Solar radiation (SR) dynamics have a profound effect on plant growth, development, and ecosystems, and they act as a primary energy source and important environmental signal that plants perceive through their photoreceptors, which primarily sense critical wavelength ratios (CWRs). The diffuse fraction of solar radiation (DF) is a key factor affecting the quality and distribution of light within the plant canopy. We analyzed one year of SR spectral observations measured by a ground-based rotating shadow-band spectroradiometer to evaluate the potential effects of DF and CWRs on plants in an outdoor environment in Fukuoka, Japan. The daily mean DF and all considered CWRs showed significant seasonal variations regardless of the solar meridian altitude. Cloudy or partly cloudy skies were prevalent throughout the year. The ratios of ultraviolet-A (UV-A)/UV-B, red (R)/blue (B), and R/green (G) increased during winter and decreased during summer. Conversely, the ratios of photosynthetically active radiation (PAR)/global solar radiation (GSI), UV/GSI, UV/PAR, B/G, R/far-red (FR), and UV-B/B increased during summer and decreased during winter. Most of the CWR correlated with DF. Furthermore, our study discovered a synergistic influence of air mass (AM), water vapor pressure (VP), and DF on specific CWRs, which play a crucial role in plant light signal processing. This highlights the potential influence of AM, VP, and DF on plant light signals, thereby opening avenues for developing innovative plant growth models and ecological responses incorporating plant photoreception.