot days, I selected two adjacent stations in Missouri. Mexico, Missouri took their readings in the morning during July, 1936 and Warrenton, Missouri took their readings in the afternoon that month.
According to TOBS theory, we should see more 100 degree days in Warrenton than in Mexico, but we see the opposite. Mexico, Missouri consistently shows more 100 degree days than Warrenton, though the patterns are nearly identical. There is no indication that TOBS theory has any basis in reality.