Surface air temperatures
measured at a single location are known to show non-constant systematic errors over time because of variations in solar heating and wind - speed effects.
In an article I wrote, the N.A. SST is compared with the directly measured atmospheric pressure at Stykkisholmur weather station / Reykjavik, since 1860's, I see no reason to suspect accuracy of it, since it was
measured at a single location by professionals.
Not exact matches
The difference over time of temperature
measured at a consistent
location has much lower systematic error than the use of that
single location's temperature as a proxy for the overall temperature of the region.