Solar radiation (SR) dynamics have a major impact on plant growth, development, and ecosystems, acting as a primary energy source and a powerful environmental signal that plants perceive through their photoreceptors, which primarily sense critical wavelength ratios (CWRs).  Most SR research has focused on seasonal variations for energy assessments, often overlooking the detailed aspects of spectral solar irradiance (SSI) and CWRs in the context of plant ecophysiology. Therefore, this research investigated the annual variation in plant-perceived critical WRs to fill an important knowledge gap spanning several areas of plant physiology to ecophysiology and agriculture. Data were collected during 2021 in Fukuoka, Japan, using a ground-based shadow-band spectroradiometer with a rotating arm. Significant annual variation was observed in all CWRs considered. Ultraviolet-A (UV-A)/UV-B, red (R)/blue (B) and R/green (G) ratios were found to increase in winter and decrease in summer. In contrast, photosynthetically active radiation (PAR)/global solar irradiance (GSI), UV/GSI, UV/PAR, B/G, and R/far red (FR) ratios increased in summer and decreased in winter. The relative differences in the variation of R/PAR and B/PAR ratios were comparatively small (<6%). These changes in CWRs may be crucial for plant behavior and ecological responses.