Solar Radiation Changes in the United States during the Twentieth Century: Evidence from Sunshine Duration Measurements

Abstract

Changes in sunshine duration (SS) measured in the conterminous United States during the past century were used as a proxy to explore changes in shortwave forcing at the earth’s surface when and where accurate measurements of global irradiance (Eg) were not available. Yearly totals of SS from the 106 Weather Bureau stations with 70 or more years of complete measurements between 1891 and 1987 were analyzed after establishing that the two changes in instrumentation during that period had not significantly influenced the measurements. Annual totals of SS were highly correlated (r2 = 0.86) with annual totals of global irradiance (Eg↓) measured at the 26 U.S. pyranometer stations during the 1977–80 period when the Solar Radiation Network (SOLRAD) was operating at its maximum accuracy. The linear relationship between annual totals of Eg↓ and SS was highly significant (P < 0.001), with each additional hour of sunshine duration equivalent to an increase of 0.0469 ± 0.002 W m−2 (or 1.48 ± 0.07 MJ m−2 solar... Abstract Changes in sunshine duration (SS) measured in the conterminous United States during the past century were used as a proxy to explore changes in shortwave forcing at the earth’s surface when and where accurate measurements of global irradiance (Eg) were not available. Yearly totals of SS from the 106 Weather Bureau stations with 70 or more years of complete measurements between 1891 and 1987 were analyzed after establishing that the two changes in instrumentation during that period had not significantly influenced the measurements. Annual totals of SS were highly correlated (r2 = 0.86) with annual totals of global irradiance (Eg↓) measured at the 26 U.S. pyranometer stations during the 1977–80 period when the Solar Radiation Network (SOLRAD) was operating at its maximum accuracy. The linear relationship between annual totals of Eg↓ and SS was highly significant (P < 0.001), with each additional hour of sunshine duration equivalent to an increase of 0.0469 ± 0.002 W m−2 (or 1.48 ± 0.07 MJ m−2 solar...