Changes in instruments and sites affecting historical weather records: A case study

All long historical climate records are based on measurements that experienced shifts in instrumentation, site characteristics, or locations. How such changes affect the quality of past data remains an uncertainty for the thousands of historical records, confounding efforts to assess climate change. Fortunately, one station in Illinois with 118 yr of records has also kept detailed records of all such shifts plus overlapping measurements of temperatures and precipitation, allowing exact measurements of how conditions changed over time. This study examined these data and found varying discontinuities of 0.1 degrees-0.9 degrees C in annual temperatures due to various shifts, but no changes in daily precipitation related to site shifts. However, hourly precipitation amounts from recording rain gauges did undergo a considerable shift due to changes in rain gauge types. Similar studies need to be made of other stations with comparable historical records of station and instrument shifts and with overlapping measurements when shifts were made.