What Is an Inversion? When the Air Gets Warmer as You Go Up

Through the lowest part of the atmosphere, temperature falls with height at a rate averaging around six and a half degrees per kilometre, and that arrangement allows convection, since a parcel of warm air rising from the surface finds itself warmer than its surroundings and keeps rising. An inversion is a layer where the temperature increases with height instead, and it acts as a lid, because a rising parcel reaches it, finds itself cooler than the surrounding air and sinks back. The layer is stable in the technical sense, resisting vertical motion rather than permitting it, and the practical consequence is that the air beneath is sealed off from the air above and is not being exchanged or diluted.

Several mechanisms produce the reversal and they occur in different settings:

• •Radiative cooling at night, where the ground loses heat and chills the air immediately above it, producing a shallow inversion by morning
• •Valley drainage, where cold dense air flows downhill and pools in the bottom under warmer air
• •Subsidence, where air sinking in a high pressure system warms by compression and caps the air below
• •Warm air advected over a cold surface, common over snow, over cold seas and along coasts
• •Frontal inversions, where warm air rides over a wedge of colder air at a front
• •The marine layer along certain coasts, where cool ocean air sits under warm subsiding air for months at a time

The consequence that matters most to people is the accumulation of whatever is emitted underneath. Vehicle exhaust, domestic heating, industrial emissions and smoke from burning all enter a layer that may be only a few hundred metres deep and that is not exchanging with the air above, so concentrations rise through the day and continue rising as long as the inversion persists. Cold still weather produces inversions and also increases emissions from heating, which compounds the problem. The worst recorded episodes followed exactly this pattern, including the London smog of December 1952, which killed thousands over several days under a persistent inversion, and comparable events in other industrial cities. Valley cities and basin cities are particularly prone because the topography holds the cold air in place, which is why certain places have chronic winter air quality problems regardless of how much they regulate.

An inversion ends when something supplies the energy or motion to overturn it, and which of those happens determines how long the episode lasts. Solar heating of the ground warms the air at the surface until it is warm enough to rise through the layer, which clears a shallow night-time inversion by mid-morning in summer and may fail entirely in midwinter at high latitudes where the sun is too weak. Wind mixes the layers mechanically, which is why a breeze clears trapped pollution rapidly and why calm conditions are the dangerous ones. A change of air mass sweeps the whole arrangement away. Subsidence inversions can persist for a week or more because the sinking air that produces them is continuously renewed, which is why the serious pollution episodes are associated with stationary high pressure systems rather than with any particular emission event.

Inversions do several more things that are visible or audible. Fog forms readily under them, since the trapped air is cooled to saturation and has nowhere to go, which is why valley fog sits in a defined layer with clear air above. Sound carries unusually far, because the temperature structure bends sound waves back towards the ground rather than letting them escape upwards, which is why distant trains and traffic are audible on still cold nights. Smoke from a chimney flattens out and spreads horizontally on reaching the lid rather than rising, which is a direct visual indicator. Radio signals can propagate far beyond their normal range by ducting along the layer. And pilots encounter the boundary as a distinct change in visibility and turbulence, with smooth air above and hazy air below.

Air normally cools with height, which permits convection, and a layer where it warms instead acts as a lid that rising air cannot penetrate. Night-time cooling, cold air pooling in valleys, and sinking air in high pressure all produce them. Everything emitted underneath accumulates, which caused the deadly urban smog episodes and explains chronic winter pollution in valley cities.