Fog persistence and dissipation over the Eastern Gangetic Plains insights from boundary layer thermodynamics during 16 to 31 December 2025

This study examines the persistence, dissipation, and driving mechanisms of winter fog at Patna Airport (Indo-Gangetic Plain) during 16–31 December 2025, integrating surface visibility and runway visual range(RVR) observations, radiosonde profiles, ERA-5 reanalysis, and a rule-based diagnostic classification. Two fog episodes were identified: a synoptically forced event (16–19 December) linked to an upper-level trough and sustained subsidence, and a more persistent, quasi-equilibrium event (23–27 December) marked by extreme boundary-layer stability and collapsing ventilation. Radiosonde and boundary-layer diagnostics indicate that fog persistence depends less on inversion strength alone than on the coordinated suppression of turbulence, ventilation, and near-surface mixing depth; a strong but elevated inversion late in the period failed to sustain fog, underscoring the primacy of surface-atmosphere coupling over inversion intensity. Synoptic analysis attributes the contrast between episodes to differing large-scale regimes, with active subsidence forcing giving way to a more radiative-thermodynamic, diurnally sensitive boundary layer. Sequential threshold-based classification of dissipation mechanisms across 49 fog events shows that mechanically driven processes, acting alone or jointly with other boundary-layer dynamics, dominate fog erosion, with subsidence-modulated and radiative pathways contributing secondary roles and a minority of cases remaining diagnostically ambiguous. Independent verification against observed fog occurrence confirms genuine, positive predictive skill beyond climatology, with a bias toward high precision over recall. A companion Fog Dissipation Potential Index offers short-lead operational guidance, distinguishing transient from persistent fog and clarifying predictability limits under extreme stability, providing a transferable diagnostic template for continental fog-prone airports.