The Farmington thermometer is very poorly sited next to a busy paved road that gets busier every year. They moved it closer to the road in the 1980s and a large parking lot went up not too far away at about the same time. It has other problems as well.
So, editing your comment a bit and ignoring the fact that the thermometer got moved closer to the roadway, we could say that "the continuing encroachment of human activity and fossil fuel burning vehicles has even affected the temperature trends in formerly rural Farmington, Maine. All you have to do is look at the past couple of decades of data!"
This is helpful. Perhaps there is a way of logging where these stations are. Surrounding a weather station with asphalt must surely have some effect?