Slide 8 in this series.
in 2016, 42% of the station data was missing so they just made up the missing data. As one can see in the curve on the right, the adjustment highly correlates to the CO2 concentration. Clearly this cannot be due to the inaccuracy of any measurement.
This appears as a blatant attempt to prove AGW by manipulating the data.
USHCN: U.S. Historical Climatology Network, NOAA
NEXT: More NOAA data manipulation on NCDC data fraud.
This section on data fraud offers mulitple cases and angles on the general topic. It is however not exhaustive.
USHCN Monthly Temp Adjustments
The list can expand each week given the claims of AGW and their intent.