Spectral observation of the 24 solar terms on the Qinghai–Tibet Plateau: A case study of Lhasa, Tibet

Abstract

Twenty-four solar terms, which were created by farmers in ancient China, are based on the Earth's orbit around the Sun and reflect the changes in climate, natural phenomena, agricultural production and other aspects of human life. In this study, we conducted observations and analysed the solar spectrum of the 24 solar terms in Lhasa, Qinghai–Tibet Plateau using a high-precision solar irradiance metre. The objective was to investigate the relationship between solar radiation variations and geophysical processes in the Qinghai–Tibet Plateau region. The results showed that during the period from the Autumnal Equinox on September 23, 2022 to the Qingming Festival on April 5, 2023, the lowest environmental humidity was only 4.7 %. The impact of the atmospheric water vapor and clouds on solar radiation was relatively small, and the solar spectrum exhibited good regularity. However, during the remaining solar terms, the solar spectrum exhibited fluctuations caused by temperature, humidity and other factors. Among the 24 solar terms in Lhasa during the Summer Solstice in 2022 and 2023, the peak values of monochromatic light were 2.53 W m⁻²·nm⁻¹ and 2.71 W m⁻²·nm⁻¹, respectively, considering the combined effects of sunlight reflection and scattering caused by discrete clouds. These values exceed the peak value of the AM0 monochromatic light. By comparing the Fraunhofer lines during the ‘Two Equinoxes and Two Solstices’ in Lhasa, the absorption peaks of the Fe, Ca, H, Mg, Na and O elements were observed at wavelengths of 358 nm, 431 nm, 486 nm, 517 nm, 589 nm and 687 nm, respectively. These results provide a foundation for further investigation of solar radiation variations on the Qinghai–Tibet Plateau and their impact on the Earth and can be used as a reference for sustainable research in the Qinghai–Tibet Plateau region of China.