Abstract Cloud‐fog formation substantially reduces near‐surface solar radiation and modulates land‐atmosphere interactions, but quantifying such reduction is limited by sparse mountain observations. Using a 1‐km geostationary satellite‐retrieved product, we apply the Reduction Ratio of Solar Radiation (RRSR), an empirical normalized metric that quantifies near‐surface solar radiation deficits relative to a local maximum solar radiation baseline. Applying RRSR over Taiwan reveals a strong seasonal contrast. Summer RRSR exhibits a distinct land‐type gradient (montane cloud forests > other forests > non‐forests), peaking within the mid‐elevations (500–2,500 m a.s.l.). In winter, prevailing northeasterly flow overrides local land‐type influences. Strong northeasterly events enhance RRSR > 1.5‐fold in windward lowlands (<1,000 m a.s.l.), producing light limitation comparable to, or exceeding, that of typical montane cloud forests. Overall, RRSR provides a practical diagnostic for ecohydrology and surface energy budgets, although its uncertainties are larger under cloudy or foggy conditions in complex terrain.