If you can find the info, it would be worth knowing whether the obs site has stayed at the same place throughout. I've spent many hours dissecting the data of the Farmington co-op, records back thru Jan 1893, and there was a significant change when the site was moved in Sept 1966 from the fully built-up section of the small (5k) town to a site 1.5 miles to the north, where houses are spaced apart and nearly all on the same road. The biggest change was summertime maxima - for instance, the co-op has recorded 14 days reaching 100+, but only one since the move: 101 on "Hot Saturday" in August 1975. The average temps have risen significantly during the current century but there is still a disproportional amount of warmth records, especially warm minima, set prior to that move to the north.