An Observational Approach to Investigate the Climatological Trends and Characteristics of Fog Over Indo Gangetic Plain

This study investigates the climatological trend in fog events over the Indo-Gangetic Plain (IGP) in light of fog-favoring meteorological factors and land use land cover due to urbanization. Long-term trends in fog frequency at nine IGP stations were studied (1975-2022), with Delhi (Megacity) and Hisar (Semi-Rural) examined to assess variations in fog patterns over urban regions. The fog frequency showed a significant positive trend across both stations in general, but a Regime Shift Analysis indicated distinct shifts at specific intervals at each site. Interestingly, Delhi shows a 10% reduction in fog frequency in the third regime (2014-2022), whereas Hisar shows an 18.7% increase (2013-2022). The Relative Importance Method was employed to examine the contribution of meteorological variables on the fog variations. The influence of wind speed on fog increased predominantly by 16.47% in Regime 3. The rise in temperatures and decline in relative humidity also played a significant role. The reduction in fog (urban holes) is also detected in MODIS/TERRA satellite and visibility changes in the IMDAA reanalysis dataset. The satellite-based analysis suggested that the fog holes across Delhi have increased from 2004 to 2014, and new fog holes have started appearing over several urban areas in the upper and middle IGP, such as Ludhiana, Jalandhar, Aligarh, and Bareilly. The increase in urban and built-up regions appears to be synchronized with the fog frequency. The fog frequency at Delhi and Hisar stations show strong positive correlations (0.79 and 0.82, respectively) with the urban and built-up areas and a high negative correlation (-0.85 at Delhi) with barren and sparsely vegetated surfaces. Based on the climatological analysis, this study suggests that fog variability is associated with alteration in meteorological parameters, most likely owing to human activities like urbanization.