Does thermal inversion occur in the winter or in the summer?

Pollutant low altitude thermal inversion happens in the winter. In this period of the year the sun heats the soil less and the natural upward move of warm air reduces. Thus the pollutants form a low altitude layer among the cold air layer near the ground and another layer of warmer air above. The pollutant layer over industrial areas or big urban concentrations decreases the penetration of the sun's energy and the air bellow takes an even longer time to warm.