File:Smoke in Pakistan and India (MODIS 2018-11-24).jpg

English: Dense smoke shrouds northern India and Pakistan in mid-November, 2018. The Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Aqua satellite acquired this true-color image on November 19.

The time of year and location strongly suggests this smoke originates from agricultural fires just south of the image. Punjab state in India is a particularly rich agricultural region with two main cropping seasons: the Kharif (July-October) and Rabi (October-March). A shorter season for summer crops begins in March and ends in July. At the start of the main cropping season, farmers often use fire to clear debris from old crops from the field, return nutrients to the soil, and prepare the ground for a new growing season.

While fire helps enhance crops and grasses for pasture, the fires also produce smoke that degrades air quality. The smoke released by any type of fire (forest, brush, crop, structure, tires, waste or wood burning) is a mixture of particles and chemicals produced by incomplete burning of carbon-containing materials. All smoke contains carbon monoxide, carbon dioxide and particulate matter or soot. The area south of the Himalayas frequently has air inversions, where a layer of high-level air stays fixed in place, trapping the lower-level air close to the ground. Such inversions can last for many days, helping to create poor air quality near the ground, especially when smoke is present.