But then comes winter. Pollution in any city mixes vertically in the atmosphere, and the height at which this happens shrinks by more than half in the winter, raising the concentration of pollution. Two new sources also enter the mix. By the end of October, when the rains have ceased, the winds begin to blow in from the northwest, carrying fumes from burning fields. Then there is the Diwali, the popular festival lights, where millions burst fire crackers to celebrate.