In normal conditions, Temperature of air decreases with the increasing heights, till the inversion layer. Normally, Air temperature decreases at a rate of 3.5°F for every 1,000 feet (or roughly 6.4°C for every kilometre), in this condition air is called unstable due to it constant movement between cold and warm layers. Temperature inversion layers is the layer at which the normal decrease in air temperature with increasing altitude is reversed i.e. the air above is warmer compared to the air below thus restricting the vertical movement of air.
If the inversion level is high, the air is better able to mix and spread around pollutants. In some areas where ground is very cool (Due to rapid radiation on ground in clear nights), inversion may happen at ground as well.
It also one of the reason behind increasing air pollution in Delhi. With te onset of winters in north, inversion level started lowering. Now Air pollutants are flowing at the lower level and their density in the air has increased.