The study area itself is almost engineered for fog. The Eastern Gangetic Plain spans roughly 94,000 square kilometres of flat land lying mostly below 100 metres, wedged between the Himalayas to the north, with peaks exceeding 8000 metres, and the Chotanagpur Plateau to the south. This basin-like geometry traps moisture and aerosols, while westerly and northwesterly winds in winter funnel pollutants from western India and Pakistan into the region like a low-level atmospheric river, supplying abundant condensation nuclei. Add elevated soil moisture from irrigation and river systems, recurrent cold-wave episodes, and weak pressure gradients that keep horizontal winds sluggish, and the ingredients for persistent dense fog assemble almost automatically each December and January. At Jay Prakash Narayan International Airport in Patna, situated centrally in the basin at about 53 metres above sea level, runway visual range regularly falls below operational minima, producing cascading delays, diversions, and cancellations.

Two contrasting fog episodes dominated the study window, and their differences turned out to be scientifically revealing. The first, from 16 to 19 December, was synoptically forced: a deep, positively tilted upper-level trough anchored over northwest India drove strong southwesterly jet flow above 20 metres per second, cold-air advection, and a spatially coherent signature of subsidence at 700 hectopascals, with pressure velocities between minus 10 and minus 20 times ten to the minus two pascal-seconds. This descending branch of an active western disturbance reinforced a capping inversion over the fog layer. The second episode, from 23 to 27 December, unfolded after the trough moved eastward and quasi-geostrophic forcing for ascent weakened. Vertical motion over Patna became fragmented and nearly neutral, and the boundary layer shifted into a radiative-thermodynamic regime, exquisitely sensitive to the diurnal cycle, yet paradoxically more stable and longer-lived than the first.

The turbulence diagnostics from the second episode were extraordinary. Turbulent kinetic energy, which peaked near 60 square metres per second squared around 23 and 24 December, collapsed to nearly zero by 25 December and stayed there. Wind shear fell from about 10 times ten to the minus three per second to between 3 and 5 times the same unit. The bulk Richardson number, a dimensionless ratio weighing buoyant suppression of mixing against mechanical shear production, climbed to roughly +300 on 24 December and then held between +100 and +200 for three days, the highest sustained values in the entire record. Ventilation coefficients, the product of boundary-layer height and 10-metre wind speed, crashed from about 6000 square metres per second to 1000 to 1500, indicating the most severe atmospheric dispersion limitation observed. Mixing heights were squeezed to just 250 to 350 metres. In effect, every mechanism that could erode the fog was switched off simultaneously.

From the radiosonde analysis emerged what the authors call a double-lock mechanism for fog persistence. Persistent dense fog required not one condition but a coordinated set: surface-based temperature inversions with bases below roughly 150 to 200 metres, moderate thermal strengths of 2 to 6 degrees Celsius, vertical depths exceeding 2000 metres, bulk Richardson numbers above 10 to 20, and ventilation coefficients below about 5000 to 6000 square metres per second. When normalized inversion strength and depth both exceeded 0.5 alongside these stability and ventilation constraints, persistent fog reliably followed. The 2000-metre depth threshold proved especially diagnostic, separating brief events lasting under six hours from prolonged ones exceeding fifteen hours, consistent with mixed-layer theory in which deeper capping inversions resist turbulent erosion far longer than shallow ones.

Perhaps the most striking single observation came on 29 December, after the main episodes had ended. A powerful inversion with a thermal strength of 10.5 degrees Celsius sat with its base above 700 metres, and it produced no fog at all. The reason is geometric rather than thermodynamic: an elevated inversion decouples the stable layer from surface moisture, so however strong the temperature jump, it cannot trap water vapour where fog droplets need to condense. Inversion placement, the study concludes, matters more than inversion intensity. This finding directly challenges forecasting practice that weights inversion magnitude alone, and it underscores the primacy of surface-atmosphere coupling in continental fog formation. During the same transition, the Richardson number swung from +60 to -50 within twenty-four hours, ventilation coefficients surged to 8000 to 10000 square metres per second, and inversion strength collapsed from 10.5 to below 2 degrees Celsius, a coordinated multi-parameter disaggregation triggered by synoptic rearrangement.

To understand how fog actually dies, the researchers built a rule-based classification framework assigning each of 49 dissipation events to radiative, subsidence-driven, mechanical, or mixed categories, ordered by precedence from synoptic vertical motion through surface wind and shear to residual radiative signatures. The result was unambiguous: mechanically driven processes, whether acting alone or jointly with other boundary-layer dynamics, dominated fog erosion. Mixed-mechanical events accounted for 51.0 percent of cases and pure mechanical dissipation another 20.4 percent, meaning wind shear intensification and turbulent mixing governed roughly 71 percent of all fog clearance at Patna. Mixed-subsidence pathways contributed 14.3 percent, while pure radiative, pure subsidence, and mixed-radiative mechanisms together managed only about 8 percent, a reflection of winter’s short days, low solar angles, and optically thick fog layers that starve the surface of warming radiation. A small residual of 6.1 percent remained diagnostically ambiguous, clustered under extreme stability where all dissipative signals fell below detection thresholds.

The framework was stress-tested and verified rather than merely asserted. A sensitivity analysis perturbing each classification threshold by plus or minus 10 and 20 percent of its interquartile range showed that radiative thresholds were highly stable, with zero reclassification for several parameters, while mechanical thresholds, ironically governing the dominant category, were the least robust, reclassifying up to 39 percent of events under the largest perturbation. Binary verification of the fog-favourable criterion, a transitional inversion with base between 50 and 500 metres, against 384 matched surface observations yielded an accuracy of 0.77, precision of 0.76, recall of 0.33, an F1-score of 0.46, and a Heidke Skill Score of 0.34, confirming genuine predictive skill beyond climatology. The high-precision, low-recall profile positions the tool as a conservative, low-false-alarm mechanism-attribution instrument rather than an exhaustive all-height fog forecaster, a distinction with real operational consequences for aviation safety managers who must balance disruption costs against missed warnings.

The practical payoff is a companion Fog Dissipation Potential Index that distills the multi-parameter picture into short-lead operational guidance, distinguishing transient from persistent fog and clarifying where predictability simply runs out under extreme stability. The authors benchmarked their diagnostic approach against a complementary decision-tree nowcasting system developed for Delhi’s Indira Gandhi International Airport, noting that the two converge on similar wind-speed regime boundaries and could be paired operationally, since the decision tree’s known failure mode, fog onset without relative humidity saturation at tower level, is precisely the signature this framework would attribute to subsidence or mechanical forcing. The team is candid about limitations: aerosol indicators such as PM2.5 and cloud condensation nuclei were deliberately excluded, turbulent kinetic energy cannot be resolved from standard ERA5 fields, thresholds are site- and season-specific, and some moisture parameters are collinear by construction. Even so, for one of the most fog-burdened and economically vital regions on Earth, the study delivers something rare: physically interpretable, quantitatively verified thresholds that explain not just when the fog comes, but why it stays, and what finally sets the plains free.

Subject of Research: Boundary-layer thermodynamic controls on winter fog persistence and dissipation over the Eastern Gangetic Plains

Article Title: Fog persistence and dissipation over the Eastern Gangetic Plains insights from boundary layer thermodynamics during 16 to 31 December 2025

Article References: Shankar, A., Kant, S., Anand, A., & Sarthi, P. P. (2026). Fog persistence and dissipation over the Eastern Gangetic Plains insights from boundary layer thermodynamics during 16 to 31 December 2025. Discover Geoscience, 4(1), Article 307. https://doi.org/10.1007/s44288-026-00684-2

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00684-2

Keywords: fog, Indo-Gangetic Plain, boundary layer, temperature inversion, Richardson number, ventilation coefficient, radiosonde, ERA5 reanalysis, aviation meteorology, fog dissipation, Patna Airport, western disturbances