LAUSR

Abstract The behavior of the surface solar radiation (SSR) during the first half of 20th century is an open issue due to the scarcity of SSR data during this period. This paper focuses on the re-evaluation of the pyrheliometer measurements in Madrid between September 1910 and January 1929. These records were formerly analyzed by Anton et al. (2014), reporting no statistically significant trends in the atmospheric column transparency. In the present work, metadata regarding the state of the solar disk during each pyrheliometer record have been used in order to remove measurements affected by thin clouds. The re-analysis of the atmospheric column transparency from the pyrheliometer data showed two periods with opposite sign in the trends of this variable: a decrease of (−2.9 ± 0.2)% per decade between 1910 and 1925, followed by a strong increase of (+15.0 ± 2.5)% per decade up to 1929. Both trends were statistically significant at the 95% confidence level. These results suggest a decrease of direct SSR in Madrid during the second decade of the 20th century, in agreement with the final stage of the “early dimming” which was observed at several European sites from the end of the 19th century. Additionally, the marked positive trend of the atmospheric transparency found in this work in the second half of the 1920s is in accordance with the beginning of the SSR recovery in Europe which was observed up to late 1940s (“early brightening”). Therefore, from the results of this study, the use of metadata is strongly required for an accurate assessment of the trends in early pyrheliometer measurements.