Delhi Temperature Inversion: Why Winter Pollution Builds Up
A temperature inversion occurs when temperature increases with height through a layer of air. This can suppress vertical mixing and allow pollution to accumulate nearer the ground. It changes how emissions disperse; it does not create those emissions.
NOAA explains the trapping mechanism. The exact height, strength and duration of an inversion vary with weather. A fixed “100–500 metre lid” is not a measurement for every Delhi night.
Why winter can bring difficult conditions
Cool nights and weak winds can restrict the volume of air available to dilute pollution. At the same time, the sources and chemistry of pollution change through the year. It is therefore misleading to say July and November have identical emissions and that the concentration difference is entirely due to an inversion.
Delhi’s air also receives pollution transported from elsewhere. Local emissions, regional transport, chemical formation and dispersion interact. See how to read Delhi source studies for why these contributions cannot be reduced to one permanent ranking.
Is the afternoon always better than the morning?
No. Daytime heating can improve mixing, but it does not guarantee cleaner air at a particular hour. Wind, cloud, local traffic, fires and other pollutants can change the pattern. A fixed claim that 2 pm is safe or that 6 am is always the worst hour could lead to a poor decision.
For outdoor activity, check a current local reading and advisory rather than the clock alone. A weather forecast is useful context, but it is not a substitute for pollutant measurements. Move or reduce strenuous activity when local guidance calls for it.
Does an inversion mean the air cannot move at all?
No. “Lid” is an analogy for restricted mixing, not an impermeable roof. Horizontal transport continues, and the atmosphere changes over time. A rising AQI alone cannot tell you the inversion height or identify which source caused the increase.
What to check indoors during a pollution episode
Record indoor PM2.5, whether windows are open, cooking activity and the filtration setting. Check airflow and filter condition if indoor readings rise. Keep necessary cooking exhaust and ventilation working; do not seal a home without accounting for replacement air and combustion safety.
A filtered fresh-air system reduces particles in its supply air. The indoor outcome still depends on openings, indoor sources, distribution and maintenance. Outdoor deterioration is not a reason to assume the indoor reading will stay exactly the same. aqi0’s technical note explains the relevant design and performance conditions.
Read the Delhi seasonal air-quality guide for the broader picture.