Event-based fog climatology and typology for cities in Indo-Gangetic plains

Dense fog blankets Indo-Gangetic Plains (IGP) during winter affecting the lives and livelihoods of millions of its inhabitants. This study investigates fog climatology and typology of winter fog in IGP using half-hourly surface observations of visibility at seven North Indian cities from 1991 to 2020. It identifies fog events and characterises them based on time of occurrence and duration, and analyses their frequency at seasonal, monthly, and daily scales. Next, the fog events are classified based on fog formation processes. The analysis shows that the majority (-84%) of the events are radiation fog, followed by advection fog (-13%). The highest number of fog events occur in January. The fog onset timing shows an east-west trend, with western cities having delayed onset. On the other hand, the fog intensity exhibits a north-south trend, with northern cities having more intense fog events. The time-series analysis indicates that seasonal fog hours show a higher inter-annual variability, while durations of fog events exhibit inter-city variability. A comparison of fog characteristics with other regions of the world reveals that IGP has a rela-tively higher percentage of radiation fog and longer-duration fog events.