The thermometer data shows no trend in US temperatures since 1900, with the three hottest years being 1921, 1934 and 1931.
That information won’t raise a lot of funding, so USHCN alters the data to make it look like the US is warming.
This is done in several steps. The conversion from raw daily to “raw” monthly mysteriously lowers most pre-1980 temperatures by about 0.2 degrees. Then they knock another 0.6 degrees off of the older raw temperatures for the TOBS adjustment, and then another huge chunk gets knocked off older records for things like anti-UHI effects. (Cities were of course much hotter 100 years ago, due to all the farting horses and lack of air conditioning.)
By the time these buffoons are done, they have created a man-made warming trend (red line) of about 1.5 degrees – which accounts for all of the claimed warming in the published USHCN data.