Climatological trends and variability of fog characteristics and meteorological parameters over cities along the Indo-Gangetic Plain

The Indo-Gangetic Plains (IGP) in the northern region of India experience widespread and persistent winter dense fog with significant variability and intensity that severely disrupts the transport and impacts human health. The present study investigated the climatological of trends of fog hours, days, duration and intensity, over five city regions; Amritsar (31.42° N, 74.47° E), Delhi (28.56° N, 77.1° E), Lucknow (26.7655° N, 80.8854° E), Patna (25.5941° N, 85.1376° E) and Gaya (24.7914° N, 85.0002° E) using half-hourly surface observations of visibility and meteorological observations during winter months from 1991 to 2024. The non-parametric statistical methods, including the Mann-Kendall test and Theil-Sen's slope estimator, were used to assess trends in fog variations during the winter months of the study period. The classification of fog events into radiation and advection types reveals a dominant contribution of radiation fog, accounting for more than 80 % of total fog events across all five cities. Higher and statistically significant increasing trends in fog hours and days are noticed throughout the cities, even during November and February. Short-duration fog events show predominantly declining trends, especially in Amritsar and Delhi, whereas moderate-duration and long-duration events show increasing trends in Amritsar, Delhi, and Lucknow. A higher percentage of short, moderate, and long duration fog events are noticed in shallow and moderate intensity fog cases compared to dense and very dense fog conditions. The analysis reveals a significant trend in shallow, moderate, dense, and very dense intensity of events is noticed in western IGP cities (Amritsar, Delhi, and Lucknow) compared to eastern IGP cities (Patna and Gaya). Climatological trends of air temperature exhibit increasing (decreasing) trends in air temperature during all fog intensity conditions over western IGP (eastern IGP) cities, whereas relative humidity reveals an overall increasing trend in the winter months.