Why is Delhi's air worse in winter?

Delhi does not emit vastly more pollution in winter; it loses its ability to get rid of what it emits, and a few seasonal sources arrive at exactly the wrong moment.

The 2024 bulletins show the shape plainly. January averaged an AQI of 355, November 374 and December 294. July and August, the heart of the monsoon, averaged 84.¹ The year calendar shows every day.

Why does cold air trap pollution?

Normally air near the ground is warmer than the air above it, so it rises and mixes, carrying pollution up and away. On still winter nights the ground cools quickly and the layer of air touching it becomes colder than the air above. That upside-down arrangement is a temperature inversion: long nights, clear skies and calm winds favour it, and the warmer air above acts as a lid that keeps emissions near the ground.²

Measurements at the Delhi Aerosol Supersite at IIT Delhi found that Delhi frequently has shallow inversion layers, less than 100 m deep, in winter, especially at night and in the morning. The height of the mixed layer multiplied by wind speed gives a ventilation coefficient, a measure of how fast the air flushes out; in winter it was four to six times slower than in summer.³ The same emissions, released into a much smaller and slower box of air, produce much higher concentrations.

The study also found that particles formed in the air from gases, the “secondary” particles, made up roughly half to 70% of the measured fine particle mass in winter and spring.³ The authors concluded that cleaning Delhi's air needs both local and regional action.³

Why does Delhi's geography make it worse?

Delhi sits in the Indo-Gangetic Plain, the broad, flat plain of northern India. When winds are light, pollution from across the region lingers over it rather than clearing. TERI–ARAI's modelling estimated that about two-thirds of Delhi's winter PM2.5 originated outside the city.⁴ Delhi is not a polluted island in clean country; it is a bright spot in a polluted region.

What does crop burning add, and when?

After the post-monsoon harvest in Punjab and Haryana, some farmers burn the leftover crop residue, known in Hindi as पराली (parali), and the burning overlaps the onset of winter weather. IIT Kanpur's study linked a rise in biomass-burning particles in early November to crop-residue fires in Punjab and Haryana, carried to Delhi on north-westerly winds.⁵

Its contribution is large but brief. In November 2024, estimates put stubble's share of Delhi's PM2.5 at about a fifth on average, peaking near two-fifths on the worst days.⁶ The following year shows why stubble cannot be the whole story: in November 2025 its estimated share fell to 7%, yet Delhi's average PM2.5 that month was 215 µg/m³, about twice October's.⁶ In December 2025, with the farm-fire share near zero, the city's PM2.5 averaged 210 µg/m³.⁷

Laws now treat stubble burning differently from other pollution offences: under the 2021 law that set up the Commission for Air Quality Management, farmers are excluded from its criminal penalties but can be charged environmental compensation.⁸

How much do Diwali firecrackers matter?

Diwali fell on 31 October in 2024.⁹ Delhi's bulletin AQI was 307 the day before, 328 on Diwali and 339 the day after, all Very Poor.¹ The year's worst readings came more than two weeks later: 494 on 18 November, part of a run of Severe days from 13 to 20 November.¹

That needs careful reading. A 24-hour city average smooths over the sharp overnight spike that firecrackers produce at individual stations, so the daily bulletin understates what people breathe on Diwali night. But the daily data also shows that Diwali is not what makes November November. A single night cannot explain a season-long pattern.

What does the data show, overall?

Read the 2024 calendar from left to right and the pattern is not a single spike but a slow closing and opening. The air starts improving in February, sits mostly in the Moderate band through spring, clears in the monsoon, and closes again from mid-October.¹ Emissions from traffic, industry, construction and households run all year. What winter changes is the weather that dilutes them, with stubble and Diwali stacked on top for a few weeks.

Rain helps for a day, not a season; the chapter on myths looks at 2024's first monsoon downpour. For what the authorities do when the lid comes down, see GRAP, CAQM and NCAP.