Linking Fog Microphysics to Visibility Reduction: Insights from Continuous Wintertime 2025–2026 Measurements in Bucharest

Fog is a meteorological phenomenon that significantly affects transportation safety by reducing horizontal visibility through the presence of suspended water droplets or ice particles. Besides its operational impact, fog also influences the surface radiation budget and local atmospheric processes. Understanding the relationship between fog microphysics and visibility is therefore essential for improving fog monitoring and forecasting.

This study investigates the microphysical characteristics of fog and their relationship with horizontal visibility using continuous in situ measurements collected during the winter of 2025–2026 in Bucharest, Romania. Observations were performed using a Fog Monitor FM-120 (Droplet Measurement Technologies/ENVEA Group), which provided real-time measurements of fog droplet size distributions in the 2–50 μm diameter range. Concurrent meteorological variables, including air temperature, atmospheric pressure, relative humidity, wind speed, and wind direction, were measured with a WS600-UMB weather station. The measurement site is located in a densely urbanized area characterized by intense road traffic, which may influence fog formation and evolution through enhanced aerosol concentrations and condensation nuclei availability.

The analysis focuses on the temporal evolution of fog droplet size distributions and key microphysical parameters, including droplet number concentration (Nd), effective diameter (ED), liquid water content (LWC), and mean volume diameter (MVD), and examines their relationships with observed visibility variations. Particular attention is given to the evaluation of several visibility parameterizations based on fog microphysical properties, aiming to identify the formulations that best represent fog conditions in the Bucharest urban environment. In addition, a local visibility parameterization as a function of LWC and Nd was developed and compared with previously proposed relationships. The results provide new observational evidence on the microphysical drivers of visibility reduction and contribute to the development of regionally adapted visibility parameterizations, supporting efforts to improve fog characterization, forecasting, and risk mitigation for transportation and aviation operations.