Index of /pub/data/ghcn/daily/hcn/
There are 863 US HCN stations which have been continuously active since at least 1930. The six hottest years (measured by number of all-time daily records at those stations) all came during the 1930’s.
Note that 2012 looks like it was a hot year, but that was primarily due to a lot mild springtime temperatures – rather than summer heat.
Makes sense absolutely, both the AMO and PDO were strong positive at that time.
The aa-index was also large especially in the late 1930’s
https://twitter.com/dhm4444/status/383400316839804928/photo/1
much more than the present levels.
The spike of 2012 is anomalous, the trend of this century is downward.