Urban Fog Microphysics and Visibility Parameterization Based on Winter 2025–2026 In Situ Measurements in Bucharest

Fog is a type of cloud that forms in direct contact with the Earth’s surface. It is composed of extremely small water droplets or ice particles suspended in the air, similar to those found in clouds. For atmospheric conditions to be classified as fog, horizontal visibility must be reduced to less than 1 kilometer due to the presence of these fine particles, which scatter and absorb light and significantly limit what can be seen near the ground. Fog also plays an important role in the Earth system because it influences the surface radiation budget, in daytime causing cooling and in nighttime causing warming. Fog is a significant phenomenon that impacts the safety of terrestrial, maritime, and especially aviation transportation.

This study investigates variations in fog microphysics and the correlations with horizontal visibility. The analysis is performed on datasets gathered during in situ continuous measurements conducted in wintertime 2025-2026 in Bucharest using the Fog Monitor FM-120 from Droplet Envea Group. A weather station from Luft (WS600-UMB) monitored meteorological parameters: temperature, pressure, humidity, wind direction and wind speed.

The measurements were taken at the National Institute for Aerospace Research (INCAS) in Bucharest (coordinates: 44.4672° N, 26.0814° E), that is located in a Bucharest area with high traffic likely providing plenty of condensation nuclei. We present very recent observational evidence on the fog droplet signature in real time, linking temporal droplet size distribution changes and visibility evolution. We focused on assessing the microphysical parameters of fog, including number concentration (N), effective diameter (ED), liquid water content (LWC), and mean volume diameter (MVD), across a dimensional spectrum from 2 to 50 µm. The observational datasets were then used to test some visibility parameterizations, with the goal of determining a specific parameterization, linking visibility to the fog microphysics, best suited for the Bucharest area.

The results add to the past studies aiming to contribute to a better understanding of fog characteristics and visibility parameterization using regional characteristics, ultimately aiding in improving safety measures in various transport sectors.