What kind of weather is the air better?

The air affected by strong cold air is the best in winter. The reason is that the north wind behind the cold front is relatively strong and can blow away the dirty air. More importantly, the downward airflow in the cold high pressure behind the cold front moves the clean air at high altitude to the ground. The clear weather and good air in autumn are all similar reasons: the downward flow under high pressure brings high-altitude clean air near the ground, with less impurities, freshness and high visibility.