Seasonal variability and cloud impacts in diffuse ratio, clearness index, and spectral characteristic of solar radiation in a temperate monsoon region

Abstract Solar radiation (SR) dynamics significantly influence plant growth, development, and ecosystems, serving as a primary energy source and an essential environmental signal. Plants sense SR through photoreceptors, with the ratios between specific wavelength ranges being particularly significant. Therefore, we refer to these as critical wavelength ratios (CWRs). The diffuse fraction of solar radiation (DF) is a key factor shaping light quality and distribution within the plant canopy. To evaluate the potential effects of DF and CWRs on plants in an outdoor environment, we analyzed 1 year of SR spectral data collected using a rotating shadow‐band spectroradiometer in Fukuoka, Japan. Cloudy or partly cloudy skies dominated throughout the year. The ratios of UV‐A/UV‐B, red (R)/blue (B), and R/green (G) increased in winter and decreased in summer. In contrast, the ratios of photosynthetically active radiation (PAR)/global solar radiation (GSR), UV/GSR, UV/PAR, B/G, R/far‐red, and UV‐B/B increased during summer and decreased in winter. Most CWRs were significantly correlated with the DF. The clearness index (CI) exhibited a strong correlation with DF and CWRs. A synergistic effect of air mass (AM), atmospheric water vapor pressure (VP), and CI on specific CWRs was revealed. Future climate change‐driven increases in VP and DF, coupled with reductions in CI, are expected to trigger complex changes in plant growth and flowering by altering light signals and enhancing photosynthesis through diffuse light fertilization. Integrating VP, AM, DF, and CI dynamics into climate models could enhance predictions of atmospheric, ecological, and plant physiological responses.