File:Fires and Smoke in India (MODIS 2021-12-15).jpg

English: A heavy haze hung over the plains at the foot of the Himalaya Mountains in early December 2021. The Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Terra satellite acquired a true-color image of part of the haze on December 14, 2021.

The shroud of smoke began in northern Pakistan, to the east, and stretched more than 1,200 miles (1,930 km) to the foothills of the Eastern Hills, running between India and Myanmar (Burma). The thick gray haze also covered most of Northern and Central India, stretching from the Himalayas to the Arabian Sea (in the west) and over Bangladesh and the Bay of Bengal (east). In many locations, the brownish-gray haze was so dense that it obscured the ground from view.

At least part of the haze is made up of smoke from agricultural fires, which are widely used in northwestern India and northern Pakistan to manage crops. The major season for burning stubble from fields is from September to late November. Although it is late in the season, other MODIS images confirm that clusters of fires still burn in the Punjab region of India, contributing significantly to the smoke and haze in this image.

Smoke is undoubtedly a major contribution to the gray pall over India, but it is likely that both urban and industrial emissions contribute to the haze. The atmospheric conditions also make a contribution to the density and extent of the blanket of smoke. During the long burning season, smoke is near-constant in northern India but it not often as dense as seen here. In warm weather, the air nearer the ground is warmer than the air above it, so smoke produced near the ground rises upward and disperses high above. As cold weather arrives in the Himalayas, cold air drops from the mountainside to linger over the agricultural plain. This creates a temperature inversion, where a layer of warmer air lies over a low-level cooler air layer. The warm layer acts like a lid, effectively trapping the colder air —and any pollutants in that layer—underneath. As a result, thick haze and smoke continues to build until the inversion lifts.