Directional Dynamics of Fog: Irreversibility and Causal Coupling With Turbulence

Abstract Fog prediction remains challenging because the physical processes governing its life cycle evolve across time scales and do not follow reversible or stationary dynamics. Using high‐frequency visibility observations from Sable Island, Canada, this study analyzes fog intensity and turbulent kinetic energy (TKE) for their time irreversibility and causal relations. Fog intensity exhibits temporal asymmetry in all stages, while TKE remains nearly reversible. The lead–lag structure between the two variables is stage dependent: TKE leads fog intensity during formation, the coupling becomes symmetric during the mature phase, and fog intensity leads TKE during dissipation. Notably, during fog formation, the strength of fog's intrinsic irreversibility increases linearly with the strength of its causal linkage to TKE, revealing that fog initiation is governed by a directional sequence of turbulence–moisture interactions. These findings demonstrate that fog is a non‐equilibrium, time‐asymmetric system, and that capturing its stage‐dependent directionality is required for enhanced fog prediction.