One of the big problems with the earlier, legacy climate monitoring stations is that they are poorly sited near artificial sources of heat or cold and they are located in urban heat islands where the temperature are artificially high due the heat absorption of concrete and blacktop pavement and the heat generated by burning fossil fuels and waste heat from electrical use, all in a concentrated area.
Many of these stations were originally located in rural areas when they were installed many decades ago, but have become increasingly surrounded by higher and higher density urban development.
This encroachment accounts for much of the observed temperature increase over time , as well as the gradual increase in measured temperature over time as the locations became increasingly surrounded by urban development and the corresponding increase in the urban heat island effect
You will see it often on the WUWT website.