MC Explains: What is thermal inversion and how it impacts Delhi's air pollution

The phenomenon responsible for rise in air pollution is recognised as thermal inversion.

As the air pollution in the national capital continues to be classified as `severe,’ on November 5, state education minister Atishi Marlena instructed primary schools in Delhi to remain closed till November 10, and gave them the option of holding online classes for those in grades 6 to 12.

In the afternoon of Wednesday, November 1, there was a brief, windy spell, which slightly cleared the hazy and polluted sky. However, that was fleeting, and as night approached, pollution levels started increasing steadily, and throughout Thursday, the haze thickened and sunlight grew dimmer. By nightfall, the air quality index (AQI) had surpassed 400, and the situation was categorised as `severe.' On Friday, November 3rd, conditions deteriorated further.

One of the reasons for this is a phenomenon called thermal — or temperature — inversion. This refers to the reversal of the usual temperature patterns on earth. Thermal inversion traps pollutants such as vehicle and industrial emissions, dust, smoke from farm fires in Punjab and Haryana, etc., under an invisible layer instead of allowing them to disperse freely.

To grasp this phenomenon, it's essential to understand the natural cycle of air warming and cooling. When the ground heats up during the day, the air near the surface also warms up, and rises, creating space for cooler air from above to take its place. The height at which this warm-cool air circulation takes place is termed the mixing layer height. The greater the height, the farther pollutants are dispersed upwards, leading to cleaner air near human settlements.

However, during thermal inversion, this vertical air circulation is disrupted. In winter, the ground cools down more rapidly than the air, causing the air near the ground to become cold and dense. In India, this season coincides with people lighting bonfires to stay warm, releasing smoke that contributes to pollutants. The next day, when the sun rises, the thick haze blocks the sunlight from penetrating the air and warming the ground. As a result, the layer above the haze, where the sky is clearer, becomes warm. Thus, the atmosphere experiences thermal inversion, and the invisible barrier lowers the mixing layer.

Last Wednesday, Delhi's surface winds, which occur between 10-100 metres above ground level, remained nearly still until 2 pm. Simultaneously, winds at higher altitudes, ranging from 500-1,000 metres, were moving at speeds of 5-8 kilometres per hour. These winds, originating from the northwest, continuously carried smoke from farm fires into the area.

Kuldeep Srivastava, a scientist at Indian Meteorological Department (IMD), noted that the visibility at Safdarjung was approximately 600 meters around 7 am, while it was 500 meters at Palam. "By 2 pm, visibility improved to 1,500 meters at Palam and 1,000 meters at Safdarjung," he said.

Mahesh Palawat, vice president at Skymet Meteorology, said that on Friday, inversion was taking place at a height of approximately 500 metres, trapping pollutants within this level. “The temperature at the top was around 25°C, and at the surface, it was closer to 18°C in the morning. Normally, this should be the opposite — the higher we go, the lower the temperature should be. This is the reason we saw the dense layer of pollution,” he said.

Thermal inversion is largely attributed to low temperature, and the city recorded a minimum temperature of 16.8°C on Friday, 3rd November.

The Air Quality Index of 500 has become the new normal with pollution hitting the severe mark and air quality reaching the hazardous stage.