Blowing in the wind

The way wind disperses pollution is a determining factor in the overall air quality management in regions

- ANUMITA ROYCHOWDHURY

It has become our unconscious habit during the winter months to keep looking at the trees for signs of leaves swaying in the wind. This is our way of knowing if there is wind in the city to blow away and lighten the heavy blanket of smog that enwraps Delhi and the entire Indo Gangetic Plain (IGP) during winter. Stillness of the air is bad news.

The killer smog experienced every winter is the result of what is commonly known as 'winter inversion'. Cooler earth surfaces during winter reduce temperatures near the ground, and prevent the air from rising up to disperse. The upper layer of warmer air caps and traps the cold air beneath. This leads to a massive trapping of pollution under the inversion layer.

In such a situation, the wind is the only saviour. Pollution can disperse only if the wind blows in the city and the region. Wind speed and direction, combined with the changes in air temperature, influence the pollution concentration across the land surface. The temperature changes in the air cause air pressure differences. The warm air rises and moves and leaves behind low pressure areas. The gases move from high to low pressure areas and disperse pollution. While gases are less dense in low pressure areas, they are highly concentrated in high pressure ones.

The insidious link between wind patterns and pollution concentration came out starkly from a study carried out by Delhi-based Centre for Science and Environment during the winter of 2023-24 in Delhi. Even though the crop fire incidents in Punjab and Haryana during that winter had not changed much compared to the previous winter, there was no other unusual increase in episodic pollution activities in the region. At the same time, there were considerable rains during that season. But the concentration of PM2.5 still increased in Delhi, impacting the overall annual levels.