File:Fires and Smoky Haze over India (MODIS 2021-11-11).jpg

English: A thick shroud of smoke stretched over northern India on November 8, 2021, when the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite acquired this true-color image of the region. On that date, the layer of haze and smoke was so thick that it obscured the land from view in several areas. It also hovered over parts of Pakistan, spread across India, and blanketed Bangladesh—a swath that extended more than 1,200 miles (1,930 km). At the same time, smoke and haze extended more than 1,600 miles (2,575 km) from the foot of the Himalaya Mountains to far over the Arabian Sea.

Every year, farmers in northwest India light large numbers of small fires between September and December to burn off rice stalks and straw leftover after harvest, a widespread practice known as stubble or paddy burning. The many dozens of small red dots that can be seen in northwest India are areas where the thermal bands on the MODIS instrument detected high temperatures. When combined with typical smoke, as in this image, such hot spots mark actively burning fire.

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.