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. 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.
During thermal inversion, however, this vertical circulation pattern is broken. As winter arrives, the ground cools faster than the sky, making the air close to it cold and heavy. In India, this is also the time when people make bonfires to keep warm, sending up smoke that adds to pollutants. The following day, when the sun comes out, the layer of haze (thickened at present with smoke from farm fires too) prevents the sun from breaking through and warming up the ground. At the same time, the layer right above the haze, where the sky is clear, becomes warm.
Thus, the atmosphere is in a thermal inversion, with cold and still air sitting close to the ground, and the invisible lid bringing the mixing layer down.
Mahesh Palawat, vice president at Skymet Meteorology, said inversion was occurring at a height of around 500 metres on Friday — within which pollutants were getting trapped. “The temperature at that height was around 25°C and at the surface, was closer to 18°C in the morning. Normally, this should be the opposite — the higher we go, the more temperature drops. This is the reason we saw the dense layer of pollution,” Palawat said.
The inversion manifested in another manner. Delhi’s surface winds, which are between the 10-100m altitude, were almost calm till 2 pm. At the same time, upper-level transport winds, blowing at a height of 500-1000m logged speeds of 5-8k/hr. These winds, blowing from the northwest, kept bringing in a steady supply of farm fire smoke.
Kuldeep Srivastava, scientist at IMD, said while the visibility was around 600m at Safdarjung around 7am, it was at 500m at Palam. “This increased to 1,500m at Palam by 2pm and to 1,000m at Safdarjung,” he said, adding even though wind speeds had picked up, they were likely to become calm late at night.
Data from the Decision Support System estimated the highest contribution of Delhi’s PM2.5 was smoke from farm fires, which made up 35.4% of the pollution. This was followed by 11.6% from Delhi’s vehicular sector at 6.8%. Thermal inversions are in a large part due to low temperature. Delhi’s minimum was recorded at 16.8°C on Friday.
Experts said strong external winds are needed in such a scenario to break the inversion pattern. “Inversion is stagnation of the air with calm winds. In such a scenario, there is considerable built-up of pollutants and visibility starts to drop, because of the density of pollutants. Wind speed picked up on Friday afternoon, which was ultimately able to help disperse pollutants. As daytime temperature rises, this lid also begins to go higher,” said Mukesh Khare, an air pollution expert and professor at IIT Delhi