The thick cover of smog that covers Delhi during the winter months is back — only much sooner this time. The Indian capital, which was ranked as the world’s most polluted city by the World Health Organization (WHO) in 2014, has seen a sharp drop in its air quality over the last few days. The culprit is the annual burning of crop residues by farmers in the neighbouring state of Punjab.

On October 30, NASA captured this image of fires burning in the Indian state of Punjab. Red outlines indicate hot spots where the sensor detected unusually warm surface temperatures associated with agricultural fires.

A recent satellite image from NASA’s Earth Observing System Data and Information System (EOSDIS) showed fire spots, which denote blazes on the ground, all across Punjab and northern Haryana. The smoke in Delhi has been attributed to these agricultural fires, which are lit by farmers to clear their fields before the farming season in November. The practice is illegal, but persists despite efforts by the state governments to stop crop burning. However, environmental experts also say that Delhi’s toxic air is caused by growing vehicular emissions.

Smoke rises from fire made by burning grass and other dried plants in a field. Image: Altaf Qadri/Associated Press

A truck gives out smoke as it drives past a car in New Delhi. Image: Altaf Qadri/Associated Press

Last year’s WHO survey had revealed that Delhi had the most unhealthy air in the world, because of a high concentration of PM 2.5 and PM 10, tiny particulate matter with a diameter of less than 2.5 microns and 10 microns respectively. These fine particles can penetrate into the lungs, causing respiratory diseases. In the last few days, PM 2.5 levels have varied from 200 to 500 micrograms per cubic metre, though the safety limit is between 60 and 100 micrograms per cubic metre.

So far, the efforts to combat pollution have had mixed results. The Delhi government experimented with a car-free day for a few hours on 22 Oct., which led a short-term drop in dangerous pollutants. Last month, the Supreme Court ordered the imposition of an environmental cess on trucks travelling through Delhi, but subsequent investigations have revealed that the tax is still not being imposed in Delhi. On the other hand, the Supreme Court also denied a request to ban firecrackers during Diwali, which falls on Nov. 11.

Smoke rises from chimneys of brick kilns on the outskirts of New Delhi. Image: Altaf Qadri/Associated Press

Indian policemen wear masks as they control traffic in New Delhi. Image: Altaf Qadri/Associated Press

Studies from the last few years indicate that Delhi's air pollution only worsens during the cold months. With a long winter ahead, Delhi’s pollution woes may just get worse.