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Weakened Baroclinic Activity Causes an Abrupt Rise in Fog in the Indo‐Gangetic Plain

Dense and widespread winter fog in the Indo‐Gangetic Plain (IGP) causes a significant reduction in visibility and worsened air pollution. Despite extensive observational campaigns, the key processes in the formation and persistence of fog are not yet clear. The global increase in surface temperatures led to a decline in the incidence of fog, whereas the IGP experienced a sudden increase in fog incidence. It is shown here that abruptly reduced activity of the western disturbances (WD) around 1996–1997 resulted in a sudden decline of the cloud cover in the region, with a concomitantly enhanced radiative cooling of the surface and the atmosphere. These abrupt changes, aided by the impacts of human activities, altered the dynamics and thermodynamics of the boundary layer to favor fog formation. Also, the critical role of the extensive deep subsidence in the modulation of stable stratification and turbulence, which are essential for fog formation, is elucidated.