Delhi's Air Pollution Explained!

An unusual confluence of factors caused unprecedented pollution and smog in the Delhi region soon after Deepavali, and experts warn of a similar episode in the days to come.

- R. Ramachandran

An unusual confluence of factors, the most important among them being meteorological, is what caused the unprecedented polluted atmosphere and smog in and around Delhi that hung low in the air for over a week following Deepavali (October 30).

Come winter, parts of north India, particularly the capital region of Delhi, are enveloped in smog during the night and early morning hours, which clears up in the morning as the sun moves higher up. This is owing to the formation of what is known as the “inversion layer”, a meteorological phenomenon that occurs during winter months, when the normal decrease in temperature with height is reversed across an atmospheric layer that forms close to the surface of the earth, at heights of 100-200 metres, and whose thickness can be a few hundred metres. “During November-December, this can be much lower, under 50 m at night and hovering around 200-500 m during the day,” said Sarath Guttikunda, founder of UrbanEmissions.Info.

Under normal conditions the air near the surface of the earth is warmer than the air above it because the atmosphere is heated from below as insolation warms the earth’s surface, which in turn warms the layer of the atmosphere directly above it. When the air is colder near the earth’s surface, like it is during winter, a warmer, less-dense air mass moves over the cooler, denser air mass forming an inversion layer. (Inversion layers form in summers too but they are formed at much greater heights of two to three kilometres.)