Zeke Hausfather says: May 12, 2014 at 3:00 pm The difference is straighforward enough. Even if you use monthly rather than annual averages of absolute temperatures, you will still run into issues related to underlying climatologies when you are comparing, say, 650 raw stations to 1218 adjusted stations. You can get around this issue either by using anomalies OR by comparing the 650 raw stations to the adjusted values of those same 650 stations. The reason why the 1218 to 650 comparison leads you astray is that NCDC’s infilling approach doesn’t just assign the 1218 stations a distance-weighted average of the reporting 650 stations; rather, it adds the distance-weighted average anomaly to the monthly climate normals for the missing stations. This means that when you compare the raw and adjusted stations, differences in elevation and other climatological factors between the 1218 stations and the 650 stations will swamp any effects of actual adjustments (e.g. those for station moves, instrument changes, etc.). It also gives you an inconsistant record for raw stations, as the changing composition of the station network will introduce large biases into your estimate of absolute raw station records over time. Using anomalies avoids this problem, of course.

Zeke Hausfather says: May 12, 2014 at 3:00 pm The difference is straighforward enough. Even if you use monthly rather than annual averages of absolute temperatures, you will still run into issues related to underlying climatologies when you are comparing, say, 650 raw stations to 1218 adjusted stations. You can get around this issue either by using anomalies OR by comparing the 650 raw stations to the adjusted values of those same 650 stations. The reason why the 1218 to 650 comparison leads you astray is that NCDC’s infilling approach doesn’t just assign the 1218 stations a distance-weighted average of the reporting 650 stations; rather, it adds the distance-weighted average anomaly to the monthly climate normals for the missing stations. This means that when you compare the raw and adjusted stations, differences in elevation and other climatological factors between the 1218 stations and the 650 stations will swamp any effects of actual adjustments (e.g. those for station moves, instrument changes, etc.). It also gives you an inconsistant record for raw stations, as the changing composition of the station network will introduce large biases into your estimate of absolute raw station records over time. Using anomalies avoids this problem, of course. — Draft