Chennai and its surrounding districts form India’s automotive capital, with the production of automobiles, heavy vehicles, motorbikes, and tractors from some of the biggest manufacturers in the world, including Ford, Hyundai, BMW, Daimler, Renault, Volvo, and Caterpillar, and also regional players such as Force and Ashok Leyland. An industrial powerhouse, the state also hosts two petroleum refineries and several other heavy industrial units, and over 7,000 small and medium manufacturing industrial units.

Tamil Nadu boasts of multiple coal-fired power plants and Chennai has the third highest number of automobiles in India, next only to Delhi-NCR and Bengaluru. If you have never been to Chennai, you would imagine it to be sitting in a dense grey soup of smog fed by the chimneys and tailpipes of over half a million automobiles, buses, and trucks, much like Delhi during the Great Smog.

And yet, this mega-city largely has breathable air, with moderate levels of particulate matter, and is devoid of the kinds of smog Delhi and much of northern India sees. Being a coastal city, the wind carries the air pollutants away from it.

The most fundamental parameter that impacts air quality is the speed and direction of wind. Winds from the Bay of Bengal therefore act like a continuous “reset” button to the city’s air quality.

Cities in northern India, unfortunately, have no such luck. While there are several contributory sources of air pollution in the region, pollution levels remain at critical levels to a considerable extent due to geographic and meteorological conditions. And that is Delhi’s—and northern India’s—geographical and meteorological misfortune.

The lack of winds that can carry away pollutants is one of the most important factors impacting air quality.

In Delhi, the average wind speed in winter ranges between one and three metres per second, which is nearly one-third of the average speeds in the summer months and much lower than the average wind speed in Chennai. A study by Sarath Guttikunda and Bhola Gurjar considering nineteen years of Delhi’s meteorological conditions revealed that in Delhi, pollution levels are 40% to 80% higher in the winter months compared to the rest of the year. Even on average, the PM (particulate matter) pollution levels exceed the national ambient standard of 100 micrograms per cubic metre by two to three times. The lack of winds that can carry away pollutants is one of the most important factors impacting air quality.

To make matters worse, Delhi is cursed with poor geography as far as air pollution is concerned. The capital city lies to the north-east of the Thar Desert, to the north-west of the central plains and to the south-west of the Himalayas. As winds arrive from the coasts, bringing with them pollutants picked up along the way, they get ‘trapped’ right before the Himalayas. The air pressure pushes from one direction, and with the inability to escape quickly in the other, the particulate matter accumulates over the northern plains. This accumulation and entrapment affect not only Delhi, but the entire expanse between Punjab in the west to West Bengal in the east. Imagine this as a “bowl” that collects pollutants, with only a narrow outlet for it to escape.

Particulate matter—especially dust—is carried not only from the Thar Desert, but can originate from much farther-off distances. For instance, the smog during Nov. 6-14, 2017, was fed by activities thousands of kilometres away. Between these dates, the Air Quality Index (AQI) had been consistently in the red zone, at hazardous levels of pollution. According to SAFAR (System of Air Quality and Weather Forecasting And Research), 40% of the particulate pollution in Delhi on those specific days could be sourced to a “multi-day dust storm” that originated in the Middle East (or from India’s perspective, the “Middle West”) a few days earlier in October 2017 and was carried to India with cool winds. These were also the days when stubble burning was in progress. However, as per SAFAR, such burning contributed to 25% of the particulate matter, which is much lower than that originating from the Gulf dust storm.

India’s northern plains are not unique in this geographic and meteorological misfortune.

And it is not just dust that these winds carry into Delhi. The north-westerly winds that come into Delhi transport sulphur dioxide (SO2) emitted from large power plants and refineries that are situated upwind of Delhi. There are, after all, thirteen power plants that run on fossil fuels in a 300km radius of the capital. India’s northern plains are not unique in this geographic and meteorological misfortune. Several global regions have to face the brunt of dust storms that originate far away and winds that carry pollutants from other regions. The issue is critical in cities such as Los Angeles and Ulaanbaatar, as both form a valley terrain, owing to which emissions linger on for longer even when wind conditions are strong.

The issue is so endemic in Los Angeles in particular that starting from the very first written accounts of the area there have been descriptions of the haze that blankets the region. In the 1540s—way before modern industry and fossil fuel-based transport came into being—a Spanish ship captain named the area “Baya de los Fumos,” which translates to “Bay of Smoke.”

The smoke itself originated from the coastal villages, but it was no different from the smoke from any cluster of villages in the world at that time. What made it special was that there were several inland valleys in the region that captured the smoky air and an “atmospheric lid” prevented it from escaping. This invisible lid is a consequence of a meteorological phenomenon called “temperature inversion,” where cool marine air cannot rise through the warm overland air beyond the valleys.

North India’s air suffers from similarly challenging topography. In fact, India happens to be in the middle of a large transcontinental “arc” with high particulate pollution. This “arc” is the mass of land with high air pollution levels starting from Mauritania, Algeria, and Mali in the north-west of Africa, right up to China on the east of Asia. Countries in between this vast geographical landscape—including Libya, Egypt, Saudi Arabia, Iran, Afghanistan, Pakistan, and India—all have high particulate pollution.

North Africa and the Middle East had high concentrations mainly due to high levels of windblown mineral dust. Countries like India and Bangladesh, however, had high particulate emission levels owing to a multiplicity of factors including household cooking, coal-fired power plants, agricultural and other open burning and industrial and vehicular emissions. PM2.5 concentrations, when weighted by population, have been steadily rising in the case of India and Bangladesh, while they have remained more or less stable (but high) for other countries such as China and Pakistan.

That northern India is cursed with such factors beyond its control should not be an excuse to do nothing.

Importantly, even though energy use and greenhouse gas emissions are significantly higher in North America and Europe, air quality is generally better and in the safe zone in most places. This is both a result of cleaner burning and comprehensive policies that address air pollution, but also due to the geographical and meteorological fortunes of these regions.

That northern India is cursed with such factors beyond its control, however, should not be an excuse to do nothing. After all, even after poor geography, meteorology, and natural sources of dust are accounted for, there are still significant pollutants that arise from anthropogenic sources—in other words, from human activity. These anthropogenic factors include manufacturing activity, power generation, construction, and transport. While these economic activities keep contributing to the emissions stock around the year, the system is given a “shock” every winter from the agricultural sector owing to the burning of crop residue.

Excerpted with the permission of Penguin Random House India from The Great Smog of India by Siddharth Singh. We welcome your comments at ideas.india@qz.com.