Behind bad air trap, phenomenon that only strong winds can negate

On Wednesday, there was a brief window during the afternoon when winds strengthened slightly. Not exactly perceptible as a breeze, it helped clear up the hazy, polluted sky ever so slightly.

- Jasjeev Gandhiok

But as night fell, the pollutant levels began rising steadily. The following day, there was no brief clearing up through Thursday, the haze only thickened and the sunlight only weakened. By the night, the air quality index had crossed 400, reaching in the "severe category". On Friday, it only got worse.

While local pollutants - tailpipe gases, dust and industrial emissions in Delhi - and smoke from farm fires in Punjab and Haryana are the primary causes for the crisis, an atmospheric phenomenon called thermal inversion is making matters worse, creating an invisible lid that is trapping pollutants instead of letting them disperse.

To understand this phenomenon, it must be remembered that air follows a cycle of warming and cooling. As the ground heats up in the day, the air closest to it warms up and rises, making way for colder air that sinks from above. The maximum height till which this warm-cold air circulation occurs is known as the mixing layer height-the higher it is, the further aloft pollutants are dispersed, helping the air be cleaner closer to the settlements.