Temperature Inversion

A temperature inversion occurs when air temperature increases with height through a layer, instead of decreasing as it normally does in the lower atmosphere. This creates stable atmospheric conditions that resist upward air movement, allowing pollutants to accumulate beneath or within the inversion layer.

Why an inversion suppresses vertical mixing

Vertical dispersion depends on rising air currents and turbulent mixing carrying pollutants away from their sources. An inversion weakens these processes by reducing the buoyancy of rising air.

• An air parcel rising through the atmosphere expands and cools as pressure decreases. Within an inversion, the surrounding air becomes warmer with height, so the parcel becomes cooler and denser than its surroundings.
• This difference produces a restoring force that opposes further ascent, suppressing convection and limiting the upward transport of pollutants.
• The inversion acts as a cap on vertical mixing, not as a solid barrier. Mechanical turbulence can still produce some mixing, depending on wind conditions and inversion strength.

Conditions that create pollutant-trapping inversions

The height, strength and duration of an inversion determine the volume of air available to dilute emissions. An inversion close to the ground is especially important for pollution from surface sources.

• A radiation inversion can develop on clear, calm nights when the ground loses heat rapidly and cools the air immediately above it, while air higher up remains relatively warmer.
• In valleys, cold, dense air can drain downslope and collect on the valley floor beneath warmer air, creating a valley inversion.
• A subsidence inversion forms when sinking air warms by compression above cooler lower air, placing a stable layer over air near the surface.

Winter pollution over northern Indian cities

During northern India's winter, long nights and weak daytime heating favour rapid surface cooling and persistent inversions. Emissions are then confined within a shallow mixing layer, increasing near-surface concentrations even without an increase in emissions.

• Transport, industry, power generation, construction and road dust, domestic fuel use, waste burning and regional biomass burning supply particulate matter and gaseous pollutants that an inversion can trap.
• Weak horizontal winds compound the problem because they reduce ventilation and the transport of polluted air away from cities.
• Repeated cold nights can recreate the inversion before accumulated pollution has dispersed, producing multi-day episodes of poor air quality and reduced visibility.
• An inversion does not itself generate pollution. Pollution severity depends on emissions, inversion strength and duration, mixing-layer depth and horizontal ventilation.
• Daytime surface heating can weaken or remove a nocturnal surface inversion by restoring convection, although strong or elevated inversions may persist. Stronger winds can also improve dispersion.