Homogenization of the Observed Surface Relative Humidity From 1959 to 2020 in China

In the early 2000s, China replaced manual psychrometers with capacitive sensors for surface relative humidity (RH) measurements despite the latter's known stability issues and the need for frequent calibration. This study employs sparse-input reanalysis as a reference to detect and correct potential inhomogeneities in observed surface RH records from 2380 stations across China (1959–2020). Systematic inhomogeneities are primarily linked to the nationwide replacement of psychrometers with capacitive sensors in the 2000s and subsequent sensor instability. Raw data suggest an artificial decreasing trend of −0.43% per decade in RH particularly pronounced in southern China. Homogenization effectively removes this spurious trend, yielding stable RH records and eliminating regional discrepancies. Manual psychrometers derive RH from measurements of dry-bulb and wet-bulb temperatures using a psychrometric coefficient—a function of wind speed that is typically assumed constant. In this study, we calculated the psychrometric coefficient based on wind speed and applied it to derive RH, revealing that the raw manual psychrometer data overestimate RH by approximately 1.5% in China. This result supports our homogenization corrections and aligns with findings from nearby stations used as references. Because ERA5 and JRA-55 assimilate raw data, they inherit these artificial trends in their surface-level analyses. These findings have important implications for the reported RH decline over land in the 21st century given that the instrument replacement occurred worldwide.