USHCN thermometers show a significant decline in US temperatures since 1920, with 1921, 1931 and 1934 being the three hottest years.
NOAA decided that this simply wouldn’t do, so about a decade ago they started adjusting the data in the four steps shown below. Remarkably, each adjustment step causes the past to get cooler.
By the time they are done corrupting the data, they have hidden the decline and created a strong warming trend, with 1998, 1999 and 2006 being the three hottest years. Prior to corruption, 1999 and 2006 aren’t even in the top ten.
You can think of USHCN as being the Enron of US temperature data. The graph below shows the total adjustment. Note the massive cheat going on in 2012, which is needed to proclaim 2012 as the hottest year ever.