New Delhi is suffocating. A shroud of toxic smog has enveloped the city for days, forcing residents to remain indoors and city officials to cancel school, institute vehicle restrictions and take other emergency measures due to the thick blanket of haze. What is taking place is a combination of out-of-control emissions from a variety of sources and the perfect recipe of conditions that’s trapping pollutants near the surface, causing air quality indexes to skyrocket.

The meteorology behind what’s turning Delhi into what its chief minister is referring to as a “gas chamber” includes the roles played by a temperature inversion, the towering Himalayan mountains to the north and multiple pollution sources.

Where is the pollution coming from?

Harvest season is wrapping up across northwestern India and neighboring Pakistan. In Punjab and Haryana states, northwest of Delhi, tens of thousands of farmers are burning the leftover stalks of their harvest. The most common cash crops in this part of India include wheat, maize and rice.

Because of the quick turnaround before the winter harvest, farmers have to clear the leftover vegetation from their land to be able to plant their next round of crops. Clearing the remnants, chopping them up and grinding them into the ground to mulch the soil would avoid causing significant air pollution, but these steps would also be time-consuming. A simpler solution? Burn it.

A look from above, via instruments on a NASA satellite, shows these fires burning as of Nov. 4.

All that smoke from agricultural fires has to go somewhere. That’s where the weather at this time of year becomes a problem.

The weather: A perfect storm to trap pollutants

Around the same time of year as agricultural burning in the northwest surges, seasonal shifts in the weather make for ideal conditions to wrap Delhi, a city of about 19 million that ranks as the most polluted major city in the world, in a poisonous veil.

Northern India is flanked by the Himalayan mountains to the north, and the Tibetan Plateau sits north of those jagged peaks. Even though it’s cold over the plateau, the high-altitude land mass heats the air above it so that this air layer becomes slightly warmer than air at the same altitude farther away.

In the summer, that inland heating over the mountains, contrasted with cooler, moist air over the Indian Ocean south of the country, helps spur the monsoonal deluges that routinely soak India. But in the mid-autumn, as the monsoon retreats southward, Delhi experiences more northerly breezes off the mountains.

A particular increase in the north-northwesterly component of the wind was observed in the past several weeks. That helps draw in both smoke-laden air from the northwest and air from the Tibetan Plateau and Himalayas.

When that mountain and plateau air is drawn toward Delhi, it sinks and warms. This creates an inversion — a layer in which the atmospheric potential temperature actually warms with height. (Most of the time the opposite is true, and the air gets cooler the higher one goes.)

This inversion prevents air near the ground from rising and mixing out air pollutants, thereby trapping air near the surface and allowing pollutants to accumulate with nowhere to go.

A weather balloon launched above Delhi early Sunday shows such an inversion taking place. At the surface, there may be some subtle cold air drainage within a few meters of the ground. Moreover, calm winds (indicated by the empty circle) allow for slight cooling at the surface.

The weak northerly winds exist above the surface and all the way through about 3,000 feet (though they’re not shown on the image), eventually turning more to the west around 3,900 feet. Within those first few thousand feet, the northerly winds drain air from off the mountains, compressing and warming it, and leading to that “lid” on the atmosphere about half a mile above the ground. Beneath it, pollutants are trapped.

The result? Surface pollutants are trapped beneath that layer of warm air, accumulating in the lowest half-mile of the atmosphere without any means of escape. Meanwhile, the calm near-surface winds don’t carry it anywhere, either.

What’s in the smoke?

Paddy burning releases soot, carbon and greenhouse gases — such as carbon dioxide, nitrogen oxide and methane — in addition to aerosols, including volatile organic compounds and fine particulate matter. Agricultural burning has been a problem for years, with Punjab’s pollution in particular dubbed “a matter of serious concern not only for [greenhouse gas] emission but also for problems of pollution, health hazards and loss of [soil] nutrients,” according to a paper from the Indian Agricultural Research Institute.

Between 2003 and 2017, the number of these agricultural fires in Punjab, Haryana and western Uttar Pradesh, which border the Himalayas, tripled.

Meanwhile, the poisonous cloud hanging over Delhi is reinforced by industry and transportation, which can boost the presence of gases and tiny particulate matter. On the worst day of the latest pollution crisis, Nov. 3, the average concentration of fine particulate matter, known as PM2.5, across the city was measured at about 25 times the World Health Organization’s guidelines.

Such particles can enter human lungs and the bloodstream, aggravating respiratory and other ailments, and can prove deadly.

Last year, the number of vehicles in Delhi alone surpassed 10 million, including 7 million mopeds and motorcycles cramming city streets. The latter are known for their poor emissions standards. Many of these vehicles are older, with incomplete or dirty combustion injecting small metallic compounds into the air. Transportation combines with industry as other major sources of air pollution in Delhi. AirVisual lists 15 Indian cities in its compilation of the 20 most polluted in the world.

The NDTV news broadcaster reported that Delhi’s 24-hour average air quality index stood at 494 on Sunday afternoon, the highest and most dangerous value in nearly three years.