A multi-parameter diagnostic framework for radiation fog and nocturnal visibility

Fog formation is a complex boundary-layer phenomenon resulting from the combined effects of thermodynamic saturation, radiative cooling, surface moisture availability, and turbulence suppression. Many existing fog diagnostics evaluate these controlling factors separately, which limits their applicability under rapidly evolving nocturnal conditions. This study presents a multi-parameter diagnostic method designed to assess radiation fog occurrence, intensity, and associated visibility regimes from a climatological perspective. The proposed method integrates 38 coupled diagnostic relationships representing near-surface thermodynamics, radiative deficit, cloud–radiation interactions, synoptic moisture and pressure anomalies, humidity evolution, wind-induced turbulence suppression, latent cooling, and dew-point depression. These components are combined into a unified Radiation Fog Coefficient (RFC) and a normalized Radiation Fog Unit (RFU), enabling threshold-based classification of fog and reduced-visibility conditions. The diagnostic method is evaluated using seven documented nocturnal reduced-visibility events across different regions of Türkiye, including both radiation-fog and deliberately selected non-radiation-fog events. This selection explicitly demonstrates the method’s ability to discriminate between true surface-based radiation fog and other types of visibility reductions. The analysis focuses on atmospheric conditions observed between 03:00 and 04:00 local time, corresponding to the period of strongest nocturnal radiative cooling when dew-point depression typically approaches its minimum values, creating thermodynamic conditions most favorable for radiation fog formation. Compared to traditional single-parameter diagnostics, the method demonstrates improved skill in representing the coupled thermodynamic and radiative evolution leading to fog formation. Overall, the proposed diagnostic approach provides a physically interpretable and scalable framework for climatological analyses of radiation fog occurrence and visibility variability under nocturnal boundary-layer conditions.