My guess is up until around 1980ish all weather stations were probably using mercury thermometers.
Up until 1980, I think most thermometers had to be eyeballed by some guy who wrote the reading down on paper. You really think the guy is actually going to go out there in a blizzard? Or is he just going to write down something that looks reasonable?
Not only the type of thermometer but the location of the “official” thermometer.
For example: The “official” temperatures were/are recorded at the Customs House in most port cities. Years ago that was on the waterfront. As piers were extended, sections of harbor filled to build warehouses and access, the Custom House was no longer on the waterfront but somewhat inland. Paved streets and buildings surround and enclose the “official” thermometer but it still records the “official” temperature for the location.
Might that have something to do with rising temperature readings?