Interannual Variability of Wintertime Fog over a Rapidly Urbanising City in the Indo-Gangetic Plains

Wintertime fog over the Indo-Gangetic Plains (IGP) exhibits large variability in frequency, duration, and intensity, with significant implications for transportation, air quality, and human health. This study examines the interannual variability of fog hours, fog days, duration, and intensity over Lucknow (26.85° N, 80.95° E), a rapidly urbanizing city in the central IGP, using half-hourly METAR visibility observations and associated meteorological parameters during 2016–2023. Rapid urban expansion and increasing anthropogenic emissions in Lucknow have the potential to modify near-surface thermodynamic conditions, moisture availability, and boundary-layer stability, thereby influencing fog formation and persistence.The results show that January experiences the highest occurrence in fog hours and days, followed by December and February. Fog hours and fog days increased until 2017, after which fog hours declined by approximately 19%, while fog days increased by about 12% up to 2023. Analysis of the data reveals an increase in shallow and moderate fog events, whereas dense and very dense fog hours exhibit a significant decreasing trend. In contrast, fog days show an increasing tendency across most intensity categories, except for moderate fog days. The results reveal a significant increasing trend in shallow and moderate intensity, whereas a significant decreasing trend in dense and very dense fog hour events, and a similar increasing trend has been noticed in fog intensity days, except in moderate intensity. The study reveals that there is a significant decline in persistence and intensity of fog amidst rising event frequency. These findings indicate a transition toward more frequent but shorter-duration and less intense fog events, suggesting a weakening of long-duration fog persistence and severity over the study period, likely linked to evolving urban and boundary-layer processes in the IGP.Keywords: Winter fog, Interannual variability, METAR Visibility, Urbanisation