The study, carried out by Pravat Rabi Naskar of the Meteorological Watch Office Kolkata at the India Meteorological Department and Gyan Prakash Singh of the Department of Geophysics at Banaras Hindu University, draws on radiosonde and radiowind observations collected at 00 UTC for every winter season from December 1990 through February 2022. These balloon-borne instruments, launched routinely to profile temperature, humidity and wind through the depth of the troposphere, provide one of the longest continuous records of vertical atmospheric structure available anywhere in the region. The authors supplemented this archive with ERA5 reanalysis vertical velocity data from the Copernicus Climate Change Service, allowing them to connect the observed inversion behavior to the large-scale motions of the atmosphere above the Gangetic delta.

Thermal inversions come in several flavors, and distinguishing them matters enormously for forecasting. A surface-based inversion forms on clear, calm nights when the ground radiates heat to space and chills the air in contact with it. A subsidence inversion, by contrast, forms aloft when air in a high-pressure system sinks slowly from above; as the descending air compresses, it warms at a rate faster than the environmental lapse rate, creating a warm layer that floats above the cooler air below. The Kolkata analysis found that the winter inversions between the 1000 and 500 hectopascal pressure levels — a layer stretching from near the surface to roughly five kilometers altitude — are overwhelmingly of the subsidence type, pointing to the dominant role of large-scale atmospheric descent rather than purely local nighttime cooling.

One of the most striking trends the researchers documented concerns multiplicity. The percentage of winter days on which more than one thermal inversion appears within that 1000 to 500 hectopascal column is increasing at a rate of approximately one percent per year. Multiple stacked inversion layers are a well-known signature of complex vertical structure in the lower troposphere, and studies from eastern China and northwest China have linked such multilayer configurations to severe haze episodes, because each warm layer acts as a separate barrier to vertical mixing. Over Kolkata, the steady rise in double-inversion days suggests that the lower troposphere is becoming increasingly stratified in winter, a structural change that could compound the region’s already serious wintertime pollution problems.

The geometry of the inversions is changing too, and in a consistent direction. Over the 33-winter record, the height of the inversion top has been dropping at roughly 28 meters per year, while the base of the inversion has descended at about 20 meters per year. Because the top is falling faster than the base, the inversion layer itself is thinning by approximately 7 meters annually. A thinner inversion layer is not a weaker one; on the contrary, compressing the temperature contrast into a narrower vertical span can sharpen the stability of the layer, making it a more effective lid on the atmosphere beneath. The authors also found that inversion intensity — the temperature difference across the inversion — is increasing, although the inversion gradient, the temperature change per unit height, shows no statistically significant trend, presumably because the intensifying temperature jump is being spread across a shrinking layer.

What physical mechanism could drive this systematic downward migration and intensification? The most likely candidate is a change in the large-scale subsidence pattern over the region, which the ERA5 vertical velocity data were brought in to examine. Sinking motion in the mid-troposphere warms the air through compression, and the altitude at which that warming is strongest determines where the inversion forms. If the descending branch of the regional circulation has shifted lower, or if subsidence has strengthened in the layers just above the boundary layer, the inversion would be expected to form closer to the ground and to grow warmer relative to the air beneath it. Such changes in atmospheric stagnation have been flagged in recent work on Indian air pollution, including studies linking circulation-induced stagnation changes to surface PM2.5 concentrations across the subcontinent.

Perhaps the most consequential finding of the study concerns the planetary boundary layer — the shallow, turbulent skin of the atmosphere that responds directly to the surface and within which nearly all human emissions of pollutants are first released. The researchers found that the planetary boundary layer height over Kolkata airport sits consistently below the base of the subsidence inversion, and that it has been decreasing significantly at a rate of around 5.5 meters per year over the study period. This relationship is physically coherent: a subsidence inversion whose base lies just above the boundary layer suppresses the turbulent entrainment that normally allows the boundary layer to grow during the day. As the inversion base sinks, it effectively presses down on the boundary layer, compressing the volume of air into which vehicle exhaust, industrial emissions and dust are injected.

The implications for air quality are direct and sobering. A lower boundary layer means a smaller mixing volume, so the same quantity of emissions produces higher surface concentrations. A stronger, thinner inversion overhead makes that confinement even more effective by preventing any exchange of air across the stable layer. Research from around the world — from the Sichuan Basin in China to Hanoi, Istanbul, Tehran and the Canary Islands — has repeatedly documented the tight coupling between inversion characteristics and particulate pollution, and a recent national cohort study in China even linked thermal inversion exposure to increased risk of incident cardiovascular disease. For Kolkata, a megacity whose winter air already ranks among the most polluted anywhere, a trend toward stronger and lower inversions represents a slow tightening of the atmospheric vise.

Fog and aviation are equally implicated. The same authors previously documented a dense fog episode over Kolkata airport in January 2025 and traced its synoptic origins, and the present study provides the climatological backdrop against which such events unfold. Inversions trap moisture near the surface overnight, and shallow, intense inversions favor the radiative cooling and moisture convergence that produce dense fog — the single most disruptive weather phenomenon for airport operations in northern and eastern India. A boundary layer that is steadily losing height, capped by an inversion that is steadily losing altitude and gaining strength, is a boundary layer primed for more frequent and more persistent fog, with cascading effects on visibility, flight delays and safety margins during the December-to-February peak travel season.

What makes this study particularly valuable is its length and its focus on a single, operationally critical location. Thirty-three winters of consistent 00 UTC soundings from one site remove much of the noise that afflicts shorter or geographically scattered records, and the airport setting ties the results directly to a real-world stakeholder. The picture that emerges is of a lower troposphere in structural transition over the Bengal delta: inversions multiplying, sinking, thinning and strengthening, while the boundary layer beneath them contracts year by year. Whether these trends are driven by shifting regional circulation, by warming sea surface temperatures in the Bay of Bengal, by aerosol-induced changes in the radiation balance, or by some combination of all three remains an open question — but the direction of change is unambiguous, and it points toward winters in which the air over Kolkata becomes steadily harder for the atmosphere to flush clean.

Subject of Research: Long-term trends in winter thermal inversion characteristics and planetary boundary layer height over Kolkata airport

Article Title: Characteristics of winter thermal inversions over Kolkata airport

Article References: Naskar, P. R., & Singh, G. P. (2026). Characteristics of winter thermal inversions over Kolkata airport. Theoretical and Applied Climatology, 157(10), Article 645. https://doi.org/10.1007/s00704-026-06575-3

Image Credits: AI Generated

DOI: 10.1007/s00704-026-06575-3

Keywords: thermal inversion, Kolkata airport, radiosonde, planetary boundary layer, subsidence, air pollution, fog, ERA5 reanalysis, winter climate, atmospheric stability, India, boundary layer height

The jet stream is a narrow, meandering current of air near the tropopause, the boundary between the troposphere and the stratosphere, typically located several kilometres above the surface and flowing from west to east at speeds that can exceed 50 metres per second in winter. Over the Atlantic–European sector, this current is maintained by the temperature contrast between the warm subtropical ocean and the cold polar region, a contrast that generates pressure gradients at upper levels of the atmosphere. Because the jet acts as a kind of atmospheric conveyor belt, guiding storm systems from the ocean toward the continent, even modest shifts in its strength or position can reshape winter weather across Britain, Scandinavia, central Europe and the Mediterranean.

Understanding how the jet responds to greenhouse warming has been one of the more contested questions in climate dynamics. One influential line of argument holds that rapid Arctic amplification—the fact that the high northern latitudes are warming several times faster than the globe as a whole—reduces the equator-to-pole temperature gradient near the surface. A weaker gradient, in this view, should sap the energy available to the jet, producing weaker winds, larger meanders and a greater tendency toward persistent blocking patterns that lock cold air over Europe for weeks at a time. Another line of argument emphasizes that the upper troposphere warms more strongly than the surface in the tropics and mid-latitudes, which increases the vertical shear of the winds and can actually strengthen the jet aloft while shifting it poleward.

The new analysis addresses this debate by looking not only at the average behaviour of the Atlantic–European jet in winter but at the full distribution of its variability, including the extreme tail. Using observational and reanalysis datasets—long records of atmospheric conditions reconstructed from weather observations and satellite data—the researchers tracked jet strength over recent decades and examined how frequently the jet reached its most intense values. The central result is a nuanced one: the typical winter jet has become stronger, consistent with the expectation that upper-level warming and changes in the temperature gradient enhance the mean flow, but the extremes of jet strength have not kept pace. In relative terms, the jet is becoming a steadier current rather than a more violent one.

This distinction between the mean and the extremes matters for how scientists and society interpret jet stream change. A stronger mean jet is generally associated with a more vigorous storm track, which can bring more frequent passages of Atlantic low-pressure systems and the mild, wet, windy weather they deliver to northwestern Europe. Extreme jet episodes, by contrast, are often linked to exceptional weather: an unusually intense jet can coincide with explosive cyclogenesis, in which storms deepen rapidly and cause damaging windstorms, while an unusually weak or displaced jet can coincide with prolonged cold spells or drought. If the mean strengthens while the extremes moderate, the net effect on hazardous winter weather may be less dramatic than either a simple strengthening or a simple weakening story would suggest.

The physical reasoning behind such a divergence can be traced to how different components of the climate system respond to warming. The mean jet strength is governed largely by broad-scale temperature gradients and by the vertical structure of tropospheric warming, both of which evolve smoothly and predictably with rising greenhouse gas concentrations. Extreme jet episodes, however, often depend on transient processes: the phasing of planetary-scale Rossby waves, the life cycles of individual baroclinic storms, and episodic coupling with the stratospheric polar vortex. Some of these transient drivers may weaken or become less variable under warming, damping the upper tail of the jet distribution even as the background flow intensifies. The study’s results are consistent with such a mechanism, in which variability about the mean contracts even as the mean itself rises.

The findings also speak to a long-running scientific controversy about Arctic influence on mid-latitude weather. Over the past decade, a vigorous debate has played out in the journals over whether sea-ice loss and Arctic warming make European winters more prone to blocking and severe cold. Proponents of this view point to episodes such as the cold winters of recent decades and to modelling experiments in which reduced sea ice favours a wavier jet. Critics counter that the observational record is short, that internal variability is large, and that model projections more often show a strengthened, poleward-shifted jet with reduced waviness in winter. By documenting a strengthening yet less extreme jet, the new work lends weight to the second camp, while underscoring that the answer may differ by season, by region and by which aspect of the jet is measured.

Methodologically, the study illustrates the value of going beyond simple averages when assessing climate change signals. Jet stream behaviour is notoriously noisy: individual winters can differ enormously, and the Atlantic–European sector in particular is influenced by modes of variability such as the North Atlantic Oscillation, which swings between phases that strengthen or weaken the westerlies on timescales from weeks to decades. Separating a forced climate signal from this internal noise requires careful statistical treatment, long datasets and, in many cases, large ensembles of climate model simulations in which many parallel realizations of the same warming scenario are run to isolate the common response. The researchers’ focus on the distribution of jet strength, rather than a single metric of average speed, allows a more complete picture of how the flow is evolving and reduces the risk of drawing conclusions from a few memorable extreme winters.

The implications extend to practical forecasting and adaptation. Winter storm risk assessment for Europe depends on assumptions about how often severe windstorms strike, and those assumptions are typically based on historical statistics. If the character of the jet is changing—stronger on average but with moderated extremes—then the historical record may be an imperfect guide to the coming decades. Insurers, infrastructure planners and emergency services all have an interest in knowing whether the tail risks of winter weather are growing or shrinking. The new results suggest that for jet-driven wind extremes, at least, the most catastrophic outcomes may not intensify as rapidly as the mean conditions do, although the researchers caution that other hazards, such as precipitation extremes associated with a moister, warmer atmosphere, continue to worsen independently of the wind field.

There remain open questions. The observational record of the upper atmosphere is only a few decades long, and reanalysis products carry uncertainties, particularly in the earlier satellite era and in the data-sparse regions of the North Atlantic. Disentangling the roles of tropical upper-tropospheric warming, Arctic amplification, stratospheric variability and ocean circulation in shaping the jet will require further modelling work. Nevertheless, the study offers a clear and somewhat reassuring refinement of the picture: the Atlantic–European winter jet is not collapsing into weakness and chaos, nor is it becoming uniformly more ferocious. Instead, it is strengthening as a background current while its wildest excursions become relatively less extreme—a reminder that climate change rarely moves every aspect of a system in the same direction, and that the most useful science is often found in the details between the averages and the extremes.

Subject of Research: Winter strengthening and reduced extremity of the Atlantic–European jet stream

Article Title: A strengthening yet less extreme Atlantic–European jet during winter

Article References: Brönnimann, S., Brugnara, Y., & Kallabis, P. (2026). A strengthening yet less extreme Atlantic–European jet during winter. Nature Geoscience. https://doi.org/10.1038/s41561-026-02069-z

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02069-z

Keywords: jet stream, Atlantic–European jet, winter climate, North Atlantic Oscillation, Arctic amplification, storm track, climate variability, reanalysis, stratosphere, extreme weather, climate change, geoscience